Inside the guidance system and computer of the Minuteman III nuclear missile

The Minuteman missile was introduced in 1962 as a key part of America's nuclear deterrent. The Minuteman III missile is currently the only US land-based intercontinental ballistic missile (ICBM), with 400 missiles ready for launch, spread across five central states.1 The missile contains a precision guidance system, capable of delivering a warhead to a target 13,000 km away (8000 miles) with an accuracy of 200 meters (660 feet).

The diagram below shows the guidance system of the Minuteman III missile (1970). This guidance system contains over 17,000 electronic and mechanical parts, costing $510,000 (about $4.5 million in current dollars). The heart of the guidance system is the gyro stabilized platform, which uses gyroscopes and accelerometers to measure the missile's orientation and acceleration. The computer uses the measurements from the platform to determine the missile's position and guide the missile on its trajectory to the target. Other key components are the missile guidance set controller, which contains electronics to support the gyro stabilized platform, and the amplifier, which interfaces the computer with the rest of the missile. In this blog post, I take a close look at the components of the guidance system that was used until the early 2000s.2

The Minuteman III guidance system (NS-20). Click on this image (or any other) for a larger version. Original image from National Air and Space Museum.

Fundamentally, the guidance computer constantly compares the missile position to the desired trajectory and generates the appropriate steering commands to keep the missile on track.3 The diagram below shows how directing the engine nozzles causes the missile to rotate around its three axes: roll, pitch, and yaw.4 In the silo, the roll angle (the azimuth) is aligned with the direction to the target. The missile takes off vertically and then the missile gradually rotates along the pitch axis to tilt over toward the target. During flight, adjustments along all three axes keep the missile on target. The Minuteman III has four rocket stages so the guidance computer jettisons each rocket stage and ignites the next stage in sequence.

The roll, pitch, and yaw axes for the Minuteman missile. The engine diagrams show how the nozzles are directed to rotate around each axis, Modified from A Simulation of Minuteman Trajectories, with changed axes.

The guidance platform

The idea behind inertial navigation is to keep track of the missile's position by constantly measuring its acceleration. By integrating the acceleration, you get the velocity. And by integrating the velocity, you get the position. Inertial navigation is self-contained, a big advantage for a missile since the enemy can't jam your navigation. The hard part is measuring the acceleration and angles with extreme accuracy, since even tiny errors are multiplied as the missile travels.

In more detail, the Minuteman's inertial guidance is built around a gyroscopically stabilized platform, which is kept in a fixed orientation. The platform is mounted on two beryllium gimbals. Feedback from gyroscopes drives three torque motors to rotate the gimbals to keep the stable platform in exactly the same orientation no matter how the missile twists and turns.

The Minuteman III stable platform. Original image from National Air and Space Museum.

The diagram below shows the components of the stable platform, in approximately the same orientation as the photo above. Three accelerometers are mounted on the stable platform to measure acceleration. The accelerometers are oriented along three perpendicular axes so each one measures acceleration along one axis. (The accelerometer axes are not aligned with the platform axes; this distributes the acceleration (mostly "up") across the accelerometers, increasing accuracy.) The two alignment mirrors allow the stable platform to be aligned with a precise device called an autocollimator, as will be described below. The gyrocompass uses the Earth's rotation to precisely determine North, providing a backup alignment technique. Both the alignment mirrors and the gyrocompass can be rotated to a precise angle, reported by the resolver.

The stable platform for Minuteman II and III. Modified from Minuteman weapon system history and description.

To target a Minuteman I missile, the missile had to be physically rotated in the silo to be aligned with the target, an angle called the launch azimuth. This angle had to be extremely precise, since even a tiny angle error will be greatly magnified over the missile's journey. Aligning the missile was a tedious process that used the North Star to determine North. Since the star was not visible from inside the silo, a complex surveying technique was used, using a surveyor's theodolite to measure the angles between the North Star and three concrete monuments outside the silo. Inside the silo, the closest monument was visible through a sighting tube, allowing the precise angle measurement to be transferred to the silo. After many more measurements inside the silo, a special device called an autocollimator was positioned precisely 90° from the desired launch azimuth. The autocollimator shot a beam of light through a window in the side of the missile, where it bounced off a mirror on the stable platform and returned to the autocollimator. If the returning beam wasn't exactly parallel, the autocollimator sent a signal to the missile, causing the stable platform to rotate as needed. The result of this process was that the stable platform was exactly aligned with the desired angle to the target.5

The guidance platform was completely redesigned for Minuteman II and III, eliminating the time-consuming alignment that Minuteman I required. The new platform had an alignment block with rotating mirrors. Instead of rotating the missile, the autocollimator remained fixed in the East position and the mirror (and thus the stable platform) was rotated to the desired launch azimuth. The new guidance platform also added a gyrocompass under the alignment block, a special compass that could precisely align itself to North by precessing against the Earth's rotation. At first, the gyrocompass was used as a backup check against the autocollimator, but eventually the gyrocompass became the primary alignment. For calibration, the alignment block also includes electrolytic bubble levels to position the stable platform in known orientations with respect to local gravity.6

The alignment block with mirrored surfaces. Image from National Air and Space Museum.

The photo above shows the alignment block on top of the gyrocompass. The front and back of the block are the precision mirrors that reflect the light beam from the autocollimator. The circles on top of the block and at the right are two level detectors, with set screws for exact adjustment. The platform has four level detectors, allowing it to be aligned against gravity in multiple positions. Like the gimbals, the gyrocompass assembly is made of beryllium due to its rigidity and light weight; it has a warning sticker because beryllium is highly toxic.

The diagram below shows how the axes align with the gimbals of the stable platform.7 Note the window at the top of the photo. Light from the autocollimator shines in through the window, reflects off the mirror on the alignment block, and returns through the window to the autocollimator. The autocollimator detects any error in alignment and signals the guidance system to correct its position accordingly.

Coordinate system for the stable platform. Note that these axes don't match the missile axes; the stable platform axes remain constant as the missile turns. Original image from National Air and Space Museum.

The stable platform uses gyroscopes to maintain its fixed orientation as the missile turns. The idea behind a gyroscope is that a spinning disk will tend to maintain its spin axis. The problem is that any friction, even from precision ball bearings, will reduce the accuracy. The solution in the Minuteman is a "gas bearing", where the gyroscope rotor is supported by an extremely thin layer of hydrogen. As shown below, the gyroscope is built around a stationary marble-sized ball (blue), fastened to the gyroscope frame at the top and bottom. The rotor (pink) is clamped around the equator of the ball and spins at high speed, powered by an induction motor (windings green, rotor yellow). If the gyroscope frame is tilted, the rotor will stay in its orientation. The resulting change in angle between the frame and the rotor is detected by sensitive capacitive pickups (purple). The gyroscope is sensitive to tilt in two axes: left-right, and front-back. Since nothing touches the rotor except the thin layer of gas around the ball, the influence of friction is minimal.

A gas-bearing gyroscope. Based on patent 3,025,708.

A gas-bearing gyroscope has the problem that when it starts or stops, the gas layer dissipates, allowing the rotor and the bearing to rub. The Minuteman missile's guidance system was kept continuously running, so starts and stops were infrequent. Moreover, when the gyroscope did need to be started, the electronics gave it a 40-volt jolt to get it up to speed quickly. Because the Minuteman's guidance system was always running—and its solid-fuel engines didn't require fueling—the missile could be launched in under a minute.

To summarize the guidance trajectory, a Minuteman flight is typically about 35 minutes,8 but only the first few minutes are powered by the rockets; the warheads coast most of the way on a ballistic trajectory. The first three rocket stages are active for just 180 seconds; this completed the boost phase for Minuteman I and II. However, the innovation of Minuteman III was that it held three warheads, a system called MIRV (Multiple Independently-targeted Reentry Vehicles). To direct these warheads to their targets, Minuteman III has a fourth stage, called PSRE (Propulsion System Rocket Engine), mounted just below the guidance system. The PSRE was active for 440 seconds, directing each warhead on its specific path. (Meanwhile, a retro-rocket sent the third stage in a random direction. Otherwise, it would tag along with the warheads, acting as a giant radar beacon for enemy anti-ballistic-missile systems.) The warheads travel very high, typically over 800 nautical miles (1500 km), more than three times the altitude of the International Space Station. As for the multiple-warhead MIRV, the Minuteman III missiles were converted back to single warheads as part of the New START arms reduction treaty, with the last MIRV removed in June 2014.

A MIRV configuration with three W78 warheads on the Minuteman III MK-12A reentry vehicle system. The conical reentry vehicles are smaller than you might expect, just under 6 feet tall (181 cm). In comparison, the Titan II had a reentry vehicle that was 14 feet long (4.3 m), holding a massive 9-megaton warhead. Photo from GAO-21-210.

The Minuteman D-17B computer

The guidance computer has a key role in the Minuteman missile, determining the missile's position from the stable platform data, executing a guidance algorithm, and steering the missile on the desired trajectory. Before explaining the D-37 computer used in Minuteman II and III, I'll start by discussing the D-17B computer used in the first Minuteman, since its characteristics strongly influenced the later computers. The Minuteman I computer was very primitive by modern standards. Although it was a 24-bit machine, it was a serial computer, operating on one bit at a time. The big advantage of serial processing is that it dramatically reduces the hardware requirements. Since the computer only processes one bit at a time, it uses a one-bit ALU. Moreover, the buses and datapaths are one bit wide rather than 24 bits. The disadvantage, of course, is that a serial computer is slow; the D-17B took 27 clock cycles (24 bits and three overhead) to perform any operation. At best, the computer could perform 12,800 additions per second.

The computer has an unusual cylindrical structure, 29 inches (74 cm) in diameter, designed to fit the diameter of the Minuteman missile. The computer itself is the bottom half of the cylindrical shell. The top half is the electronic equipment chassis, holding the power supplies for the computer and the stable platform, as well as servo control amplifiers, oscillators, and converters.

The Minuteman I guidance computer. The computer itself is the bottom half of the cylinder, with the disk drive in the 4 o'clock position. The upper half is electronics to drive the IMU and rocket. The IMU itself would be mounted in the center. Photo by Steve Jurvetson, CC BY 2.0.

The computer doesn't have any RAM. Instead, all instructions, data, and registers are stored on a hard disk, but not like a modern hard disk. The disk has separate, fixed heads for each track so it can access tracks without seeking. (This approach is similar to a computer built around drum memory, except the drum is flattened.) In total, the disk held just 2727 24-bit words (approximately 8 Kbytes). The computer's serial processing and its disk-based storage worked well together. The disk provided data one bit at a time, which the computer would process serially. The results were written back to the disk, one bit at a time as calculation proceeded. The write head was positioned just behind the read head so a value could be overwritten as it was computed.

The photo below shows the numerous read and write heads for the D-17B's hard disk. Note that the heads are fixed (unlike modern hard drives), and the heads are widely distributed across the surface. (There is no need for different tracks to be aligned.) I believe that the green and white heads in pairs are for the "regular" tracks, while the heads with other spacings implement registers and short-term storage called loops.9

Disk head assembly from the D-17B. Photo by LaserSam, CC BY-SA 40.

The D-17B computer was transistorized. The photo below shows one of its circuit boards, crammed with transistors (the black cylinders), resistors, diodes, and other components. (This board is a read amplifier, amplifying the signals from the hard disk.) The computer used diode-resistor logic and diode-transistor logic to minimize the number of transistors; as a result, it used 6282 diodes and 5094 resistors compared to 1521 silicon and germanium transistors (source).

A read amplifier circuit board from the D-17B. Photo from bitsavers.

The computer supported 39 instructions. Many of the instructions are straightforward: add, subtract, multiply (but no divide), complement, magnitude, AND, left shift, and right shift. The computer handled 24-bit words as well as 11-bit split words, so many of these instructions had "split" versions to operate on a shorter value. One unusual instruction was "split compare and limit", which replaced the accumulator value with a limit value from memory, if the accumulator value exceeded the limit.

The focus of the computer was I/O with 48 digital inputs, 26 incremental inputs, 28 digital outputs, 12 analog voltage outputs, and 3 pulse outputs for gyro control. The computer had special instructions to support the various inputs and outputs.10 For example, to integrate pulse signals from the stable platform, the computer had instructions to enter and exit "Fine Countdown" mode, which caused two special registers to operate as digital integrators, in parallel with regular computation (details).

The D-37 computer

For the Minuteman II missile, Autonetics built the D-37 computer, one of the earliest integrated circuit computers. By using integrated circuits, the guidance computer was dramatically shrunk, increasing range, functionality, and accuracy. The photo below compares the size of the older D-17B computer (half-cylinder) with the D-37B (held by the engineer).

The Minuteman D-17B computer (cylinder) and D-37B computer (being held). From Microcomputer comes off the line, Electronics, Nov 1, 1963. Using modern definitions, the computer was a minicomputer, not a microcomputer.

Although the main task of the computer is guidance, with the increased capacity of the D-37, the computer took over many of the tasks formerly performed by ground support equipment. The D-37 managed "ground control and checkout, monitoring, communication coding and decoding, as well as the airborne tasks of navigation, guidance, steering, and control" (link).

The D-37 had several models. The D-37A was the prototype system, while the D-37B was deployed in the first 60 Minuteman II missiles. The Air Force soon realized that nuclear radiation posed a threat to the computer, so they developed the radiation-hardened D-37C.11 The Minuteman III used the D-37D, an improved and slightly larger version. Even with additional disk space, program memory was so tight that software features were dropped to save just 47 words.

As far as architecture and performance, the D-37 computer is almost the same as the D-17B, but extended. Most importantly, the D-37 kept the serial architecture of the D-17B, so it had the same slow instruction speed. The D-37 kept the instruction set of the D-17B, with additional instructions such as division, logical OR, bit rotates, and more I/O, giving it 58 instructions versus 39 in the older computer. It expanded the hard disk storage, but with a double-sided disk providing 7222 words of storage in the D37-C.12 The D-37 included division implemented in hardware (which the D-17B didn't have), along with a faster hardware implementation of multiplication, improving the speed of those instructions.13 The D-37C added more I/O lines, as well as radio input and 32 analog voltage inputs.

The diagram below shows the D-37C computer, used in the Minuteman II. At the left is the hard disk that provides the computer's memory. Most of the computer is occupied by complex circuit boards covered with flat-pack integrated circuits. At the right is the advanced switching power supply, generating numerous voltages for the computer (±3, 6, 9, 12, 18, and 24 volts). The connectors at the top provide the interface between the computer and the rest of the system. Because the computer has so many digital (discrete) and analog signals, it uses multiple 61-pin connectors (details).

The D-37C computer. Image courtesy Martin Miller, www.martin-miller.us.

The D-37C computer was built from 22 different integrated circuits, custom-built by Texas Instruments for the Minuteman project. These chips ranged from digital functions such as NAND gates and a flip-flop to linear amplifiers to specialized functions such as a demodulator/chopper. Texas Instruments sold the Minuteman series integrated circuits on the open market, but the chips were spectacularly expensive ($55 for a flip-flop, over $500 in current dollars) and not as popular as TI's general-purpose integrated circuits.14 The circuit boards were very complex for the time, with 10 interconnected layers. Each board was about 4 × 5½ inches and held about 150 flatpack integrated circuits, with components on both sides.

The growth of the integrated circuit industry owes a lot to the Minuteman computer and the Apollo Guidance Computer, both developed during the early days of the integrated circuit. These projects bought integrated circuits by the hundreds of thousands, helping the IC industry move from low-volume prototypes to mass-produced commodities, both by providing demand and by motivating companies to fix yield problems. Moreover, both computers required high-reliability integrated circuits, forcing the industry to improve its manufacturing processes. Finally, Minuteman and Apollo gave integrated circuits credibility, showing that ICs were a practical design choice.

The Minuteman III used the D-37D computer, which had about twice the disk capacity, 14,137 words. The layout is similar to the D-37C above, with the disk drive on the left and the power supply on the right. Since the computer is mounted "upside down", the boards are not visible inside, blocked by the interconnect board.15 Note the use of flexible PCBs, advanced technology for the time, soldered with low-melting-point indium/tin solder.

The D-37D computer. Image from National Air and Space Museum.

By 1970, the D-37 computer had made the cylindrical D-17B obsolete. The government gave away surplus D-17B computers to universities and other organizations for use as general-purpose microcomputers. Dozens of organizations, from Harvard to the Center for Disease Control to Tektronix jumped at the chance to obtain a free computer, even if it was slow and difficult to use, forming a large users group to share programming tips.

The P92 amplifier

The amplifier provides the interface between the computer and the rest of the missile. The amplifier sends control signals to the missile's four stages, controlling the engines and steering. (The electronic circuitry from the Minuteman I's nozzle control units was moved to the amplifier, simplifying maintenance.) Moreover, the Minuteman has explosive ordnance in many places, ranging from small squibs that activate valves to explosives that separate the missile stages. The amplifier sends the high-current (30 amp) signals to detonate the ordnance, while monitoring the current to detect faults.16 The amplifier acts as a safety device for the ordnance, blocking signals unless the amplifier has been armed with the proper code. The amplifier sends control signals to the reentry system (i.e. the warheads) as well as the chaff dispenser, which emits clouds of wires to jam enemy radar. The amplifier also sends and receives signals through the umbilical cable from the ground equipment.

The PS 92A amplifier. Image from National Air and Space Museum. Click this (or any other image) for a higher-resolution version.

The photo above shows the amplifier with its cover removed. The amplifier is constructed as two stacks of six circuit boards, on top of a double-width power supply board. At the top and bottom of each board, connectors with thick cables connect the boards to the rest of the system. Each board is a multi-layer printed-circuit board built on a thick magnesium frame for cooling. The amplifier has five power switching boards, a valve driver board, three servo amplifier boards, and an ACTR control board (whatever that is). The system board is visible on the left, with large capacitors and precision 0.01% resistors. To its right is the decoder board, presumably decoding computer commands to select a particular I/O device. Note the extensive use of Texas Instruments flat-pack integrated circuits on this board, the tiny white rectangles.

Missile Guidance Set Control

The Missile Guidance Set Control (MGSC) contains the electronics to power and run the inertial measurement unit (IMU), providing the interface to the computer. The MGSC handles the platform servo loop, accelerometer server loops, gyroscope torquing, gyrocompass torquing and slew, and accelerometer temperature control.17 One unexpected function of the MGSC is powering the computer's hard disk, supplying 400 Hz, 3-phase power at 27.25 volts (source).

The Missile Guidance Set Control with the modules labeled. Original image from National Air and Space Museum.

The MGSC is constructed from hinged metal modules, each with a particular function, shown above. The modules are constructed around printed circuit boards. Two large connectors at the right of the MGSC provide electrical connectivity with the IMU and computer. At the top and bottom of the MGSC are connections for coolant. The MGSC is roughly equivalent to the top half of the Minuteman I's cylindrical guidance system, opposite the computer half. The MGSC is unchanged between the Minuteman II and Minuteman III. The MGSC is normally covered with a metal cover that provides radiation protection, but the cover is missing in the photo above.

Battery

The battery in the Minuteman Guidance System is very unusual, since it is a "reserve battery", completely inert until activated. It is a silver/zinc battery with the electrolyte stored separately, giving the battery an essentially infinite shelf life. To power up the battery during a launch, a gas generator inside the battery is ignited by a squib. The gas pressure forces the potassium hydroxide electrolyte out of a tank and into the battery, energizing the battery in under a second. The battery can only be used once, of course, and you can't test it. The battery was built by Delco-Remy (a division of General Motors) (details). It provides 28 volts at 14.5 Amp-hours, powering the guidance system and most of the missile; a separate battery powers the first-stage rocket.

The battery inside the Minuteman III. Original image from National Air and Space Museum.

The photo above shows the battery mounted inside the guidance system. Note the two thin wires attached to the posts on the left front of the battery to enable the battery, and the thick power wires bolted to the posts on the right. Above these posts is an "electrolyte vent port"; I'm not sure what prevents caustic electrolyte from spraying out under high pressure.

The photo below shows the construction of a Minuteman I battery, similar but with two independent battery blocks. The two round gas generators on the front of the electrolyte tube force the electrolyte into the battery sections.

Inside the remotely-activated SE12G battery. (source)

Squib-activated switch

Another unusual component is the squib-activated switch. This switch is activated by a small explosive squib; when fired, the squib forces the switch to change positions. This switch may seem excessively dramatic, but it has a few advantages over, say, an electromagnetic relay. The squib-activated switch will switch solidly, while the contacts on a relay may "chatter" or bounce before settling into their new positions. An electromagnetic relay may require more current to switch, especially if it has large contacts or many contacts. However, like the battery, the squib-activated switch can only be used once.

The squib-activated switch, next to a coolant line. The manufacturer of this part is Boeing, as indicated by the Cage Code 94756 on the part. Image from National Air and Space Museum.

The purpose of the switch is to disconnect important signals, known as critical leads, during launch. The Minuteman missile has an umbilical connection that provides power, cooling, and signals while the missile is in the silo. Just before the umbilical cable is disconnected, the switch severs the connections for the master reset signal along with an enable and disable signal. Presumably, these control signals are cleanly disconnected to avoid stray signals or electrical noise that could cause problems when the umbilical connection is pulled off.

The photo below shows the umbilical cable connected to a Minuteman II missile in its silo. Also note the window in the side of the missile to allow the light beam from the autocollimator to reflect off the guidance platform for alignment.

A Minuteman II missile in its silo. Photo by Kelly Michals, CC BY-NC 2.0.

Cooling

The guidance system is water-cooled while in the silo, using a solution of sodium chromate to inhibit corrosion. After launch, the guidance system operated for just a few minutes before releasing the warheads, so it operated without water cooling. (The stable platform has a fan and heat exchanger to keep it cool during flight.) The diagram below highlights the cooling lines. Coolant is provided from the ground support equipment through the umbilical connector in the upper right. It flows through the computer, diode assembly, MGSC, and stable platform. Finally, the coolant exits through the umbilical connector.

Original image from National Air and Space Museum.

Diode assembly

In the middle of the guidance system, the diode assembly consists of seven power diodes. These diodes control the power flow when switching from ground power to battery power. The photo below shows the diode assembly, with coolant connections at the top and bottom. The thick gray wire in the center of the diode assembly receives power from the battery just to the left.

The diode assembly. Image from National Air and Space Museum.

Permutation plug

The Permutation plug (or P-plug) was the key cryptographic element of the guidance system, defining the launch codes for a particular missile. The P-plug looked similar to a hockey puck and plugged into a 55-pin socket attached to the amplifier. The retaining bar held the P-plug in place.

The connector that receives the Permutation plug. Image from National Air and Space Museum.

Because the security of the missile hinged on the P-plug, the P-plug was handled in a highly ritualized way, transported by a two-person team, an airman and an officer, both armed (source). After the guidance system underwent maintenance, the P-plug team would ensure that the plug was properly installed, just before the missile was bolted back together. There was also a lot of ritual around the disk memory, since it held security codes and targeting information.18 Before anyone could work on the computer, a special team would come to the silo and erase the memory. Afterward, another team would load up the computer from a magnetic tape (in the case of Minuteman III) or punched tape (earlier).19

The missile launch codes are said to be split between the hard disk and the permutation plug. In particular, the missile software holds a two-word code for each of the five launch control facilities.22 The launch code in an Execute Launch Command (ELC) must match the combination of the P-plug value and the site-specific value on disk.23 Thus, the launch code is unique to each launch control site and each missile.24 As another security feature, a launch requires messages from two launch control sites, unless only one was available.25

Transient current detector

A nuclear blast has many bad effects on semiconductors and can cause transient errors. A rather brute-force approach was used to minimize this risk in the D-37C and D-37D computers: if a nuclear blast is detected, the computer stops writing to disk until the burst of radiation passes by. When the radiation level drops, the computer carries on from where it left off, extrapolating to make up for the lost time26 to minimize the error. Since all data is stored on the hard disk, the system doesn't need to worry about memory corruption as could happen with semiconductor RAM.

The Minuteman documents euphemistically refer to "operating in a hostile environment" for the ability to handle large pulses of radiation from a nearby nuclear explosion. Another euphemism is "seismic environment", when a nuclear blast near a silo could disturb the missile's targeting alignment. To get an idea of the expected forces, note that the launch officers were strapped into their seats with four-point harnesses to protect against the seismic environment.27

The Transient Current Detector. Image from National Air and Space Museum.

The "transient current detector" above detects dangerous levels of radiation. I couldn't find any details, but I suspect that it contains a semiconductor and detects transient current through the semiconductor induced by radiation. It would make sense to use a semiconductor similar to the ones in the computer so the detector's response matches the response of the computer, perhaps a matching Texas Instruments IC.

The Minuteman III also has two "field detectors" mounted on the outside of the guidance ring. These presumably detect large fluctuations in the electromagnetic field, indicating an electromagnetic pulse (EMP), different from the ionizing radiation picked up by the Transient Current Detector.

Conclusions

The Minuteman guidance system is full of innovative technologies. Among other things, Minuteman I used an early transistorized computer, and Minuteman II used one of the first integrated circuit computers. The Minuteman missile isn't just something from the past, though. There are currently 400 Minuteman missiles in the United States, ready to launch at a moment's notice and create global devastation. Thus, its technical achievements can't be glorified without reflecting on the negativity of its underlying purpose. On the other hand, Minuteman has succeeded (so far) in its purpose of deterrence, so it can also be viewed in a positive, peacekeeping role. In any case, the Minuteman technology is morally ambiguous, compared to, say, the Apollo Guidance Computer.

I plan to write more about the role of Minuteman and Apollo in the IC industry, so follow me on Mastodon as @[email protected] or RSS for updates. Probably the best overview of Minuteman is Minuteman weapon system history and description. The book Minuteman: A technical history has thorough information. For information on the missile targeting and alignment process, see Association of Air Force Missileers Newsletter, December 2006. The Minuteman guidance system is described in detail in The evolution of Minuteman guidance and control. Much of the imagery in this article is from the National Air and Space Museum. Thanks to Martin Miller for providing a detailed D-37C photo. He has taken amazing photos of nuclear equipment, published in his book Weapons of Mass Destruction: Specters of the Nuclear Age, so check it out.

Notes and references

1. The Minuteman missile was introduced in 1962, followed by the improved Minuteman II in 1965 and the Minuteman III in 1970. From 1966 to 1985, the US had 1000 Minuteman missiles fielded, but the number has been reduced since then due to various arms control agreements. At present, there are 400 active Minuteman III missiles spread among 450 launch sites. The Minuteman guidance system was updated in the early 2000s to a platform called the NS-50, using a computer based on a MIL-STD-1750A microprocessor. I'm not discussing that system in this post for reasons of space.

   Although the Minuteman has undergone modernization projects, it is reaching the end of its life and is scheduled to be replaced by the Sentinel missile. The Sentinel program is encountering delays and is over budget by 80%, raising the risk of cancellation but the Sentinel program is proceeding as of July 2024. ↩

2. Disclaimer: This information is all from published sources. There's nothing secret, and it's mostly obsolete from 60 years ago. I don't have access to a Minuteman system (unlike the Titan), so this post is based on publications and photos, rather than hands-on experience. I've tried to be accurate, but I'm sure there are errors. ↩

3. Different guidance algorithms can be used, such as Q-guidance, delta guidance, explicit guidance, and numerical integration; the more advanced algorithms require better computers but provide easier targeting, better accuracy, and more ability to correct for course deviations (see Present and Advanced Guidance Techniques). Q-guidance uses a precomputed "Q matrix" to constantly determine the direction in which velocity needs to be gained, while delta guidance attempts to keep the missile along a precomputed trajectory by using polynomials. In explicit guidance, the equations of motion are solved to determine the steering direction. Minuteman used delta guidance at first, but moved to "hybrid explicit" guidance when the computer became more advanced. See Minuteman: A technical history, page 234 for more on targeting algorithms. ↩

4. On Minuteman I, the three stages were steered by changing the direction of the rocket nozzles. Minuteman II, however, used a single fixed nozzle on the second stage but injected fluid into the exhaust to steer the missile, a technique called liquid injection thrust vector control. The Minuteman III used this technique on the third stage as well, injecting a strontium perchlorate solution. (Small nozzles powered by a gas generator are used for roll control, since directing the exhaust won't produce roll motion.) The thrust control liquid was Freon 114B2, which turned out to be harmful to the ozone layer, so it was replaced in the 1990s with perfluorohexane. ↩

5. Strictly speaking, the launch azimuth wasn't aimed at the target. Because the Earth rotated during the missile's flight, the launch azimuth was aimed at where the target would be when the warhead landed. Another factor was the Minuteman I had a limited ability to steer off the launch azimuth, about 10°, allowing the missile to switch between two targets at launch time. ↩

6. The Minuteman guidance system is designed to achieve as much accuracy as possible. One problem is that the gyroscopes and accelerometers aren't perfect, but have small errors due to friction and other factors. Moreover, the construction of the stable platform isn't exact; components that should be parallel or perpendicular will have tiny angle errors. To deal with these problems, the missile performs periodic calibrations ranging from some every 15 minutes to some every few months.

   To assist with calibration, the guidance platform contains electrolytic bubble levels, similar to an ordinary carpentry level, but extremely sensitive. Each bubble level contains wires positioned partially in the bubble and partially in the conductive electrolyte fluid. As the bubble shifts, the amount of wire in the fluid changes, changing the measured resistance. These levels are so sensitive that The levels allow the stable platform to be rotated to known positions relative to gravity for calibration.

   The top of the gyrocompass has two mirrors for calibration, allowing the missile platform to rotate exactly 180° relative to the autocollimator. Every 15 minutes, the platform would flip over to measure the gyroscope and accelerometer signals in the opposite orientation. This allowed much better calibration, canceling out errors and improving the missile accuracy. Other calibrations were performed less frequently, such as checking each accelerometer in the up and down positions. Every 90 days, a calibration called PSAT (Perturbation Self-Alignment Technique) pitched the platform by 90° and then slowly rotated the gyrocompass around the vertical to simulate the Earth's rotation (details).

   Another alignment measurement checks the angle between the two mirrors. The two mirrors on the alignment block are supposed to be parallel, but they won't be exactly parallel. The guidance platform periodically rotates the mirror assembly to check one mirror and the other against the autocollimator to compute the angle between the mirrors, called zeta. (See Software Validation Study, page A-94.)

   These calibrations permitted the measurement of small biases and imperfections in the gyroscopes and accelerometers; this data was fed into the guidance calculations to squeeze out as much accuracy as possible. These measurements also provided statistical tracking of the devices so they could be replaced if their performance started to deteriorate. ↩

7. Inconveniently, I found contradictory sources about the Minuteman coordinate system. Most sources specify Z as the roll axis, but one detailed paper swaps the X and Z axes, maybe to match simulation software. Examining Figure 5 closely shows that the new axis names were drawn in by hand. ↩

8. The flight time of Minuteman depended on the distance and trajectory. The Minuteman's range is said to be 13,000 km. For a closer target, there are two possible trajectories: a high path and a low path. Being direct, the low path could take about 25 minutes, while the high path would reach over 1500 nautical miles (almost 3000 km, seven times the altitude of the ISS) and take 45 minutes. See A simulation of Minuteman Trajectories. ↩

9. The disk holds a timing track, which provides the timing for the computer, giving it a 345.6 kHz clock speed. Note that all operations in the computer are synchronized to the disk, rather than a clock inside the computer. One consequence of this is that the processor speed depends on the disk speed, so it isn't as precise as most computers, which generate the clock from a quartz crystal. The processor timing is very important for a guidance computer, since its calculations of positions depend on the time step. If the processor is running fast or slow, the position will be correspondingly wrong. The solution is that the computer calculates a parameter "tau", the ratio between processor time and wall clock time. The computer receives an interrupt exactly once per second; by counting the number of instructions executed between interrupts, the computer can compute tau and ensure that the calculations are accurate. ↩

10. The computer has 8-bit analog-to-digital converters. The D-37C supports 32 analog inputs with a range of +/- 10 volts (source). It also has four digital-to-analog outputs with 8-bit accuracy, also +/- 10 volts.

    In the D-17B, nine analog outputs control the rocket steering, providing roll, pitch, and yaw to the three stages, while three analog outputs go to the stable platform, probably positioning the gimbals. ↩

11. The housing for the stable platform provides radiation shielding; it is one of the few parts of the guidance system that is officially secret, but is said to be tantalum sheeting (see Minuteman: A technical history page 224). Although the computer is also said to have radiation shielding, it is curiously not on the secret list. ↩

12. Sources give different memory capacities. The reason is that in addition to the regular memory, part of the disk is used for special purposes including registers and rapid access loops. The problem with the regular memory is that the processor may need to wait for an entire disk revolution to access a particular word. The solution is rapid access loops: by putting the write head just upstream of the read head, the data can be accessed more rapidly. For instance, if the write head is positioned one word length upstream, the word can be read (and rewritten) every cycle, providing immediate access to a single word. Putting the write head further upstream allows storage of longer values, with a corresponding longer wait. The D-37C has ten rapid-access channels of one to 16 words (source). The regular memory in the D-37C consists of 56 channels (i.e. tracks) of 128 words, totaling 7168 words. Counting the loops and registers yields the higher memory capacity of 7222 words. ↩

13. The differences between the D-17B and D-37C instruction sets are described here. ↩

14. The schematic for the Minuteman's flip-flop IC is shown below. This is a complex circuit for the time, with six transistors along with numerous resistors, diodes, and capacitors.

    

    Flip-flop schematic. From Integrated circuits go operational, Electronics, Feb 15, 1963.

     ↩

15. The diagram below shows an exploded view of the D-37D computer (rotated 180° from the earlier photo).

    

    Exploded view of the D-37D computer. Modified and fixed from Minuteman weapon system history and description.

     ↩

16. The danger of these explosives is illustrated by a bizarre accident summarized by "The warhead is no longer on top of the missile." At 3:00 pm on December 5, 1964, two airmen were in the missile silo, troubleshooting a fault in the security system. One airman removed a fuse, triggering a loud explosion and the nuclear warhead fell off the missile, falling 75 feet to the floor of the silo. Nobody was injured and the warhead was hoisted out a few days later without incident.

    The problem was that the airmen used an "unauthorized tool" (a screwdriver) to remove the fuse, briefly shorting power to ground. This caused a current on a ground line connected to the missile through an umbilical cable. Inside the missile, the retrorocket for the warhead had an igniter, but a short on its connector caused another connection to ground. This ground went out through a second umbilical, closing the circuit. (Apparently, the resistance between the two grounds was high enough that the path through the two shorts had enough current to ignite the igniter.) The force of the retrorocket flung the warhead off the rocket.

    More details are in this report and this report. (This incident is not the 1980 Damascus Titan incident, where a dropped 8-pound wrench socket led to the explosion of the missile, killing one person and injuring 21 others, while flinging the warhead out of the silo. The very interesting book Command and Control discusses the Damascus incident and other mishaps with nuclear weapons.) ↩

17. The functional diagram below shows the interactions between the stable platform and the guidance set. Shaded circuits are mounted on the stable platform, while others are in the control set. This diagram is for the later NS-50 platform, but it should be mostly relevant to the NS-20 used in Minuteman III earlier. At the top are the feedback loops for the PIGA accelerometers (top). The torque motors (TM) in the middle provide feedback through the gimbals for the gyroscopes. Below that, the gyrocompass has a a feedback loop with its internal torquer. The torque motor at the bottom rotates the gyrocompass and mirrors with feedback through the optical resolver.

    

    Platform Control Functional Diagram. From Technical Reference Handbook, SELECT WS133A, D2-27524-5, Fig. 3-12, page 3-68.

     ↩

18. The Air Force was especially concerned with keeping the targeting information secret; the people launching the missiles had no idea what the targets were. It occurs to me, though, that since the Minuteman I missile had to be physically rotated in its silo to exactly line up with the target, one presumably could draw an azimuth line on the map and know the target was along the line. ↩

19. The Minuteman computer has a conditional fill mode, where the computer can't be loaded with a new program unless the first four words match the first four words in memory channel 12. This ensures that the computer can't be loaded with unauthorized software. This four-word code must be different from the P-plug value for two reasons. First, the P-plug value is not allowed to be stored in memory. Second, the filling code is four words, while the P-plug value is two words.

    The P-plug held two hardwired code words that could be read by the processor.20 For security, the two words were not allowed to be in memory (i.e. the hard drive) at the same time. I assume it is called a Permutation Plug for historical reasons; the Saturn V booster used in Apollo used a security plug that provided a permutation of the 21-character code.21 (That is, it mapped 21 inputs to 21 outputs as a permutation.) ↩

20. The processor read the P-plug code words by first triggering the discrete output #25 with the DOB 25 instruction (Discrete Output B) and then reading the value (twice for reliability). The process was repeated with output #6. Finally, the discretes were cleared with DOB 0 (reference). ↩

21. The Apollo flights used "code plugs" to protect the Range Safety system from unauthorized access, since this system was capable of blowing up the Saturn V rockets (details). Signals were transmitted in a 21-symbol "alphabet" (encoded by 2 tones out of 7). The code plug permuted the 21 symbols in an arbitrary way. This wasn't a lot of security, just a simple substitution cipher, but it was sufficient for its role. A command consisted of 11 characters (9 for the address and 2 for the command), so the odds were low of hitting a valid message by chance. ↩

22. One feature of the Minuteman missile is that the missile sites themselves are uncrewed; the missile officers who launch the missiles work remotely, handling multiple missiles to reduce the personnel required. Specifically, each group of 10 missiles (called a "flight") is controlled by an underground launch control center. A squadron consists of 50 missiles. A "wing" is the largest grouping, handling 150 to 200 missiles, and attached to a particular Air Force base. At its peak, Minuteman had 1000 missiles divided among six wings in Missouri, Montana, North Dakota, South Dakota, and Wyoming, with missiles spilling across the Wyoming border into Colorado and Nebraska. ↩

23. Information on the launch code mechanism is from Technical Reference Handbook D2-27524-5, "System Engineering Level Evaluation Correction Team, WS133A", chapter 2. ↩

24. The Command Signals Decoder provides another layer of security. It is an electromechanical stepping decoder that blocks the first-stage rocket from igniting unless it receives the proper 27-bit code as part of an Enable command. (The Enable command (ENC) happens before the Execute Launch command (ELC); see the state diagram below.) Its operation is murky; my hypothesis is that the decoder acts much like a combination lock, with the 27 code posts raised or lowered by the input bits. If all the posts are in the proper position, the inner wheel is released, allowing it to rotate to the armed position and close the electrical firing circuit for the motor igniters. Specifically, the 27 posts have a high notch on one side and a low notch on the other, so the device is programmed by rotating each pin so the desired notch faces inward. When the device receives code bits, the wheel rotates one position for each bit and a solenoid raises or lowers the pin, depending on if it is a zero or one. If all pins are in the correct positions, the inner wheel can rotate through the notches, but if any pins are incorrect, the inner wheel will bind on that pin. The 27 bits are the "CSD(M) secure code", probably consisting of 24 code bits and three padding bits. Another Command Signals Decoder on the ground "CSD(G)" provides an interlock for ground ordnance.

    

    The Command Signals Decoder, from Evolution of ordnance subsystems and components design in Air Force strategic missile systems.

    I think there are two motivations behind this complicated device. First, they want an interlock that is mechanical rather than electronic, since an electronic device can be affected unpredictably by radiation, power surges, component failure, programming errors, etc. Second, they want an interlock that physically disconnects the firing circuit so there is no path that can be triggered by stray current, lightning, EMP, etc.

    The Minuteman's P92 amplifier assembly also blocks ordnance unless armed with a code. It's unclear if this is the same enable code as the Comand Signals Decoder or a different code.

    The earlier Titan missile also had a code mechanism to prevent an unauthorized launch by blocking the engine. The Titan had a butterfly valve in the fuel line with a 6-digit code. If you don't enter the right code, the fuel line stays shut and the missile simply can't take off (video). ↩

25. A missile launch normally requires an Execute Launch Command (ELC) from two launch control sites, moving the missile to the "Launch in Process" mode. However, that raises the concern that there could only be one surviving site. The solution is that after receiving a single launch command, the missile starts a timer. If the "one-vote launch time" passes uneventfully, the missile is launched. However, another site can cancel a rogue launch during that time by sending an Inhibit Command (INC) message. The sites have a complex system to detect which sites are active and to determine the primary and secondary sites controlling each missile. (This is reminiscent of the Byzantine generals problem.)

    

    The state machine for Minuteman missile status. From Technical Reference Handbook D2-27524-5, page 2-25.

     ↩

26. After detecting a nuclear blast, the Minuteman computer shuts down for an integral number of disk revolutions. When it comes back up, it double-counts the accelerometer pulses for the same number of disk revolutions to make up for the missed time (see Minuteman: A technical history pages 220 and 223). As long as not much changed during the lost time, the accuracy loss is small. Of course, this counter would need to be outside the part of the computer that gets shut down. ↩

27. Missiles were aligned to such accuracy that even running a diesel generator nearby could shift the silo enough to cause alignment problems, as happened with a Titan site. (See Association of Air Force Missileers Newsletter, March 2007, page 6.) A "seismic event" could also be an earthquake; the enormous 1964 Alaska earthquake—9.2 on the Richter scale—caused Minuteman guidance systems to lose alignment with the autocollimator (See Minuteman: A technical history page 221). ↩

Reverse engineering the 59-pound printer onboard the Space Shuttle

The Space Shuttle contained a bulky printer so the astronauts could receive procedures, mission plans, weather reports, crew activity plans, and other documents. Needed for the first Shuttle launch in 1981, this printer was designed in just 7 months, built around an Army communications terminal. Unlike modern printers, the Shuttle's printer contains a spinning metal drum with raised characters, allowing it to rapidly print a line at a time.

The Space Shuttle's Interim Teleprinter. The horizontal rails allowed it to be mounted in a Space Shuttle stowage locker. Click this image (or any other) for a larger version.

This printer is known as the Space Shuttle Interim Teleprinter System.1 As the name "Interim" suggests, this printer was intended as a stop-gap measure, operating for a few flights until a better printer was operational. However, the teleprinter proved to be more reliable than its replacement, so it remained in use as a backup for over 50 flights, often printing thousands of lines per flight. This didn't come cheap: with a Shuttle flight costing $27,000 per pound, putting the 59-pound teleprinter in space cost over $1.5 million per flight.

Pilot Overmyer reading a printout from the teleprinter, STS-5, November 16, 1982. From National Archives. The description says that this output is from the Text and Graphics System, but the yellow paper and the date show that this is the Interim Teleprinter.

We obtained access to a Shuttle teleprinter (probably a development system that remained on the ground) and wanted to put it into operation. I had to reverse engineer three of the boards inside the printer to determine the data format the printer accepted: serial data encoded into audio. But after analyzing the printer and performing a lot of maintenance, we succeeded in getting the printer to print. In this article, I'll describe the Shuttle's Interim Teleprinter, explain its circuitry and drum-based printing mechanism, and show it in operation.

History of the Shuttle's Interim Teleprinter

The motivation for the teleprinter goes back to the Apollo program. During Apollo missions, the only way to send information to the astronauts was by talking to them over the radio and having the astronauts write down the data. NASA decided that the Space Shuttle should include a mechanism to send text and images to the astronauts, a 78-pound, high-tech fax machine called the Uplink Text & Graphics System (TAGS). A high-resolution grayscale image was sent to the Shuttle as a digital data stream. Onboard the Shuttle, a squat CRT displayed the image one line at a time and a fiber-optic faceplate transferred each line to light-sensitive silver emulsion paper. The paper was developed by passing it over a hot roller at 260ºF for 25 seconds, creating a permanent image.

The one flaw in this plan was that sending the digital image to the Shuttle required the Tracking and Data Relay Satellite System (TDRS), which due to delays wouldn't be ready until the sixth Shuttle flight. (The TDRS was a space-based replacement for the worldwide network of ground stations that was used during Apollo.) As a result, NASA decided just seven months before the first Shuttle launch that they needed an interim system "for transmission of real-time, flight-plan changes and other operational data to the crew."2

The Shuttle teleprinter is the result of this rushed effort to create a printer that could work over the existing audio channel rather than the digital TDRS satellite. Due to the time pressure, the Shuttle teleprinter needed to be based on an off-the-shelf printer. Thermal and electrostatic printers were rejected due to toxicity and flammability problems. (The Shuttle teleprinter used a roll of yellowish paper, which required a NASA waiver due to its flammability, a concern ever since the Apollo-1 disaster).

The AN/UGC-74 military communications terminal. This terminal was developed by the Army but also used by the Navy and Air Force. Image from the Operator's Manual, TM 11-5815-602-10.

The decision was made to use a military communications terminal, the the AN/UGC-743 "Tactical Teletype". The terminal's interfacing was very flexible, supporting serial data in either ASCII or Baudot format, with multiple configurations and baud rates (up to 1200 baud), using either a current-loop or voltage signals. The military terminal supported two-way communication, so it had a keyboard. Remarkably, the terminal also implemented a word processor, controlled by a Motorola 6800 microprocessor (ancestor of the famous MOS 6502). The word processor allowed messages to be composed offline, minimizing the radio transmission time, which was important in a hostile environment. As will be seen, this 100-pound military system required many large changes to be usable on the Space Shuttle, most visibly removing the keyboard.

The printing mechanism

The teleprinter uses a spinning drum with raised characters, shown below.4 To print a character, the printer fires a hammer, forcing the inked ribbon and paper against the raised character on the drum. The drum is 80 characters wide, matching the line length, and there are 80 corresponding hammers, one for each print position. The drum has 64 printable characters, wrapped around each position of the drum.

The printer's drum rotating drum has 64 raised characters in each column. The characters spiral around the drum and are in reverse order, minimizing the chance that a line will fire all the hammers near-simultaneously.

The printer prints a line at a time, not instantaneously, but during each revolution of the drum. When the drum makes one complete revolution, each of the 64 characters passes by each print position once. Printing requires precise timing of the hammers to strike the right character on the drum as it whizzes by. The printer control circuitry triggers each hammer at the proper time, when the desired character on the drum is lined up with the hammer, producing the desired text.5

The character set is slightly different between the military printer and the Shuttle printer. The military drum had 64 ASCII characters (upper-case letters only, numbers, and special characters). The drum doesn't contain an explicit space character, since nothing is printed for a space. In its place, the drum has a diamond "◊", used as a special character to indicate a parity error or other error. The drum for the Shuttle teleprinter replaces 10 ASCII special characters with symbols that are more useful to the Shuttle, such as Greek letters for angles. Specifically, the characters ;@[\]^!"#$ are replaced by θ✓‾↑↓~αβΔϕ.

With the teleprinter disassembled, the 20 hammer cards are visible at the front. Two hammer driver cards are to the right of the hammer cards.

The video below shows a closeup of the hammers as they strike the paper to print text. The text is the teleprinter's built-in test message: "THE LAZY YELLOW DOG WAS CAUGHT BY THE SLOW RED FOX AS HE LAY SLEEPING IN THE SUN". This test message is based on the traditional quick brown fox..., which is a pangram, containing all 26 letters, but the teleprinter's test sentence is missing J, K, M, Q, and V. However, the test message is exactly 80 characters long and replaces spaces with the diamond "◊", so it is effective for verifying that all 80 columns work.

The electronics

The photo below shows the circuitry inside the teleprinter, looking down from above. At the left are the three interface boards, custom boards that demodulate the incoming audio signal. In front of the interface boards are large inductors to filter the incoming power. Hidden beneath them, a solid-state relay controls the power to the rest of the printer, implementing the low-power standby mode. In the middle, the blue board is the surprisingly complex switching power supply, mounted on a thick metal plate for cooling. Normally, the large roll of paper is mounted above the power supply board. At the right, four large circuit boards implement the main logic of the printer: a printer driver board, a communications board, a memory board, and the processor board. The rotating drum is protected by the perforated black metal grill at the front.

Inside the Shuttle teleprinter, showing the electronics.

The demodulator boards

The original military teleprinter received data as a serial bitstream. However, on the Space Shuttle, data was encoded as frequencies on the audio link. Three custom boards were constructed to demodulate the audio data so the rest of the printer could handle it. These boards also performed Shuttle-specific tasks such as powering up the printer when a message comes in, and then returning the printer to standby mode. I reverse-engineered these boards to determine how they work and to determine the data encoding. (Schematics are in the footnotes.7) In this section, I'll discuss these three boards, which are on the left side of the printer.

To summarize, the serial bitstream is encoded with Frequency Shift Keying, with a 1 represented by 3600 Hz and a 0 represented by 7200 Hz.6 The serial data is transmitted at 600 baud, even parity, one stop bit. The demodulation process first converts the input audio to a digital signal by thresholding it. (That is, the input sine wave is converted to a square wave.) The digital signal is autocorrelated to distinguish the 3600 Hz and 7200 Hz signals, recovering the underlying serial data. This signal is passed to the printer's logic boards (part of the original military teleprinter), which convert the serial signal to ASCII bytes and prints them.

Signal processing starts with the "FSK input" board, shown below. First, it amplifies the input audio signal. (The two large resistors provide a 600 Ω load for the audio input.) Next, a 900 Hz high-pass filter eliminates low-frequency noise. (The filter is implemented by a two-stage Sallen-Key topology.)

The input board.

The signal bounces from board to board, going to the "output FSK demod" board next. This board has a carrier-detect circuit that turns on the rest of the printer if it detects an input signal. This allows the printer to sit idle until it receives a signal from Earth. This board also applies the threshold to the signal to turn it into a digital waveform, which goes to the "control" board.

The output board.

The output board also holds the 5-volt and 12-volt linear regulators that power the three boards; these are the metal-can ICs at the bottom of the board. To reduce the load on the regulators, two large resistors drop the input voltage (28 volts) to a lower level before it is regulated.

The control board holds the FSK decoder, an interesting circuit that converts the two FSK frequencies to binary by implementing a digital auto-correlator. It uses a 64-bit shift register to delay the digital input by 139 µs. The input and the delayed input are XOR'd together, generating a result that depends on the frequency. A 7200 Hz signal repeats every 139 µs, so the input and the delayed input match, yielding 0 from the XOR. However, a 3600 Hz square wave switches state every 139 µs, so the two XOR inputs will always differ, resulting in a 1 output. Thus, the circuit cleanly distinguishes between a 3600 Hz input and a 7200 Hz input.

The control board.

The digital demodulator avoids some of the problems of an analog FSK demodulator. It is not sensitive to signal levels, since the signal is converted to digital. The digital demodulator is also not sensitive to harmonics, which can cause problems with analog demodulators. Finally, it doesn't require the carefully-tuned filters of an analog circuit.

The demodulated signal passes from the control board back to the output board. This board applies a 400 Hz low-pass filter and then a threshold to convert the signal back to binary. If the input frequencies are not exact, the demodulator will produce the correct 0 or 1 value over most of the waveform, but there will be glitches at the edges. The low-pass filter removes these glitches. (You might be concerned that a 600-baud signal would be wiped out by a 400 Hz low-pass filter. However, the worst case signal (alternating 0's and 1's) would be 300 Hz because it takes two bits to make one cycle, so the filter has plenty of margin.) Next, the board blocks the signal unless a carrier is detected. This ensures that random noise isn't demodulated and printed. Finally, the serial binary signal leaves the custom Shuttle boards and goes to the teleprinter's communication board, part of the standard teleprinter.

I noticed two unusual things about these boards. First, they have some modifications: "bodge" wires and added components. Second, the boards are not conformal coated, which is unusual for aerospace boards. (The four logic cards, in comparison, are protected with conformal coating.) My hypothesis is that these boards were development boards, early in the design process of the Shuttle teleprinter, so they were modified as the design changed. The teleprinter is also marked "Not for flight", which supports this theory.

Mission Specialist Thagard getting output from the teleprinter. Flight STS-7, June 24, 1983. From NARA. Although the description says this is the Text & Graphics System, it is clearly the Interim Teleprinter.

The logic cards

The military teleprinter contained four logic circuit cards: a CPU card, a memory card, a communications card, and a print control card, mounted at the right rear of the teleprinter. These cards are used unchanged in the Shuttle teleprinter.

The circuitry is more complex than you might expect, with four large cards full of ICs. There are several reasons for this. First, the cards use 1970s microprocessor technology, so it takes a lot of circuitry to do anything. In particular, many simple 7400-series logic chips perform "glue" functions: decoding addresses, buffering data, latching signals, and so forth. Moreover, a drum printer is inherently complicated, since 80 hammers must be driven at the right time based on the desired characters. Third, the teleprinter is very flexible, supporting multiple signal levels and two character formats (ASCII and Baudot). Most surprisingly, the teleprinter implements a word processor, allowing messages to be composed and edited offline. Of course, since the Shuttle's teleprinter is only used to receive data, and doesn't even have a keyboard, the word processor feature is entirely useless.

The CPU card

The CPU card holds the microprocessor that controls the teleprinter. Its most important function is to convert a line of ASCII characters into print drum codes. These codes are stored in memory for use by the print control card. The CPU also implements configuration and self-test functions.

The diagram below shows some of the main components. The CPU card contains a Motorola 6800 CPU, 4 kilobytes of memory, and a ROM that holds its program code.8 Inconveniently, all the IC part numbers are military numbers so it takes some investigation to determine what a part really is. The MC6822 is a Peripheral Interface Adapter, a Motorola chip that provides two parallel I/O ports. This chip is used on three of the cards to support a variety of I/O tasks. On the CPU card, the I/O ports drive eight status lamps (most of which were removed for the Shuttle teleprinter) as well as internal status signals such as "paper low" or "keyboard present" and the baud rate setting input.

The CPU card is centered around a Motorola 6800 microprocessor.

The print control card

In a sense, the print control card is the heart of the printer, since it causes characters to be printed by firing hammers against the rotating drum. As the drum goes through one revolution, all 64 characters will spin past each of the 80 print positions. By firing hammers at the exact time, the card prints a line of text.9 In more detail, for each row on the drum, the printer card scans through the 80-character memory buffer using Direct Memory Access (DMA). If the value in memory matches the current drum row number, the hammer is fired. Note that the hammers don't fire simultaneously, but in sequence as memory is scanned.

This diagram shows how the print control board interacts with the rest of the system. From the Maintenance manual, TM 11-5815-602-24.

The diagram above shows the interaction between the drum, the print control card, and the 80 hammers. The hammers are implemented on 20 print hammer cards, each with 4 hammers. Electrically, the hammers are arranged in a matrix. One wire out of 20 (S1-S20) selects the hammer board, the group of four. Another wire selects one of four hammers (Col 1-4). This approach simplifies the electronics, since 20 + 4 driver circuits and wires are used, rather than 80 (one for each column). The print control card is synchronized to the drum by two photo-transistor sensors that detect the drum's position. One sensor is triggered on each row, while the other sensor triggers once per revolution.

The print control card is shown below, with the main functional blocks labeled. The large purple-and-gold chip is the PIA, the same I/O chip that appeared on the CPU card. It handles a variety of signals such as the self-test request, paper out, and the drum stop signal. The mode control logic generates timing signals depending on the printer's mode. The data compare logic increments the row counter on each drum pulse, and compares the row counter to the value read from memory.10 The hammer driver circuitry on the left selects one of the 20 hammer cards, while the hammer driver circuitry on the right selects one of four hammers. The ribbon circuitry raises and lowers the ribbon so the ribbon doesn't block the text when the printer is idle. The line feed circuitry advances the paper for a line feed operation.

The print control card prints data by driving the hammers.

The photo below shows one of the hammer cards, with four hammers. Each hammer has an electromagnet that pulls a lever, rotating the hammer wheel, and causing the hammer to strike the paper. (The hammers themselves are in the upper right of the photo.) A screw adjustment controls the distance between each hammer and the paper, allowing precise adjustment of the timing. (Marc had to carefully adjust all the hammers to make the print quality readable.)

One of the 20 Hammer driver cards. Photo courtesy of Marcel.

The communication card

The communication card handles the teleprinter's serial data input. The key chip is the 8251A, a USART (Universal Synchronous/Asynchronous Receiver/Transmitter). This complex chip performs the conversion between the serial data stream and the bytes that the processor uses. (Note that the military teleprinter both sent and received serial data, while the Shuttle teleprinter only receives data.) The chip has a few support chips, labeled "UART" in the diagram below. The board has another Peripheral Interface Adapter chip, providing two I/O ports. These ports have functions such as reading the serial line settings (ASCII vs. Baudot, odd or even parity, number of stop bits, and current loop levels).

The communication card converts the serial input to parallel byte data.

The board also has circuitry to generate the clock pulses for the selected baud rate. The mode circuitry handles various phases of transmit/receive. The filter/demod circuitry handles different input types, digitally filtering and demodulating as necessary.11

The memory card

The memory card supports the word-processing feature. It provides additional RAM to hold the text buffer as well as the ROM holding the software for editing. The 16 DRAM chips on the left (MK4027) provide 8 KB of RAM while the two ROM chips on the right provide 8K of ROM. The chips in the middle to the right of the resistors split the 12 address bits into row and column addresses as required by the RAM chips. The address signals go through the numerous 24 Ω resistors in the middle; I don't know why. According to the manual, the printer operates fine without this card, except without the word processor. Since the word processor was irrelevant to the Shuttle, I wonder why this card wasn't removed to reduce weight.

The memory card has additional RAM and ROM to support the word processing feature.

The power supply

The power supply board (shown earlier) implements separate power supplies for different parts of the printer.12 The supplies are implemented as switching power supplies, which were not as common at the time as now. The microprocessor supply provides +5V, +12V, and -5V, voltages required by memory chips in the 1970s. A separate switching power supply provides +5V, -8.6V, and +8.6V for the keyboard, dustcover, and interface module, components that were removed for the Shuttle teleprinter. Another supply powers the printer's status lamps.

The drum motor supply is important because its voltage is regulated to control the rotational speed of the drum. A sensor on the drum provides a feedback pulse for each row on the drum. (I think the drum speed is 868 RPM.) These pulses control the drum motor's switching supply. If the drum spins too slowly, the voltage is increased, and similarly if it spins too fast.

The hammers have an unusual constant-current power supply. When the printer is active, this power supply generates +18 V. However, the power supply is designed to use a constant current of 600 mA regardless of the hammer activity. A capacitor provides a reservoir of power that is filled by the constant current. If the hammers are using less current, the excess current is bled off through a resistor. The purpose of this is "to mask printing intelligence during periods of message traffic." In other words, if you used a teleprinter in the embassy in Moscow, for instance, spies could monitor power transients to see when hammers are firing, and perhaps figure out what is being printed. By keeping the current constant, this source of intelligence is blocked. Of course, this feature is useless on the Space Shuttle and only wastes power.

The military teleprinter accepted multiple input voltages: 22-30 VDC, 115 VAC, or 230 VAC, along with a 12 VDC battery backup. The transformers and diodes to support these voltages were part of the interface module that was removed for the Shuttle teleprinter. Instead, the Shuttle teleprinter is powered by 28 VDC.

Mechanical changes

The military teleprinter underwent significant mechanical changes to make it suitable for the Shuttle. These changes reduced its weight from 100 pounds to 59 pounds. The most visible change to the printer is the removal of the keyboard. The entire front section of the printer was replaced, removing the controls that were not needed in the Shuttle.13 The rugged frame of the original printer was replaced with a lighter-weight (but still substantial) frame. Horizontal rails were added to the frame to support the printer in the Shuttle locker.

The photo below shows the front of the Shuttle teleprinter. While the military teleprinter had numerous lights and switches on the front, the Shuttle teleprinter has just two lights and four switches.

Front view of the Shuttle teleprinter. The bar across the middle holds a paper cutter for removing the output.

NASA was concerned that the temperature of the teleprinter could become hazardous to the astronauts. To mitigate this danger, the teleprinter had a large heat-sensitive warning sticker. The yellow sticker on the left of the teleprinter changes color and displays an image if it heats up: it shows a bandaged hand and the word "HOT". Above it is an "Omegalabel" temperature monitoring sticker that shows the highest temperature the device reached. There are more of these stickers inside the teleprinter on various motors.

The Interim Teleprinter inside the Space Shuttle

The teleprinter was too large to be mounted on the flight deck, so it was mounted in a storage locker on the middeck, one level lower. The photo below shows the location of the locker that held the teleprinter (although the teleprinter was not present in this photo), looking backward (aft) toward the airlock. The locker is denoted MA9F, indicating Mid-deck Aft, position 9F (details), in the back on the right side of the Shuttle.

This photo shows the locker that held the teleprinter. Photo by DMolybdenum, panorama viewed on renderstuff.

The teleprinter was noisy because of its impact printing; even with it in a locker, the sound outside was 69.5 dB. The solution was to soundproof the locker with acoustic insulation. Various insulating materials were tested until one was found that passed the toxicity requirements. Another flammability waiver was required for the insulation.

Putting the teleprinter in an insulated locker without cooling caused another problem: overheating. The military teleprinter used 34 watts even while idle, which would cause the printer to become dangerously hot after just 6 orbits. The printer was redesigned to support a standby mode that used just 1 watt. When a signal from Earth was detected, the printer would power up while in use, and then return to standby mode. A circuit was added to send a tone back to Earth when the printer was activated, reassuring Mission Control that the printer had switched out of standby mode. These circuits were on the three custom Shuttle boards described earlier.

Putting the teleprinter in a locker made cabling difficult. The solution was a panel on the locker door with connectors for power and audio. The panel has a power switch and light as well as a light to indicate that a message has been received.

The panel on the outside of the locker, used for connection to the teleprinter. From distantsuns, NASA Space Flight forum.

The photo below shows the teleprinter locker with the connection panel on the far left. Note the cables attached to the connectors. These cables went across the back of the Shuttle to the left side, where they went up to the flight deck; the cable routing was performed before launch.14 For this flight, the neighboring locker MA16F held 3300 honeybees for a student experiment.

The teleprinter in middeck locker MA9F on flight STS-41C. The hands belong to mission specialist van Hoften. From National Archives; the description says the photo is from 1995 and shows the Thermal Impulse Printer system, but both are wrong. (STS-41C was in April, 1984.)

The teleprinter cables connect to the shuttle at panel A15 on the aft bulkhead of the flight deck on the left side of the Shuttle. In other words, if you sat in the Shuttle Commander's seat in the cockpit and turned around, this is what you would see.

The connections for the teleprinter in the flight deck. This photo shows Atlantis in the Kennedy Space Center visitor complex. In use, the Shuttle was much more cluttered.

The audio cable from the teleprinter went to the Payload Specialist communication connection on panel A15, while the power cable went to the DC power connection right below. During launch, this audio connection was needed for crew communication, so the teleprinter was plugged in after launch and the audio settings were reconfigured on panel L9. A cue card was placed above panel L9 with instructions on the teleprinter.

The teleprinter's replacements

The Shuttle teleprinter was supposed to be used for a short time until the Uplink Text and Graphics System (TAGS) entered service, but things didn't work out that way. TAGS, described earlier, was the fax-like system that could receive grayscale images, but it depended on the TDRS satellites with their support for digital data. The first TDRS satellite was launched by the sixth shuttle flight, STS-6 (1983). This allowed the use of TAGS on STS-7, but the printer promptly jammed.15 TAGS had constant problems with jamming; on STS-35, the printer jammed and then the unjamming tool broke. Due to the unreliability of the TAGS, the Interim Teleprinter was kept in service as a backup device. TAGS was mounted on a dual cold plate in avionics bay 3 of the crew compartment middeck (details), on the other side of the airlock from the teleprinter.

The Uplink Text and Graphics System, serial number 2. Photo from Smithsonian National Air and Space Museum.

After a decade, another printer, the Thermal Impulse Printer System (TIPS) was put into service, probably on flight STS-56 in 1993. Once TIPS proved its reliability, it replaced both the teleprinter and the Text and Graphics System (TAGS). The TIPS printer was installed in mid-deck locker MF28E; the F indicates the locker was on the forward wall, not the aft wall that held the Interim Teleprinter. As a backup for the TIPS, the Shuttle flew with a second TIPS.

The Thermal Impulse Printer System (TIPS) on flight STS-58. From National Archives. The description says that this device is the teleprinter but it is TIPS.

One motivation behind the TIPS thermal printer was NASA's desire to use more commercial-off-the-shelf (COTS) equipment instead of expensive custom equipment. The TIPS printer is the Raytheon TDU-850 printer (below), a commercial product that sold for $4950. A custom communication interface board inside the printer provided the interface between the printer and the Shuttle's S-Band and Ku-Band communications systems. This interface also allowed astronauts to use the TIPS as a printer for an onboard personal computer.

The Raytheon TDU-850 printer (Thermal Display Unit). From EDN, Mar 17, 1988, p.251.

The photo below shows the TIPS printer in use, printing a long stream of output that Eileen Collins is reading. Collins was the first woman to pilot the Space Shuttle; she flew on the Shuttle four times, twice as pilot and twice as commander.

Pilot Collins reading output from the TIPS printer, the gray box on the right. This is flight STS-84, Atlantis. Photo from National Archives.

The teleprinter, operational

We succeeded in making the Shuttle teleprinter operational. The printer had many mechanical problems, mainly because the rubber rollers had turned to liquid and gummed up the mechanism. Marc disassembled the printer, carefully cleaned the mechanism, and realigned everything. I won't discuss the restoration process here since there will be a video on CuriousMarc's channel. We were able to send FSK-modulated data to the printer and it was printed successfully, as shown below.

Conclusions

At first, I thought that the Shuttle's Interim Teleprinter was a terrible design. It's absurdly heavy and was in danger of overheating. Although the design started with an existing product, much of it required redesign: the front section, the new drum, the interface, and even the frame. The design inherited features it couldn't use, such as the built-in word processor. And the constant-current feature was pointless for the Shuttle and just wasted power.

When I learned that the design had to be completed in just seven months, my opinion of the teleprinter improved. Moreover, the design had many constraints, such as toxicity and flammability restrictions, that limited the potential approaches.

In the end, the teleprinter was used on over 50 flights, acting as a reliable backup to the somewhat flaky Text and Graphics System (TAGS).16 Despite its name, the Interim Teleprinter turned out to be a long-lasting solution, not interim at all. So I have to conclude that the teleprinter was a good design, working much better and much longer than intended.17

In any case, the Interim Teleprinter is an interesting piece of hardware and I hope you enjoyed this article. Follow me on Mastodon as @[email protected] or RSS. Thanks to Marcel for providing the printer. Restoration performed with CuriousMarc, Eric Schlapefer, and Mike Stewart.

Notes and references

1. References for the teleprinter:
   The Interim Teleprinter and its development is described in detail in: M.D. Schuette, “Space Shuttle Interim Teleprinter System,” in Conference record: NTC ’82, Systems for the Eighties, IEEE. (I'll call this the "teleprinter paper" for short.)
   The Shuttle Crew Operations Manual has extensive information on the shuttle and some information on the teleprinter.
   The teleprinter is briefly discussed here.
   Some teleprinter information is in the "Crew Systems Equipment Workbook" via RR Auction.
   The layouts of the Shuttle panels are in Orbiter OV-102 Display and Control Panel Configuration.
   The lockers are described in Orbiter middeck/paylod standard interfaces control document.
   The manuals for the AN-UGC/74 are at RadioNerds.
   An enormous collection of Shuttle documents is at gandalfddi. ↩

2. The teleprinter paper mentions that Shuttle had one other option for receiving hardcopy data: the Text Uplink to Mass Memory System (TUMMS). This allowed text to be displayed on a CRT and the crew could take a Polaroid photo. This was obviously an impractical solution. I couldn't find any other references to TUMMS, so TUMMS may be a proposal that wasn't implemented. ↩

3. Specifically, the Shuttle teleprinter was based on the Honeywell Model AN/UGC-74A9(V)3 Communications Terminal. ↩

4. The mechanism of a drum printer is similar to a chain printer such as the IBM 1403 line printer: each print position has a hammer that fires when the correct character is in that position. However, chain printers have better print quality than drum printers, due to the effect of timing errors. In a drum printer, a small timing error on a hammer will cause the character to be printed too high or too low. In a chain printer, however, a timing error will cause the character to be shifted to the left or right. Vertical mispositioning is obvious and looks terrible. Horizontal mispositioning is much less noticeable since character spacing is normally slightly variable. ↩

5. To be precise, the hammer is fired 1.5 characters early due to its travel time. By the time the hammer hits the drum, the drum has rotated enough to put the desired character in place. Each hammer has a screw to adjust its distance to the drum, necessary to get the timing exact. It's amazing that this system works and doesn't produce a smudged mess. ↩

6. After reverse-engineering the boards, I found a paper on the Shuttle teleprinter that specified the FSK frequencies as 1600 Hz for a 0 and 2057 Hz for a 1, different from what we used. Perhaps the frequencies were changed during development. ↩

7. I created schematics of the three Shuttle-specific boards. Click an image for a larger (readable) version.

   

   Schematic of the input board.

   

   Schematic of the control board.

   

   Schematic of the output board.

    ↩

8. The block diagram below shows the main functional blocks of the CPU card.

   

   CPU block diagram. From Maintenance Manual, TM 11-5815-602-24, p3-6

    ↩

9. I expected that a line would be printed during one drum revolution but looking at the print pattern, it appears to take multiple revolutions per line. Perhaps the printer is avoiding hammers firing too close together to minimize current spikes. Moreover, the published print speed of 60 characters per second is considerably slower than one revolution. Or perhaps the hammer pattern is randomized so spies can't listen in and determine what is being printed. I'm still investigating. ↩

10. Looking at the circuitry, I think the memory buffer holds the drum row number for each position, and the print control card fires the hammer if the value matches the current row number. In contrast, the "obvious" approach would put the character values in the memory buffer and the print control card would match against the current drum character. The implemented solution puts less work on the print control card, which only needs to update the target comparison value once per line, rather than every character. However, it requires the CPU card to transform the input characters into row values. ↩

11. The teleprinter accepts two types of inputs: NRZ and D10. NRZ (Non-Return to Zero) is the straightforward encoding of the serial signal as 0's or 1's. The manual doesn't define D10, but I think it is Manchester encoding, using a 01 sequence for a 0 and a 10 sequence for a 1 (or inverted). The D10 signal is self-clocking, since each bit contains a transition. The demodulation circuit converts the D10 signal into a straight bit sequence. An NRZ signal can either use an external clock or an internal clock from the baud rate generator. With the internal clock, the input is sampled four times and digitally filtered since the input may not exactly line up with the internal clock. ↩

12. The power supply is explained in the Maintenance Manual. The fold-out power supply schematics in that manual were not scanned for some reason but can be found in the B&C Maintenance Manual. ↩

13. The military teleprinter contained a large interface module at the back, providing the signal and power connections to the terminal. The serial-line signals could be a 20-milliamp current loop, a 60-milliamp current loop, or MIL-STD-188/144 (similar to RS-422). The interface module converts these signals to the TTL signals used internally. The interface module also contains a power supply for the interface circuitry. Since this interfacing was not required for the Shuttle, the interface module was discarded and replaced with the Shuttle's custom FSK interface cards. The AC power supply and filtering was also removed. ↩

14. I was a bit surprised that the teleprinter cables would run for a long distance through the Shuttle. But the Shuttle is full of wires and cables running in all directions, as shown in the photo below. This photo is from the same angle as the earlier diagram showing where the teleprinter is connected. This flight was after the teleprinter was retired, but the teleprinter would have been plugged in behind the exercise equipment.

    

    The aft flight deck of Discovery during STS-116. From National Archives.

     ↩

15. One source says that the inaugural flight of TAGS was STS-29 (March 1989). Another source says that testing of the "new" TAGS system continued on STS-29. Contradicting this, TAGS was used on STS-7 (June 1983), jamming after the first page. TAGS was also used on STS-8 (August 1983) but failed after five pages. The TAGS unit was not flown on STS-41B (Feb 1984, the next Challenger flight after STS-8). (Note that STS-41B was the tenth flight, considerably before STS-29, the 28th flight. The Space Shuttle mission numbers are a mess.) It's hard to reconcile these statements. Probably, TAGS was still in the testing stage as late as STS-29 due to reliability problems. ↩

16. The teleprinter had a few problems during use. On flight STS-6, the teleprinter got stuck in high power mode. On flight STS-30, messages were illegible (link). ↩

17. The teleprinter shows the risk of building an interim solution that turns out to last much longer than expected. This also happened with the Interim Upper Stage (IUS), a launch system to boost Shuttle payloads to a higher orbit. The Interim Upper Stage was designed as a temporary solution until a space tug became available. Eventually, NASA realized that nothing was replacing the IUS, so it was renamed to "Inertial Upper Stage", preserving the acronym.

    I'll mention that this also happened with the 8086 processor. It was intended as an interim processor until the iAPX 432 "micro-mainframe" processor was ready. The iAPX 432 turned out to be a disaster, while the "stopgap" 8086 is still with us as the x86 architecture. ↩

Inside an IBM/Motorola mainframe controller chip from 1981

In this article, I look inside a chip in the IBM 3274 Control Unit.1 But before I discuss the chip, I need to give some background on mainframes. (I didn't completely analyze the chip, so don't expect a nice narrative or solid conclusions.)

Die photo of the Motorola/IBM SC81150 chip. Click this image (or any other) for a larger version.

IBM's vintage mainframes were extremely underpowered compared to modern computers; a System/370 mainframe ran well under 1 million instructions per second, while a modern laptop executes billions of instructions per second. But these mainframes could support rooms full of users, while my 2017 laptop can barely handle one person.2 Mainframes achieved their high capacity by offloading much of the data entry overhead so the mainframe could focus on the "important" work. The mainframe received data directly into memory in bulk over high-speed I/O channels, without needing to handle character-by-character editing. For instance, a typical data entry terminal (a "3270") let the user update fields on the screen without involving the computer. When the user had filled out the screen, pressing the "Enter" key sent the entire data record to the mainframe at once. Thus, the mainframe didn't need to process every keystroke; it only dealt with complete records. (This is also why many modern keyboards have an "Enter" key.)

A room with IBM 3179 Color Display Stations, 1984. Note that these are terminals, not PCs. From 3270 Information Display System Introduction.

But that was just the beginning of the hierarchy of offloaded processing in a mainframe system. Terminals weren't attached directly to the mainframe. You could wire 16 terminals to a terminal multiplexer (such as the 3299). This would in turn be connected to a 3274 Control Unit that merged the terminal data and handled the network protocols. The Control Unit was connected to the mainframe's channel processor which handled I/O by moving data between memory and peripherals without slowing down the CPU. All these layers allowed the mainframe to focus on the important data processing while the layers underneath dealt with the details.3

An overview of the IBM 3270 Information Display System attachment. The yellow highlights indicate the 3274 Control Unit. From 3270 Information Display System: Introduction.

The 3274 Control Unit (highlighted above) is the source of the chip I examined. The purpose of the Control Unit "is to take care of all communication between the host system and your organization's display stations and printers". The diagram above shows how terminals were connected to a mainframe, with the 3274 Control Unit (indicated by arrows) in the middle. The 3274 was an all-purpose box, handling terminals, printers, modems, and encryption (if needed). It could communicate with the mainframe at up to 650,000 characters per second. The control unit below (above) is a boring beige box. The control panel is minimal since people normally didn't interact with the unit. On the back are coaxial connectors for the lines to the terminals, as well as connectors to interface with the computer and other peripherals.

An IBM 3274-41D Control Unit. From bitsavers.

The Keystone II board

In 1983, IBM announced new Control Unit models with twice the speed: these were the Model 41 and Model 61. These units were built around a board called Keystone II, shown below. The board is constructed with IBM's peculiar PCB style. The board is arranged as a grid of squares with the PCB traces too small to see unless you zoom in. Most of the decoupling capacitors are in IBM's thin, rectangular packages, although I see a few capacitors in more standard blue packages. IBM is almost a parallel universe with its unusual packaging for ICs and capacitors as well as the strange circuit board appearance.

The Keystone II board. The box is labeled Keystone II FCS [i.e. First Customer Shipment] July 23, 1982. Photo from bitsavers, originally from Bob Roberts.

Most of the chips on the board are IBM chips packaged in square aluminum cans, known as MST (Monolithic System Technology). The first line on each package is the IBM part number, which is usually undocumented. The empty socket can hold a ROS chip; ROS is Read-Only Store, known as ROM to people outside IBM. The Texas Instruments ICs in the upper right are easier to identify; the 74LS641 chips are octal bus transceivers, presumably connecting this board to the rest of the system. Similarly, the 561 5843 is a 74S240 octal bus driver while the 561 6647 chips are 74LS245 octal bus transceivers.

The memory chips on the left side of this board are interesting: each one consists of two "piggybacked" 16-kilobit DRAM chips. IBM's part number 8279251 corresponds to the Intel 4116 chip, originally made by Mostek. With 18 piggybacked chips, the board holds 64 kilobytes of parity-protected memory.

The photo below shows the Keystone II board mounted in the 3274 Control Unit. The board is in slot E towards the left and the purple Motorola IC is visible.

The Keystone II card in slot E of a 3274-41D Control Unit. Photo from bitsavers.

The Motorola/IBM chip

The board has a Motorola chip in a purple ceramic package; this is the chip that I examined. Popping off the golden lid reveals the silicon die underneath. The package has the part number "SC81150R", indicating a Motorola Special/Custom chip. This part number is also visible on the die, as shown below.

The corner of the die is marked with the SC81150 part number. Bond pads and bond wires are also visible.

While the outside of the IC is labeled "Motorola", there are no signs of Motorola internally. Instead, the die is marked "IBM" with the eight-striped logo. My guess is that IBM designed the chip and Motorola manufactured it.

The IBM logo on the die.

The diagram below shows the chip with some of the functional blocks identified. Around the outside are the bond pads and the bond wires that are connected to the chip's grid of pins. At the right is the 16×16 block of memory, along with its associated control, byte swap, and output circuitry. The yellowish-white lines are the metal layer on top of the chip that provides the chip's wiring. The thick metal lines distribute power and ground throughout the chip. Unlike modern chips, this chip only has a single metal layer, so power and ground distribution tends to get in the way of useful circuitry.

The die with some functional blocks identified.

The chip is centered around a 16-bit bus (yellow line) that connects many part of the chip. To write to the bus, a circuit pulls bus lines low. The bus lines are kept high by default by 16 pull-up transistors. This approach was fairly common in the NMOS era. However, performance is limited by the relatively weak pull-up current, making bus lines slow to go high due to R-C delays. For higher performance, some chips would precharge the bus high during one clock cycle and then pull lines low during the next cycle.

The two groups of I/O pins at the bottom are connected to the input buffer on the left and the output buffer on the right. The input buffer includes XOR circuits to compute the parity of each byte. Curiously, only 6 bits of the inputs are connected to the main bus, although other circuits use all 8 bits. The buffer also has a circuit to test for a zero value, but only using 5 of the bits.

I've put red boxes around the numerous PLAs, which can be identified by their grids of transistors. This chip has an unusually large number of PLAs. Eric Schlaepfer hypothesizes that the chip was designed on a prototype circuit board using commercial PAL chips for flexibility, and then they transferred the prototype to silicon, preserving the PLA structure. I didn't see any obvious structure to the PLAs; they all seemed to have wires going all over.

The miscellaneous logic scattered around the chip includes many latches and bus drivers; the latch circuit is similar to the memory cells. I didn't fully reverse-engineer this circuitry but I didn't see anything that looked particularly interesting, such as an ALU or counter. The circuitry near the PLAs could be latches as part of state machines, but I didn't investigate further.

I was hoping to find a recognizable processor inside the package, maybe a Motorola 6809 or 68000 processor. Instead, I found a complicated chip that doesn't appear to be a processor. It has a 16×16 memory block along with about 20 PLAs (Programmable Logic Arrays), a curiously large number. PLAs are commonly used in processors for decoding instructions, since they can match bit patterns. I couldn't find a datapatch in the chip; I expected to see the ALU and registers organized in a large but regular 8-bit or 16-bit block of circuitry. The chip doesn't have any ROM4 so there's no microcode on the chip. For these reasons, I think the chip is not a processor or microcontroller, but a specialized data-handling chip, maybe using the PLAs to interpret bits of a protocol.

The chip is built with NMOS technology, the same as the 6502 and 8086 for instance, rather than CMOS technology that is used in modern chips. I measured the transistor features and the chip appears to be built with a 3.5 µm process (not nm!), which Motorola also used for the 68000 processor (1979).

The memory buffer

The chip has a 16×16 memory buffer, which could be a register file or a FIFO buffer. One interesting feature is that the buffer is triple-ported, so it can handle two reads and one write at the same time. The buffer is implemented as a grid of cells, each storing one bit. Each row corresponds to a 16-bit word, while each column corresponds to one bit in a word. Horizontal control lines (made of polysilicon) select which word gets written or read, while vertical bit lines of metal transmit each bit of the word as it is written or read.

The microscope photo below shows two memory cells. These cells are repeated to create the entire memory buffer. The white vertical lines are metal wiring. The short segments are connections within a cell. The thicker vertical lines are power and ground. The thinner lines are the read and write bit lines. The silicon die itself is underneath the metal. The pinkish regions are active silicon, doped to make it conductive. The speckled golden lines are regions are polysilicon wires between the silicon and the metal. It has two roles: most importantly, when polysilicon crosses active silicon, it forms the gate of a transistor. But polysilicon is also used as wiring, important since this chip only has one layer of metal. The large, dark circles are contacts, connections between the metal layer and the silicon. Smaller square regions are contacts between silicon and polysilicon.

Two memory cells, side by side, as they appear under the microscope.

It was too difficult to interpret the circuits when they were obscured by the metal layer so I dissolved the metal layer and oxide with hydrochloric acid and Armour Etch respectively. The photo below shows the die with the metal removed; the greenish areas are remnants in areas where the metal was thick, mostly power and ground supplies. The dark regions in this image are regions of doped silicon. These are the active areas of the chip, showing the blocks of circuitry. There are also some thin lines of polysilicon wiring. The memory buffer is the large block on the right, just below the center.

The chip with the metal layer removed. Click to zoom in on the image.

Like most implementations of static RAM, each storage cell of the buffer is implemented with cross-coupled inverters, with the output of one inverter feeding into the input of the other. To write a new value to the cell, the new value simply overpowers the inverter output, forcing the cell to the new state. To support this, one of the inverters is designed to be weak, generating a smaller signal than a regular inverter. Most circuits that I've examined create the inverter by using a weak transistor, one with a longer gate. This chip, however, uses a circuit that I haven't seen before: an additional transistor, configured to limit the current from the inverter.

The schematic below shows one cell. Each cell uses ten transistors, so it is a "10T" cell. To support multiple reads and writes, each row of cells has three horizontal control signals: one to write to the word, and two to read. Each bit position has one vertical bit line to provide the write data and two vertical bit lines for the data that is read. Pass transistors connect the bit lines to the selected cells to perform a read or a write, allowing the data to flow in or out of the cell. The symbol that looks like an op-amp is a two-transistor NMOS buffer to amplify the signal when reading the cell.

Schematic of one memory cell.

With the metal layer removed, it is easier to see the underlying silicon circuitry and reverse-engineer it. The diagram below shows the silicon and polysilicon for one storage cell, corresponding to the schematic above. (Imagine vertical metal lines for power, ground, and the three bitlines.)

One memory cell with the metal layer removed. I etched the die a few seconds too long so some of the polysilicon is very thin or missing.

The output from the memory unit contains a byte swapper. A 16-bit word is generated with the left half from the read 1 output and the second half from the read 2 output, but the bytes can be swapped. This was probably used to read an aligned 16-bit word if it was unaligned in memory.

Parity circuits

In the lower right part of the chip are two parity circuits, each computing the parity of an 8-bit input. The parity of an input is computed by XORing the bits together through a tree of 2-input XOR gates. First, four gates process pairs of input bits. Next, two XOR gates combine the outputs of the first gates. Finally, an XOR gate combines the two previous outputs to generate the final parity.

The arrangement of the 14 XOR gates to compute parity of the two 8-bit values A and B.

The schematic below shows how an XOR gate is built from a NOR gate and an AND-NOR gate. If both inputs are 0, the first NOR gate forces the output to 0. If both inputs are 1, the AND gate forces the output to 0. Thus, the circuit computes XOR. Each labeled block above implements the XOR circuit below.

Schematic of an XOR gate.

Conclusion

My conclusion is that the processor for the Keystone II board is probably one of the other chips, one of the IBM metal-can MST packages, and this chip helps with data movement in some way. It would be possible to trace out the complete circuitry of the chip and determine exactly how it functions, but that is too time-consuming a project for this relatively obscure chip.

Follow me on Twitter @kenshirriff or RSS for more chip posts. I'm also on Mastodon occasionally as @[email protected]. Thanks to Al Kossow for providing the chip and Dag Spicer for providing photos. Thanks to Eric Schlaepfer for discussion.

Notes and references

1. The 3274 Control Unit was replaced by the 3174 Establishment Controller, introduced in 1986. An "Establishment Controller" managed a cluster of peripherals or PCs connected to a host mainframe, essentially a box that provided a "kitchen-sink" of functionality including terminal support, local disk storage, Ethernet or token-ring networking, ASCII terminal support, encryption/decryption, and modem support. These units ranged from PC-sized boxes to mini-fridge-sized boxes, depending on how much functionality was required. ↩

2. I'm serious that my laptop can barely handle one person; my 2017 MacBook Air starts dropping characters if it has even a moderate load, and I have to start one-finger typing. You would think that a 1.8 GHz dual-core i5 processor could handle more than 2 characters per second. I don't know if there's something wrong with it, or if modern software just has too much overhead. Don't worry, I upgraded and do most of my work on a faster, more recent laptop. ↩

3. The IBM hardware model had the CPU focusing on the big picture, while the hierarchy of boxes underneath processed data, performed storage, handled printing, and so forth. In a sense, this paralleled the structure of offices in that era, where executives had assistants and secretaries to do the tedious work for them: typing, filing, and so forth. Nowadays, the computer hierarchy and the office hierarchy are both considerably flatter. Maybe there's a connection? ↩

4. A ROM and a PLA are similar in many ways. The general distinction is that a ROM activates one word (row) at a time, while a PLA can activate multiple rows at a time and combine the values, giving more flexibility. A ROM generally has a binary decoder to select the row. This decoder can be recognized by its binary structure: transistors alternating by 1's, by 2's, by 4's, and so forth. ↩