Reverse engineering the mysterious Up-Data Link Test Set from Apollo

Back in 2021, a collector friend of ours was visiting a dusty warehouse in search of Apollo-era communications equipment. A box with NASA-style lights caught his eye—the "AGC Confirm" light suggested a connection with the Apollo Guidance Computer. Disappointingly, the box was just an empty chassis and the circuit boards were all missing. He continued to poke around the warehouse when, to his surprise, he found a bag on the other side of the warehouse that contained the missing boards! After reuniting the box with its wayward circuit cards, he brought it to us: could we make this undocumented unit work?

The Up-Data Link Confidence Test Set, powered up.

A label on the back indicated that it is an "Up-Data Link Confidence Test Set", built by Motorola. As the name suggests, the box was designed to test Apollo's Up-Data Link (UDL), a system that allowed digital commands to be sent up to the spacecraft. As I'll explain in detail below, these commands allowed ground stations to switch spacecraft circuits on or off, interact with the Apollo Guidance Computer, or set the spacecraft's clock. The Up-Data Link needed to be tested on the ground to ensure that its functions operated correctly. Generating the test signals for the Up-Data Link and verifying its outputs was the responsibility of the Up-Data Link Confidence Test Set (which I'll call the Test Set for short)

The Test Set illustrates how, before integrated circuits, complicated devices could be constructed from thumb-sized encapsulated modules. Since I couldn't uncover any documentation on these modules, I had to reverse-engineer them, discovering that different modules implemented everything from flip-flops and logic gates to opto-isolators and analog circuits. With the help of a Lumafield 3-dimensional X-ray scanner, we looked inside the modules and examined the discrete transistors, resistors, diodes, and other components mounted inside.

Four of the 13-pin Motorola modules. These implement logic gates (2/2G & 2/1G), lamp drivers (LD), more logic gates (2P/3G), and a flip-flop (LP FF). The modules have 13 staggered pins, ensuring that they can't be plugged in backward.

Reverse-engineering this system—from the undocumented modules to the mess of wiring—was a challenge. Mike found one NASA document that mentioned the Test Set, but the document was remarkably uninformative.1 Moreover, key components of the box were missing, probably removed for salvage years ago. In this article, I'll describe how we learned the system's functionality, uncovered the secrets of the encapsulated modules, built a system to automatically trace the wiring, and used the UDL Test Set in a large-scale re-creation of the Apollo communications system.

The Apollo Up-Data Link

Before describing the Up-Data Link Test Set, I'll explain the Up-Data Link (UDL) itself. The Up-Data Link provided a mechanism for the Apollo spacecraft to receive digital commands from ground stations. These commands allowed ground stations to control the Apollo Guidance Computer, turn equipment on or off, or update the spacecraft's clock. Physically, the Up-Data Link is a light blue metal box with an irregular L shape, weighing almost 20 pounds.

The Up-Data Link box.

The Apollo Command Module was crammed with boxes of electronics, from communication and navigation to power and sequencing. The Up-Data Link was mounted above the AC power inverters, below the Apollo Guidance Computer, and to the left of the waste management system and urine bags.

The lower equipment bay of the Apollo Command Module. The Up-Data Link is highlighted in yellow. Click this image (or any other) for a larger version. From Command/Service Module Systems Handbook p212.

Up-Data Link Messages

The Up-Data Link supported four types of messages:

• Mission Control had direct access to the Apollo Guidance Computer (AGC) through the UDL, controlling the computer, keypress by keypress. That is, each message caused the UDL to simulate a keypress on the Display/Keyboard (DSKY), the astronaut's interface to the computer.

• The spacecraft had a clock, called the Central Timing Equipment or CTE, that tracked the elapsed time of the mission, from days to seconds. A CTE message could set the clock to a specified time.

• A system called Real Time Control (RTC) allowed the UDL to turn relays on or off, so some spacecraft systems to be controlled from the ground.2 These 32 relays, mounted inside the Up-Data Link box, could do everything from illuminating an Abort light—indicating that Mission Control says to abort—to controlling the data tape recorder or the S-band radio.

• Finally, the UDL supported two test messages to "exercise all process, transfer and program control logic" in the UDL.

The diagram below shows the format of messages to the Up-Data Link. Each message consisted of 12 to 30 bits, depending on the message type. The first three bits, the Vehicle Address, selected which spacecraft should receive the message. (This allowed messages to be directed to the Saturn V booster, the Command Module, or the Lunar Module.3) Next, three System Address bits specified the spacecraft system to receive the message, corresponding to the four message types above. The remaining bits supplied the message text.

Format of the messages to the Up-Data Link. From Telecommunication Systems Study Guide. Note that the vehicle access code uses a different sub-bit pattern from the rest of the message. This diagram shows an earlier sub-bit encoding, not the encoding used by the Test Set.

The contents of the message text depended on the message type. A Real Time Control (RTC) message had a six-bit value specifying the relay number as well as whether it should be turned off or on. An Apollo Guidance Computer (AGC) message had a five-bit value specifying a key on the Display/Keyboard (DSKY). For reliability, the message was encoded in 16 bits: the message, the message inverted, the message again, and a padding bit; any mismatching bits would trigger an error. A CTE message set the clock using four 6-bit values indicating seconds, minutes, hours, and days. The UDL processed the message by resetting the clock and then advancing the time by issuing the specified number of pulses to the CTE to advance the seconds, minutes, hours, and days. (This is similar to setting a digital alarm clock by advancing the digits one at a time.) Finally, the two self test messages consisted of 24-bit patterns that would exercise the UDL's internal circuitry. The results of the test were sent back to Earth via Apollo's telemetry system.

For reliability, each bit transmitted to the UDL was replaced by five "sub-bits": each "1" bit was replaced with the sub-bit sequence "01011", and each "0" bit was replaced with the complement, "10100".4 The purpose of the sub-bits was that any corrupted data would result in an invalid sub-bit code so corrupted messages could be rejected. The Up-Data Link performed this validation by matching the input data stream against "01011" or "10100". (The vehicle address at the start of a message used a different sub-bit code, ensuring that the start of the message was properly identified.) By modern standards, sub-bits are an inefficient way of providing redundancy, since the message becomes five times larger. As a consequence, the effective transmission rate was low: 200 bits per second.

There was no security in the Up-Data Link messages, apart from the need for a large transmitter. Of the systems on Apollo, only the rocket destruct system—euphemistically called the Propellant Dispersion System—was cryptographically secure.5

Since the Apollo radio system was analog, the digital sub-bits couldn't be transmitted from ground to space directly. Instead, a technique called phase-shift keying (PSK) converted the data into an audio signal. This audio signal consists of a sine wave that is inverted to indicate a 0 bit versus a 1 bit; in other words, its phase is shifted by 180 degrees for a 0 bit. The Up-Data Link box takes this audio signal as input and demodulates it to extract the digital message data. (Transmitting this audio signal from ground to the Up-Data Link required more steps that aren't relevant to the Test Set, so I'll describe them in a footnote.6)

The Up-Data Link Test Set

Now that I've explained the Up-Data Link, I can describe the Test Set in more detail. The purpose of the UDL Test Set is to test the Up-Data Link system. It sends a message—as an audio signal—to the Up-Data Link box, implementing the message formatting, sub-bit encoding, and phase shift keying described above. Then it verifies the outputs from the UDL to ensure that the UDL performed the correct action.

Perhaps the most visible feature of the Test Set is the paper tape reader on the front panel: this reader is how the Test Set obtains messages to transmit. Messages are punched onto strips of paper tape, encoded as a sequence of 13 octal digits.7 After a message is read from paper tape, it is shown on the 13-digit display. The first three digits are an arbitrary message number, while the remaining 10 octal digits denote the 30-bit message to send to the UDL. Based on the type of message, specified by the System Address digit, the Test Set validates the UDL's response and indicates success or errors on the panel lights.

I created the block diagram below to explain the architecture and construction of the Test Set (click for a larger view). The system has 25 circuit boards, labeled A1 through A25;8 for the most part, they correspond to functional blocks in the diagram.

My block diagram of the Up-Data Link Test Set. (Click for a larger image.)

The Test Set's front panel is dominated by its display of 13 large digits. It turns out that the storage of these digits is the heart of the Test Set. This storage (A3-A9) assembles the digits as they are read from the paper tape, circulates the bits for transmission, and provides digits to the other circuits to select the message type and validate the results. To accomplish this, the 13 digit circuits are configured as a 39-bit shift register. As the message is read from the paper tape, its bits are shifted into the digit storage, right to left, and the message is shown on the display. To send the message, the shift register is reconfigured so the 10 digits form a loop, excluding the message number. As the bits cycle through the loop, the leftmost bit is encoded and transmitted. At the end of the transmission, the digits have cycled back to their original positions, so the message can be transmitted again if desired. Thus, the shift-register mechanism both deserializes the message when it is read and serializes the message for transmission.

The Test Set uses three boards (A15, A2, and A1) to expand the message with sub-bits and to encode the message into audio. The first board converts each bit into five sub-bits. The second board applies phase-shift keying (PSK) modulation, and the third board has filters to produce clean sine waves from the digital signals.

On the input side, the Test Set receives signals from the Up-Data Link (UDL) box through round military-style connectors. These input signals are buffered by boards A25, A22, A23, A10, and A24. Board 15 verifies the input sub-bits by comparing them with the transmitted sub-bits. For an AGC message, the computer signals are verified by board A14. The timing (CTE) signals are verified by boards A20 and A21. The UDL status (validity) signals are processed by board A12. Board A11 implements a switching power supply to power the interface boards.

You can see from the block diagram that the Test Set is complex and implements multiple functions. On the other hand, the block diagram also shows that it takes a lot of 1960s circuitry to implement anything. For instance, one board can only handle two digits, so the digit display alone requires seven boards. Another example is the inputs, requiring a full board for two or three input bits.

Encapsulated modules

The box is built from modules that are somewhat like integrated circuits but contain discrete components. Modules like these were used in the early 1960s before ICs caught on. Each module implements a simple function such as a flip-flop or buffer. They were more convenient than individual components, since a module provided a ready-made function. They were also compact, since the components were tightly packaged inside the module.

Physically, each module has 13 pins: a row of 7 on one side and a row of 6 offset on the other side. This arrangement ensures that a module cannot be plugged in backward.

A Motorola "LP FF" module. This module implements a J-K flip-flop. "LP" could indicate low performance, low power, or low propagation; the system also uses "HP FF" modules, which could be high performance.

Reverse engineering these modules was difficult since they were encapsulated in plastic and the components were inaccessible. The text printed on each module hinted at its function. For example, the J-K flip-flop module above is labeled "LP FF". The "2/2G & 2/1G" module turned out to contain two NAND gates and two inverters (the 2G and 1G gates). A "2P/3G" module contains two pull-up resistors and two three-input NAND gates. Other modules provided special-purpose analog functions for the PSK modulation.

I reverse-engineered the functions of the modules by applying signals and observing the results. Conveniently, the pins are on 0.200" spacing so I could plug modules into a standard breadboard. The functions of the logic modules were generally straightforward to determine. The analog modules were more difficult; for instance, the "-3.9V" module contains a -3.9-volt Zener diode, six resistors, and three capacitors in complicated arrangements.

To determine how the modules are constructed internally, we had a module X-rayed by John McMaster and another module X-rayed in three dimensions by Lumafield. The X-rays revealed that modules were built with "cordwood construction", a common technique in the 1960s. That is, cylindrical components were mounted between two boards, stacked parallel similar to a pile of wood logs. Instead of using printed-circuit boards, the leads of the components were welded to metal strips to provide the interconnections.

A 3-D scan of the module showing the circuitry inside the compact package, courtesy of Lumafield. Two transistors are visible near the center.

For more information on these modules, see my articles Reverse-engineering a 1960s cordwood flip-flop module with X-ray CT scans and X-ray reverse-engineering a hybrid module. You can interact with the scan here.

The boards

In this section, I'll describe some of the circuit boards and point out their interesting features. A typical board has up to 15 modules, arranged as five rows of three. The modules are carefully spaced so that two boards can be meshed with the components on one board fitting into the gaps on the other board. Thus, a pair of boards forms a dense block.

This photo shows how the modules of the two circuit boards are arranged so the boards can be packed together tightly.

Each pair of boards is attached to side rails and a mounting bracket, forming a unit.8 The bracket has ejectors to remove the board unit, since the backplane connectors grip the boards tightly. Finally, each bracket is labeled with the board numbers, the test point numbers, and the Motorola logo. The complexity of this mechanical assembly suggests that Motorola had developed an integrated prototyping system around the circuit modules, prior to the Test Set.

Digit driver boards

The photo below shows a typical board, the digit driver board. At the left, a 47-pin plug provides the connection between the board and the Test Set's backplane. At the right, 15 test connections allow the board to be probed and tested while it is installed. The board itself is a two-sided printed circuit board with gold plating. Boards are powered with +6V, -6V, and ground; the two red capacitors in the lower left filter the two voltages.

Boards A4 through A9 are identical digit driver boards.

The digit driver is the most common board in the system, appearing six times.9 Each board stores two octal digits in a shift register and drives two digit displays on the front panel. Since the digits are octal, each digit requires three bits of storage, implemented with three flip-flop modules connected as a shift register. If you look closely, you can spot the six flip-flop modules, labeled "LP FF".

The digits are displayed through an unusual technology: an edge-lit lightguide display.10 From a distance, it resembles a Nixie tube, but it uses 10 lightbulbs, one for each number value, with a plastic layer for each digit. Each plastic sheet has numerous dots etched in the shape of the corresponding number. One sheet is illuminated from the edge, causing the dots in the sheet to light up and display that number. In the photo below, you can see both the illuminated and the unilluminated dots. The displays take 14 volts, but the box runs at 28 volts, so a board full of resistors on the front panel drops the voltage from 28 to 14, giving off noticeable heat in the process.

A close-up of a digit in the Test Set, showing the structure of the edge-lit lightguide display.

For each digit position, the driver board provides eight drive signals, one for each bulb. The drivers are implemented in "LD" modules. Since each LD module contains two drive transistors controlled by 4-input AND gates, a module supports two bulbs. Thus, a driver board holds eight LD modules in total. The LD modules are also used on other boards to drive the lights on the front panel.

Ring counters

The Test Set contains multiple counters to count bits, sub-bits, digits, states, and so forth. While a modern design would use binary counters, the Test Set is implemented with a circuit called a ring counter that optimizes the hardware.

For instance, to count to ten, five flip-flops are arranged as a shift register so each flip-flop sends its output to the next one. However, the last flip-flop sends its inverted output to the first. The result is that the counter will proceed: 10000, 11000, 11100, 11110, 11111 as 1 bits are shifted in at the left. But after a 1 reaches the last bit, 0 bits will be shifted in at the left: 01111, 00111, 00011, 00001, and finally 0000. Thus, the counter moves through ten states.

Why not use a 4-bit binary counter and save a flip-flop? First, the binary counter requires additional logic to go from 9 back to 0. Moreover, acting on a particular binary value requires a 4-input gate to check the four bits. But a particular value of a ring counter can be detected with a smaller 2-input gate by checking the bits on either side of the 0/1 boundary. For instance, to detect a count of 3 (11100), only the two highlighted bits need to be tested. Thus, the decoding logic is much simpler for a ring counter, which is important when each gate comes in an expensive module.

Another use of the ring counter is in the sub-state generator, counting out the five states. Since this ring counter uses three flip-flops, you might expect it to count to six. However, the first flip-flop gets one of its inputs from the second flip-flop, resulting in five states: 000, 100, 110, 011, and 001, with the 111 state skipped.11 This illustrates the flexibility of ring counters to generate arbitrary numbers of states.

The PSK boards

Digital data could not be broadcast directly to the spacecraft, so the data was turned into an audio signal using phase-shift keying (PSK). The Test Set uses two boards (A1 and A2) to produce this signal. These boards are interesting and unusual because they are analog, unlike the other boards in the Test Set.

The idea behind phase-shift keying is to change the phase of a sine wave depending on the bit (i.e., sub-bit) value. Specifically, a 2 kHz sine wave indicated a one bit, while the sine wave was inverted for a zero bit. That is, a phase shift of 180º indicated a 0 bit. But how do you tell which sine wave is original and which is flipped? The solution was to combine the information signal with a 1 kHz reference signal that indicates the start and phase of each bit. The diagram below shows how the bits 1-0-1 are encoded into the composite audio signal that is decoded by the Up-Data Link box.

The phase-shift keying modulation process. This encoded digital data into an audio signal for transmission to the Up-DataLink. Note that "1 kc" is 1 kilocycle, or 1 kilohertz in modern usage. From Apollo Digital Up-Data Link Description.

The core of the PSK modulation circuit is a transformer with a split input winding. The 2 kHz sine wave is applied to the winding's center tap. One side of the winding is grounded (by the "ø DET" module) for a 0 bit, but the other side of the winding is grounded for a 1 bit. This causes the signal to go through the winding in one direction for a 1 bit and the opposite direction for a 0 bit. The transformer's output winding thus receives an inverted signal for a 0 bit, giving the 180º phase shift seen in the second waveform above. Finally, the board produces the composite audio signal by mixing in the reference signal through a potentiometer and the "SUM" module.12

Board A2 is the heart of the PSK encoding. The black transformer selects the phase shift, controlled by the "ø DET" and "ø DET D" modules in front of it. The two central potentiometers balance the components of the output signal.

Inconveniently, some key components of the Test Set were missing; probably the most valuable components were salvaged when the box was scrapped. The missing components included the power supplies and amplifiers on the back of the box, as well as parts from PSK board A1. This board had ten white wires that had been cut, going to missing components labeled MP1, R2, L1, and L2. By studying the circuitry, I determined that MP1 had been a 4-kHz oscillator that provided the master clock for the Test Set. R2 was simply a potentiometer to adjust signal levels.

Marc added circuitry to board A1 to replace the two missing filters and the missing oscillator. (The oscillator was used earlier to drive a clock from Soyuz.)

But L1 and L2 were more difficult. It took a lot of reverse-engineering before we determined that L1 and L2 were resonant filters to convert the digital waveforms to the sine waves needed for the PSK output. Marc used a combination of theory and trial-and-error to determine the inductor and capacitor values that produced a clean signal. The photo above shows our substitute filters, along with a replacement oscillator.

Input boards

The Test Set receives signals from the Up-Data Link box under test and verifies that these signals are correct. The Test Set has five input boards (A22 through A25) to buffer the input signals and convert them to digital levels. The input boards also provide electrical isolation between the input signals and the Test Set, avoiding problems caused by ground loops or different voltage levels.

A typical input board is A22, which receives two input signals, supplied through coaxial cables. The board buffers the signals with op-amps, and then produces a digital signal for use by the box. The op-amp outputs go into "1 SS" isolation modules that pass the signal through to the box while ensuring isolation. These modules are optocouplers, using an LED and a phototransistor to provide isolation.13 The op-amps are powered by an isolated power supply.

Board A22 handles two input signals. It has two op-amps and associated circuitry. Note the empty module positions; board A23 has these positions populated so it supports three inputs.

Each op-amp module is a Burr-Brown Model 1506 module,14 encapsulating a transistorized op-amp into a convenient 8-pin module. The module is similar to an integrated-circuit op-amp, except it has discrete components inside and is considerably larger than an integrated circuit. Burr-Brown is said to have created the first solid-state op-amp in 1957, and started making op-amp modules around 1962.

Board A24 is also an isolated input board, but uses different circuitry. It has two modules that each contain four Schmitt triggers, circuits to sharpen up a noisy input. These modules have the puzzling label "-12+6LC". Each output goes through a "1 SS" isolation module, as with the previous input boards. This board receives the 8-bit "validity" signal from the Up-Data Link.

The switching power supply board

Board A11 is interesting: instead of sealed modules, it has a large green cube with numerous wires attached. This board turned out to be a switching power supply that implements six dual-voltage power supplies. The green cube is a transformer with 14 center-tapped windings connected to 42 pins. The transformer ensures that the power supply's outputs are isolated. This allows the op-amps on the input boards to remain electrically isolated from the rest of the Test Set.

The switching power supply board is dominated by a large green transformer with many windings. The two black power transistors are at the front.

The power supply uses a design known as a Royer Converter; the two transistors drive the transformer in a push-pull configuration. The transistors are turned on alternately at high frequency, driven by a feedback winding. The transformer has multiple windings, one for each output. Each center-tapped winding uses two diodes to produce a DC output, filtered by the large capacitors. In total, the power supply has four ±7V outputs and two ±14V outputs to supply the input boards.

This switching power supply is independent from the power supplies for the rest of the Test Set. On the back of the box, we could see where power supplies and amplifiers had been removed. Determining the voltages of the missing power supplies would have been a challenge. Fortunately, the front of the box had test points with labels for the various voltages: -6, +6, and +28, so we knew what voltages were required.

The front panel

The front panel reveals many of the features of the Test Set. At the top, lights indicate the success or failure of various tests. "Sub-bit agree/error" indicates if the sub-bits read back into the Test Set match the values sent. "AGC confirm/error" shows the results of an Apollo Guidance Computer message, while "CTE confirm/error" shows the results of a Central Timing Equipment message. "Verif confirm/error" indicates if the verification message from the UDL matches the expected value for a test message. At the right, lights indicate the status of the UDL: standby, active, or powered off.

A close-up of the Test Set's front panel.

In the middle, toggle switches control the UDL operation. The "Sub-bit spoil" switch causes sub-bits to be occasionally corrupted for testing purposes. "Sub-bit compare/override" enables or disables sub-bit verification. The four switches on the right control the paper tape reader. The "Program start" switch is the important one: it causes the UDL to send one message (in "Single" mode) or multiple messages (in "Serial" mode). The Test Set can stop or continue when an error occurs ("Stop on error" / "Bypass error"). Finally, "Tape advance" causes messages to be read from paper tape, while "Tape stop" causes the UDL to re-use the current message rather than loading a new one.

The UDL provides a verification code that indicates its status. The "Verification Return" knob selects the source of this verification code: the "Direct" position uses a 4-bit verification code, while "Remote" uses an 8-bit verification code.15

At the bottom, "PSK high/low" selects the output level for the PSK signal from the Test Set. (Since the amplifier was removed from our Test Set, this switch has no effect. Likewise, the "Power On / Off" switch has no effect since the power supplies were removed. We power the Test Set with an external lab supply.) In the middle, 15 test points allow access to various signals inside the Test Set. The round elapsed time indicator shows how many hours the Test Set has been running (apparently over 12 months of continuous operation).

Reverse-engineering the backplane

Once I figured out the circuitry on each board, the next problem was determining how the boards were connected. The backplane consists of rows of 47-pin sockets, one for each board. Dense white wiring runs between the sockets as well as to switches, displays, and connectors. I started beeping out the connections with a multimeter, picking a wire and then trying to find the other end. Some wires were easy since I could see both ends, but many wires disappeared into a bundle. I soon realized that manually tracing the wiring was impractically slow: with 25 boards and 47 connections per board, brute-force testing of every pair of connections would require hundreds of thousands of checks.

The backplane wiring of the Test Set consisted of bundles of white wires, as shown in this view of the underside of the Test Set.

To automate the beeping-out of connections, I built a system that I call Beep-o-matic. The idea behind Beep-o-matic is to automatically find all the connections between two motherboard slots by plugging two special boards into the slots. By energizing all the pins on the first board in sequence, a microcontroller can detect connected pins on the second board, revealing the wiring between the two slots.

This system worked better than I expected, rapidly generating a list of connections. I still had to plug the Beep-o-matic boards into each pair of slots (about 300 combinations in total), but each scan took just a few seconds, so a full scan was practical. To find the wiring to the switches and connectors, I used a variant of the process. I plugged a board into a slot and used a program to continuously monitor the pins for changes. I went through the various switch positions and applied signals to the connectors to find the associated connections.

Conclusions

I started reverse-engineering the Test Set out of curiosity: given an undocumented box made from mystery modules and missing key components, could we understand it? Could we at least get the paper tape reader to run and the lights to flash? It was a tricky puzzle to figure out the modules and the circuitry, but eventually we could read a paper tape and see the results on the display.

But the box turned out to be useful. Marc has amassed a large and operational collection of Apollo communications hardware. We use the UDL Test Set to generate realistic signals that we feed into Apollo's S-band communication system. We haven't transmitted these signals to the Moon, but we have transmitted signals between antennas a few feet apart, receiving them with a box called the S-band Transponder. Moreover, we have used the Test Set to control an Up-Data Link box, a CTE clock, and a simulated Apollo Guidance Computer, reading commands from the paper tape and sending them through the complete communication path. Ironically, the one thing we haven't done with the Test Set is use it to test the Up-Data Link in the way it is intended: connecting the UDL's outputs to the Test Set and checking the panel lights.

From a wider perspective, the Test Set provides a glimpse of the vast scope of the Apollo program. This complicated box was just one part of the test apparatus for one small part of Apollo's electronics. Think of the many different electronic systems in the Apollo spacecraft, and consider the enormous effort to test them all. And electronics was just a small part of Apollo alongside the engines, mechanical structures, fuel cells, and life support systems. With all this complexity, it's not surprising that the Apollo program employed 400,000 people.

For more information, the footnotes include a list of UDL documentation16 and CuriousMarc's videos17. Follow me on Bluesky (@righto.com), Mastodon (@[email protected]), or RSS. (I've given up on Twitter.) I worked on this project with CuriousMarc, Mike Stewart, and Eric Schlapfer. Thanks to John McMaster for X-rays, thanks to Lumafield for the CT scans, and thanks to Marcel for providing the box.

Notes and references

1. Mike found a NASA document Functional Integrated System Schematics that includes "Up Data Link GSE/SC Integrated Schematic Diagram". Unfortunately, this was not very helpful since the diagram merely shows the Test Set as a rectangle with one wire in and one wire out. The remainder of the diagram (omitted) shows that the output line passes through a dozen boxes (modulators, switches, amplifiers, and so forth) and then enters the UDL onboard the Spacecraft Command Module. At least we could confirm that the Test Set was part of the functional integrated testing of the UDL.

   

   Detail from "Up Data Link GSE/SC Integrated Schematic Diagram", page GT3.

   Notably, this diagram has the Up-Data Link Confidence Test Set denoted with "2A17". If you examine the photo of the Test Set at the top of the article, you can see that the physical box has a Dymo label "2A17", confirming that this is the same box. ↩

2. The table below lists the functions that could be performed by sending a "realtime command" to the Up-Data Link to activate a relay. The crew could reset any of the relays except for K1-K5 (Abort Light A and Crew Alarm).

   

   The functions controlled by the relays. Adapted from Command/Service Module Systems Handbook.

   A message selected one of 32 relays and specified if the relay should be turned on or off. The relays were magnetic latching relays, so they stayed in the selected position even when de-energized. The relay control also supported "salvo reset": four commands to reset a bank of relays at once. ↩

3. The Saturn V booster had a system for receiving commands from the ground, closely related to the Up-Data Link, but with some differences. The Saturn V system used the same Phase-Shift Keying (PSK) and 70 kHz subcarrier as the Up-Data Link, but the frequency of the S-band signal was different for Saturn V (2101.8 MHz). (Since the Command Module and the booster use separate frequencies, the use of different addresses in the up-data messages was somewhat redundant.) Both systems used sub-bit encoding. Both systems used three bits for the vehicle address, but the remainder of the Saturn message was different, consisting of 14 bits for the decoder address, and 18 bits for message data. A typical message for the Launch Vehicle Digital Computer (LVDC) includes a 7-bit command followed by the 7 bits inverted for error detection. The command system for the Saturn V was located in the Instrument Unit, the ring containing most of the electronic systems that was mounted at the top of the rocket, below the Lunar Module. The command system is described in Astrionics System Handbook section 6.2.

   

   The Saturn Command Decoder. From Saturn IB/V Instrument Unit System Description and Component Data.

   The Lunar Module also had an Up-Data system, called the Digital Up-link Assembly (DUA) and built with integrated circuits. The Digital Up-link Assembly was similar to the Command Module's Up-Data Link and allowed ground stations to control the Lunar Guidance Computer. The DUA also controlled relays to arm the ascent engine. The DUA messages consisted of three vehicle address bits, three system address bits, and 16 information bits. Unlike the Command Module's UDL, the DUA includes the 70-kHz discriminator to demodulate the sub-band. The DUA also provided a redundant up-link voice path, using the data subcarrier to transmit audio. (The Command Module had a similar redundant voice path, but the demodulation was performed in the Premodulation Processor.) The DUA was based on the Digital-Command Assembly (DCA) that received up-link commands on the development vehicles. See Lunar Module Communication System and LM10 Handbook 2.7.4.2.2. ↩

4. Unexpectedly, we found three different sets of sub-bit codes in different documents. The Telecommunications Study Guide says that the first digit (the Vehicle Address) encodes a one bit with the sub-bits 11011; for the remaining digits, a one bit is encoded by 10101. Apollo Digital Command System says that the first digit uses 11001 and the remainder use 10001. The schematic in Apollo Digital Up-Data Link Description shows that the first digit uses 11000 and the remainder use 01011. This encoding matches our Up-Data Link and the Test Set, although the Test Set flipped the phase in the PSK signal. (In all cases, a zero bit is encoded by inverting all five sub-bits.) ↩

5. To provide range safety if the rocket went off course, the Saturn V booster had a destruct system. This system used detonating fuses along the RP-1 and LOX tanks to split the tanks open. As this happened, the escape tower at the top of the rocket would pull the astronauts to safety, away from the booster. The destruct system was controlled by the Digital Range Safety Command System (DRSCS), which used a cryptographic plug to prevent a malevolent actor from blowing up the rocket.

   The DRSCS—used on both the Saturn and Skylab programs—received a message consisting of a 9-character "Address" word and a 2-character "Command" word. Each character was composed of two audio-frequency tones from an "alphabet" of seven tones, reminiscent of the Dual-Tone Multi-Frequency (DTMF) signals used by Touch-Tone phones. The commands could arm the destruct circuitry, shut off propellants, disperse propellants, or switch the DRSCS off.

   To make this system secure, a "code plug" was carefully installed in the rocket shortly before launch. This code plug provided the "key-of-the-day" by shuffling the mapping between tone pairs and characters. With 21 characters, there were 21! (factorial) possible keys, so the chances of spoofing a message were astronomically small. Moreover, as the System Handbook writes with understatement: "Much attention has been given to preventing execution of a catastrophic command should one component fail during flight."

   For details of the range safety system, see Saturn Launch Vehicle Systems Handbook, Astrionics System Handbook (schematic in section 6.3), Apollo Spacecraft & Saturn V Launch Vehicle Pyrotechnics / Explosive Devices, The Evolution of Electronic Tracking, Optical, Telemetry, and Command Systems at the Kennedy Space Center, and Saturn V Stage I (S-IC) Overview. ↩

6. I explained above how the Up-Data Link message was encoded into an audio signal using phase-shift keying. However, more steps were required before this signal could be transmitted over Apollo's complicated S-band radio system. Rather than using a separate communication link for each subsystem, Apollo unified most communication over a high-frequency S-band link, calling this the "Unified S-Band". Apollo had many communication streams—voice, control data, scientific data, ranging, telemetry, television—so cramming them onto a single radio link required multiple layers of modulation, like nested Russian Matryoshka dolls with a message inside.

   For the Up-Data Link, the analog PSK signal was modulated onto a subcarrier using frequency modulation. It was combined with the voice signal from ground and the pseudo-random ranging signal, and the combined signal was phase-modulated at 2106.40625 MHz and transmitted to the spacecraft through an enormous dish antenna at a ground station.

   

   The spectrum of the S-band signal to the Command Module. The Up-Data is transmitted on the 70 kHz subcarrier. Note the very wide spectrum of the pseudo-random ranging signal.

   Thus, the initial message was wrapped in several layers of modulation before transmission: the binary message was expanded to five times its length by the sub-bits, modulated with Phase-Shift Keying, modulated with frequency modulation, and modulated with phase modulation.

   On the spacecraft, the signal went through corresponding layers of demodulation to extract the message. A box called the Unified S-band Transceiver demodulated the phase-modulated signal and sent the data and voice signals to the pre-modulation processor (PMP). The PMP split out the voice and data subcarriers and demodulated the signals with FM discriminators. It sent the data signal (now a 2-kHz audio signal) to the Up-Data Link, where a phase-shift keying demodulator produced a binary output. Finally, each group of five sub-bits was converted to a single bit, revealing the message. ↩

7. The Test Set uses eight-bit paper tape, but the encoding is unusual. Each character of the paper tape consists of a three-bit octal digit, the same digit inverted, and two control bits. Because of this redundancy, the Test Set could detect errors while reading the tape.

   One puzzling aspect of the paper tape reader was that we got it working, but when we tilted the Test Set on its side, the reader completely stopped working. It turned out that the reader's motor was controlled by a mercury-wetted relay, a high-current relay that uses mercury for the switch. Since mercury is a liquid, the relay would only work in the proper orientation; when we tilted the box, the mercury rolled away from the contacts. ↩

8. This view of the Test Set from the top shows the positions of the 25 circuit boards, A1 through A25. Most of the boards are mounted in pairs, although A1, A2, and A15 are mounted singly. Because boards A1 and A11 have larger components, they have empty slots next to them; these are not missing boards. Each board unit has two ejector levers to remove it, along with two metal tabs to lock the unit into position. The 15 numbered holes allow access to the test points for each board. (I don't know the meaning of the text "CTS" on each board unit.) The thirteen digit display modules are at the bottom, with their dropping resistors at the bottom right.

   

   Top view of the Test Set.

    ↩↩

9. There are seven driver boards: A3 through A9. Board A3 is different from the others because it implements one digit instead of two. Instead, board A3 includes validation logic for the paper tape data. ↩

10. Here is the datasheet for the digit displays in the Test Set: "Numerik Indicator IND-0300". In current dollars, they cost over $200 each! The cutaway diagram shows how the bent plastic sheets are stacked and illuminated.

    

    Datasheet from General Radio Catalog, 1963.

    For amazing photos that show the internal structure of the displays, see this article. Fran Blanche's video discusses a similar display. Wikipedia has a page on lightguide displays.

    While restoring the Test Set, we discovered that a few of the light bulbs were burnt out. Since displaying an octal digit only uses eight of the ten bulbs, we figured that we could swap the failed bulbs with unused bulbs from "8" or "9". It turned out that we weren't the first people to think of this—many of the "unused" bulbs were burnt out. ↩

11. I'll give more details on the count-to-five ring counter. The first flip-flop gets its J input from the Q' output of the last flip-flop as expected, but it gets its K input from the Q output of the second flip-flop, not the last flip-flop. If you examine the states, this causes the transition from 110 to 011 (a toggle instead of a set to 111), resulting in five states instead of six. ↩

12. To explain the phase-shift keying circuitry in a bit more detail, board A1 produces a 4 kHz clock signal. Board A2 divides the clock, producing a 2 kHz signal and a 1 kHz signal. The 2 kHz signal is fed into the transformer to be phase-shifted. Then the 1 kHz reference signal is mixed in to form the PSK output. Resonant filters on board A1 convert the square-wave clock signals to smooth sine waves. ↩

13. I was surprised to find LED opto-isolators in a device from the mid-1960s. I expected that the Test Set isolator used a light bulb, but testing showed that it switches on at 550 mV (like a diode) and operated successfully at over 100 kHz, impossible with a light bulb or photoresistor. It turns out that Texas Instruments filed a patent for an LED-based opto-isolator in 1963 and turned this into a product in 1964. The "PEX 3002" used a gallium-arsenide LED and a silicon phototransistor. Strangely, TI called this product a "molecular multiplex switch/chopper". Nowadays, an opto-isolator costs pennies, but at the time, these devices were absurdly expensive: TI's device sold for $275 (almost $3000 in current dollars). For more, see The Optical Link: A New Circuit Tool, 1965. ↩

14. For more information on the Burr-Brown 1506 op amp module, see Burr-Brown Handbook of Operational Amplifier RC Networks. Other documents are Burr-Brown Handbook of Operational Amplifier Applications, Op-Amp History, Operational Amplifier Milestones, and an ad for the Burr-Brown 130 op amp. ↩

15. I'm not sure of the meaning of the Direct versus Remote verification codes. The Block I (earlier) UDL had an 8-bit code, while the Block II (flight) UDL had a 4-bit code. The Direct code presumably comes from the UDL itself, while the Remote code is perhaps supplied through telemetry? ↩

16. The block diagram below shows the structure of the Up-Data Link (UDL). It uses the sub-bit decoder and a 24-stage register to deserialize the message. Based on the message, the UDL triggers relays (RTC), outputs data to the Apollo Guidance Computer (called the CMC, Command Module Computer here), sends pulses to the CTE clock, or sends validity signals back to Earth.

    

    UDL block diagram, from Apollo Operations Handbook, page 31

    For details of the Apollo Up-Data system, see the diagram below (click it for a very large image). This diagram is from the Command/Service Module Systems Handbook (PDF page 64); see page 80 for written specifications of the UDL.

    

    This diagram of the Apollo Updata system specifies the message formats, relay usages, and internal structure of the UDL.

    Other important sources of information: Apollo Digital Up-Data Link Description contains schematics and a detailed description of the UDL. Telecommunication Systems Study Guide describes the earlier UDL that included a 450 MHz FM receiver. ↩

17. The following CuriousMarc videos describe the Up-Data Link and the Test Set, so smash that Like button and subscribe :-)

    • Mystery Apollo Up-Data Box
    • Up-Data Commands
    • Up-Data Link Analog Mystery Solved
    • Looking inside Apollo components with Lumafield's 3D X-ray machine
    • UDL Grand Opening and Power Up
    • Breaking the Updata Link Code
    • Is there something wrong with our NASA Up Data Link transmitter?
    • Trying every function of the Apollo command system
     ↩

Inside the Apollo "8-Ball" FDAI (Flight Director / Attitude Indicator)

During the Apollo flights to the Moon, the astronauts observed the spacecraft's orientation on a special instrument called the FDAI (Flight Director / Attitude Indicator). This instrument showed the spacecraft's attitude—its orientation—by rotating a ball. This ball was nicknamed the "8-ball" because it was black (albeit only on one side). The instrument also acted as a flight director, using three yellow needles to indicate how the astronauts should maneuver the spacecraft. Three more pointers showed how fast the spacecraft was rotating.

An Apollo FDAI (Flight Director/Attitude Indicator) with the case removed. This FDAI is on its side to avoid crushing the needles.

Since the spacecraft rotates along three axes (roll, pitch, and yaw), the ball also rotates along three axes. It's not obvious how the ball can rotate to an arbitrary orientation while remaining attached. In this article, I look inside an FDAI from Apollo that was repurposed for a Space Shuttle simulator1 and explain how it operates. (Spoiler: the ball mechanism is firmly attached at the "equator" and rotates in two axes. What you see is two hollow shells around the ball mechanism that spin around the third axis.)

The FDAI in Apollo

For the missions to the Moon, the Lunar Module had two FDAIs, as shown below: one on the left for the Commander (Neil Armstrong in Apollo 11) and one on the right for the Lunar Module Pilot (Buzz Aldrin in Apollo 11). With their size and central positions, the FDAIs dominate the instrument panel, a sign of their importance. (The Command Module for Apollo also had two FDAIs, but with a different design; I won't discuss them here.2)

The instrument panel in the Lunar Module. From Apollo 15 Lunar Module, NASA, S71-40761. If you're looking for the DSKY, it is in the bottom center, just out of the picture.

Each Lunar Module FDAI could display inputs from multiple sources, selected by switches on the panel.3 The ball could display attitude from either the Inertial Measurement Unit or from the backup Abort Guidance System, selected by the "ATTITUDE MON" toggle switch next to either FDAI. The pitch attitude could also be supplied by an electromechanical unit called ORDEAL (Orbital Rate Display Earth And Lunar) that simulates a circular orbit. The error indications came from the Apollo Guidance Computer, the Abort Guidance System, the landing radar, or the rendezvous radar (controlled by the "RATE/ERROR MON" switches). The pitch, roll, and yaw rate displays were driven by the Rate Gyro Assembly (RGA). The rate indications were scaled by a switch below the FDAI, selecting 25°/sec or 5°/sec.

The FDAI mechanism

The ball inside the indicator shows rotation around three axes. I'll first explain these axes in the context of an aircraft, since the axes of a spacecraft are more arbitrary.4 The roll axis indicates the aircraft's angle if it rolls side-to-side along its axis of flight, raising one wing and lowering the other. Thus, the indicator shows the tilt of the horizon as the aircraft rolls. The pitch axis indicates the aircraft's angle if it pitches up or down, with the indicator showing the horizon moving down or up in response. Finally, the yaw axis indicates the compass direction that the aircraft is heading, changing as the aircraft turns left or right. (A typical aircraft attitude indicator omits yaw.)

I'll illustrate how the FDAI rotates the ball in three axes, using an orange as an example. Imagine pinching the horizontal axis between two fingers with your arm extended. Rotating your arm will roll the ball counter-clockwise or clockwise (red arrow). In the FDAI, this rotation is accomplished by a motor turning the frame that holds the ball. For pitch, the ball rotates forward or backward around the horizontal axis (yellow arrow). The FDAI has a motor inside the ball to produce this rotation. Yaw is a bit more difficult to envision: imagine hemisphere-shaped shells attached to the top and bottom shafts. When a motor rotates these shells (green arrow), the hemispheres will rotate, even though the ball mechanism (the orange) remains stationary.

A sphere, showing the three axes.

The diagram below shows the mechanism inside the FDAI. The indicator uses three motors to move the ball. The roll motor is attached to the FDAI's frame, while the pitch and yaw motors are inside the ball. The roll motor rotates the roll gimbal through gears, causing the ball to rotate clockwise or counterclockwise. The roll gimbal is attached to the ball mechanism at two points along the "equator"; these two points define the pitch axis. Numerous wires on the roll gimbal enter the ball along the pitch axis. The roll control transformer provides position feedback, as will be explained below.

The main components inside the FDAI.

Removing the hemispherical shells reveals the mechanism inside the ball. When the roll gimbal is rotated, this mechanism rotates with it. The pitch motor causes the ball mechanism to rotate around the pitch axis. The yaw motor and control transformer are not visible in this photo; they are behind the pitch components, oriented perpendicularly. The yaw motor turns the vertical shaft, with the two hemisphere shells attached to the top and bottom of the shaft. Thus, the yaw motor rotates the ball shells around the yaw axis, while the mechanism itself remains stationary. The control transformers for pitch and yaw provide position feedback.

The components inside the ball of the FDAI.

Why doesn't the wiring get tangled up as the ball rotates? The solution is two sets of slip rings to implement the electrical connections. The photo below shows the first slip ring assembly, which handles rotation around the roll axis. These slip rings connect the stationary part of the FDAI to the rotating roll gimbal. The vertical metal brushes are stationary; there are 23 pairs of brushes, one for each connection to the ball mechanism. Each pair of brushes contacts one metal ring on the striped shaft, maintaining contact as the shaft rotates. Inside the shaft, 23 wires connect the circular metal contacts to the roll gimbal.

The slip ring assembly in the FDAI.

A second set of slip rings inside the ball handles rotation around the pitch axis. These rings provide the electrical connection between the wiring on the roll gimbal and the ball mechanism. The yaw axis does not use slip rings since only the hemisphere shells rotate around the yaw axis; no wires are involved.

Synchros and the servo loop

In this section, I'll explain how the FDAI is controlled by synchros and servo loops. In the 1950s and 1960s, the standard technique for transmitting a rotational signal electrically was through a synchro. Synchros were used for everything from rotating an instrument indicator in avionics to rotating the gun on a navy battleship. A synchro produces an output that depends on the shaft's rotational position, and transmits this output signal on three wires. If you connect these wires to a second synchro, you can use the first synchro to control the second one: the shaft of the second synchro will rotate to the same angle as the first shaft. Thus, synchros are a convenient way to send a control signal electrically.

The photo below shows a typical synchro, with the input shaft on the top and five wires at the bottom: two for power and three for the output.

A synchro transmitter.

Internally, the synchro has a rotating winding called the rotor that is driven with 400 Hz AC. Three fixed stator windings provide the three AC output signals. As the shaft rotates, the voltages of the output signals change, indicating the angle. (A synchro resembles a transformer with three variable secondary windings.) If two connected synchros have different angles, the magnetic fields create a torque that rotates the shafts into alignment.

The schematic symbol for a synchro transmitter or receiver.

The downside of synchros is that they don't produce a lot of torque. The solution is to use a more powerful motor, controlled by the synchro and a feedback loop called a servo loop. The servo loop drives the motor in the appropriate direction to eliminate the error between the desired position and the current position.

The diagram below shows how the servo loop is constructed from a combination of electronics and mechanical components. The goal is to rotate the output shaft to an angle that exactly matches the input angle, specified by the three synchro wires. The control transformer compares the input angle and the output shaft position, producing an error signal. The amplifier uses this error signal to drive the motor in the appropriate direction until the error signal drops to zero. To improve the dynamic response of the servo loop, the tachometer signal is used as a negative feedback voltage. The feedback slows the motor as the system gets closer to the right position, so the motor doesn't overshoot the position and oscillate. (This is sort of like a PID controller.)

This diagram shows the structure of the servo loop, with a feedback loop ensuring that the rotation angle of the output shaft matches the input angle.

A control transformer is similar to a synchro in appearance and construction, but the rotating shaft operates as an input, not the output. In a control transformer, the three stator windings receive the inputs and the rotor winding provides the error output. If the rotor angle of the synchro transmitter and control transformer are the same, the signals cancel out and there is no error voltage. But as the difference between the two shaft angles increases, the rotor winding produces an error signal. The phase of the error signal indicates the direction of the error.

In the FDAI, the motor is a special motor/tachometer, a device that was often used in avionics servo loops. This motor is more complicated than a regular electric motor. The motor is powered by 115 volts AC at 400 hertz, but this won't spin the motor on its own. The motor also has two low-voltage control windings. Energizing the control windings with the proper phase causes the motor to spin in one direction or the other. The motor/tachometer unit also contains a tachometer to measure its speed for the feedback loop. The tachometer is driven by another 115-volt AC winding and generates a low-voltage AC signal that is proportional to the motor's rotational speed.

A motor/tachometer similar (but not identical) to the one in the FDAI.

The photo above shows a motor/tachometer with the rotor removed. The unit has many wires because of its multiple windings. The rotor has two drums. The drum on the left, with the spiral stripes, is for the motor. This drum is a "squirrel-cage rotor", which spins due to induced currents. (There are no electrical connections to the rotor; the drums interact with the windings through magnetic fields.) The drum on the right is the tachometer rotor; it induces a signal in the output winding proportional to the speed due to eddy currents. The tachometer signal is at 400 Hz like the driving signal, either in phase or 180º out of phase, depending on the direction of rotation. For more information on how a motor/tachometer works, see my teardown.

The amplifiers

The FDAI has three servo loops—one for each axis—and each servo loop has a separate control transformer, motor, and amplifier. The photo below shows one of the three amplifier boards. The construction is unusual and somewhat chaotic, with some components stacked on top of others to save space. Some of the component leads are long and protected with clear plastic sleeves.5 The cylindrical pulse transformer in the middle has five colorful wires coming out of it. At the left are the two transistors that drive the motor's control windings, with two capacitors between them. The transistors are mounted on a heat sink that is screwed down to the case of the amplifier assembly for cooling. Each amplifier is connected to the FDAI through seven wires with pins that plug into the sockets on the right of the board.6

One of the three amplifier boards. At the right front of the board, you can see a capacitor stacked on top of a resistor. The board is shiny because it is covered with conformal coating.

The function of the board is to amplify the error signal so the motor rotates in the appropriate direction. The amplifier also uses the tachometer output from the motor unit to slow the motor as the error signal decreases, preventing overshoot. The inputs to the amplifier are 400 hertz AC signals, with the magnitude indicating the amount of error or speed and the phase indicating the direction. The two outputs from the amplifier drive the two control windings of the motor, determining which direction the motor rotates.

The schematic for the amplifier board is below. 7 The two transistors on the left amplify the error and tachometer signals, driving the pulse transformer. The outputs of the pulse transformer will have opposite phases, driving the output transistors for opposite halves of the 400 Hz cycle. This activates the motor control winding, causing the motor to spin in the desired direction.8

The schematic of an amplifier board.

History of the FDAI

Bill Lear, born in 1902, was a prolific inventor with over 150 patents, creating everything from the 8-track tape to the Learjet, the iconic private plane of the 1960s. He created multiple companies in the 1920s as well as inventing one of the first car radios for Motorola before starting Lear Avionics, a company that specialized in aerospace instruments.9 Lear produced innovative aircraft instruments and flight control systems such as the F-5 automatic pilot, which received a trophy as the "greatest aviation achievement in America" for 1950.

Bill Lear went on to solve an indicator problem for the Air Force: the supersonic F-102 Delta Dagger interceptor (1953) could climb at steep angles, but existing attitude indicators could not handle nearly vertical flight. Lear developed a remote two-gyro platform that drove the cockpit indicator while avoiding "gimbal lock" during vertical flight. For the experimental X-15 rocket-powered aircraft (1959), Lear improved this indicator to handle three axes: roll, pitch, and yaw.

Meanwhile, the Siegler Corporation started in 1950 to manufacture space heaters for homes. A few years later, Siegler was acquired by John Brooks, an entrepreneur who was enthusiastic about acquisitions. In 1961, Lear Avionics became his latest acquisition, and the merged company was called Lear Siegler Incorporated, often known as LSI. (Older programmers may know Lear Siegler through the ADM-3A, an inexpensive video display terminal from 1976 that housed the display and keyboard in a stylish white case.)

The X-15's attitude indicator became the basis of the indicator for the F-4 fighter plane (the ARU/11-A). Then, after "a minimum of modification", the attitude-director indicator was used in the Gemini space program. In total, Lear Siegler provided 11 instruments in the Gemini instrument panel, with the attitude director the most important. Next, Gemini's indicator was modified to become the FDAI (flight director-attitude indicator) in the Lunar Module for Apollo.10 Lear Siegler provided numerous components for the Apollo program, from a directional gyro for the Lunar Rover to the electroluminescent display for the Apollo Guidance Computer's Display/Keyboard (DSKY).

An article titled "LSI Instruments Aid in Moon Landing" from LSI's internal LSI Log publication, July 1969. (Click for a larger version.)

In 1974, Lear Siegler obtained a contract to develop the Attitude-Director Indicator (ADI) for the Space Shuttle, producing a dozen ADI units for the Space Shuttle. However, by this time, Lear Siegler was losing enthusiasm for low-volume space avionics. The Instrument Division president said that "the business that we were in was an engineering business and engineers love a challenge." However, manufacturing refused to deal with the special procedures required for space manufacturing, so the Shuttle units were built by the engineering department. Lear Siegler didn't bid on later Space Shuttle avionics and the Shuttle ADI became its last space product. In the early 2000s, the Space Shuttle's instruments were upgraded to a "glass cockpit" with 11 flat-panel displays known as the Multi-function Electronic Display System (MEDS). The MEDS was produced by Lear Siegler's long-term competitor, Honeywell.

Getting back to Bill Lear, he wanted to manufacture aircraft, not just aircraft instruments, so he created the Learjet, the first mass-produced business jet. The first Learjet flew in 1963, with over 3000 eventually delivered. In the early 1970s, Lear designed a steam turbine automobile engine. Rather than water, the turbine used a secret fluorinated hydrocarbon called "Learium". Lear had visions of thousands of low-pollution "Learmobiles", but the engine failed to catch on. Lear had been on the verge of bankruptcy in the 1960s; one of his VPs explained that "the great creative minds can't be bothered with withholding taxes and investment credits and all this crap". But by the time of his death in 1978, Lear had a fortune estimated at $75 million.

Comparing the ARU/11-A and the FDAI

Looking inside our FDAI sheds more details on the evolution of Lear Siegler's attitude directors. The photo below compares the Apollo FDAI (top) to the earlier ARU/11-A used in the F-4 aircraft (bottom). While the basic mechanism and the electronic amplifiers are the same between the two indicators, there are also substantial changes.

Comparison of an FDAI (top) with an ARU-11/A (bottom). The amplifier boards and needles have been removed from the FDAI.

The biggest difference between the ARU-11/A indicator and the FDAI is that the electronics for the ARU-11/A are in a separate module that was plugged into the back of the indicator, while the FDAI includes the electronics internally, with boards mounted on the instrument frame. Specifically, the ARU-11/A has a separate unit containing a multi-winding transformer, a power supply board, and three amplifier boards (one for each axis), while the FDAI contains these components internally. The amplifier boards in the ARU-11/A and the FDAI are identical, constructed from germanium transistors rather than silicon.11 The unusual 11-pin transformers are also the same. However, the power supply boards are different, probably because the boards also contain scaling resistors that vary between the units.12 The power supply boards are also different shapes to fit the available space.

The ball assemblies of the ARU/11-A and the FDAI are almost the same, with the same motor assemblies and slip ring mechanism. The gearing has minor changes. In particular, the FDAI has two plastic gears, while the ARU/11-A uses exclusively metal gears.

The ARU/11-A has a patented pitch trim feature that was mostly—but not entirely—removed from the Apollo FDAI. The motivation for this feature is that an aircraft in level flight will be pitched up a few degrees, the "angle of attack". It is desirable for the attitude indicator to show the aircraft as horizontal, so a pitch trim knob allows the angle of attack to be canceled out on the display. The problem is that if you fly your fighter plane vertically, you want the indicator to show precisely vertical flight, rather than applying the pitch trim adjustment. The solution in the ARU-11/A is a special 8-zone potentiometer on the pitch axis that will apply the pitch trim adjustment in level flight but not in vertical flight, while providing a smooth transition between the regions. This special potentiometer is mounted inside the ball of the ARU-11/A. However, this pitch trim adjustment is meaningless for a spacecraft, so it is not implemented in the Apollo or Space Shuttle instruments. Surprisingly, the shell of the potentiometer still exists in our FDAI, but without the potentiometer itself or the wiring. Perhaps it remained to preserve the balance of the ball. In the photo below, the cylindrical potentiometer shell is indicated by an arrow. Note the holes in the front of the shell; in the ARU-11/A, the potentiometer's wiring terminals protrude through these holes, but in the FDAI, the holes are covered with tape.

Inside the ball of the FDAI. The potentiometer shell is indicated with an arrow.

Finally, the mounting of the ball hemispheres is slightly different. The ARU/11-A uses four screws at the pole of each hemisphere. Our FDAI, however, uses a single screw at each pole; the screw is tightened with a Bristol Key, causing the shaft to expand and hold the hemisphere in place.

To summarize, the Apollo FDAI occupies a middle ground: while it isn't simply a repurposed ARU-11/A, neither is it a complete redesign. Instead, it preserves the old design where possible, while stripping out undesired features such as pitch trim. The separate amplifier and mechanical units of the ARU/11-A were combined to form the larger FDAI.

Differences from Apollo

The FDAI that we examined is a special unit: it was originally built for Apollo but was repurposed for a Space Shuttle simulator. Our FDAI is labeled Model 4068F, which is a Lunar Module part number. Moreover, the FDAI is internally stamped with the date "Apr. 22 1968", over a year before the first Moon landing.

However, a closer look shows that several key components were modified to make the Apollo FDAI work in the Shuttle Simulator.14 The Apollo FDAI (and the Shuttle ADI) used resolvers as inputs to control the ball, while our FDAI uses synchros. (Resolvers and synchros are similar, except resolvers use sine and cosine inputs, 90° apart, on two wire pairs, while synchros use three inputs, 120° apart, on three wires.) NASA must have replaced the three resolver control transformers in the FDAI with synchro control transformers for use in the simulator.

The Apollo FDAI used electroluminescent lighting for the display, while ours uses eight small incandescent bulbs. The metal case of our FDAI has a Dymo embossed tape label "INCANDESCENT LIGHTING", alerting users to the change from Apollo's illumination. Our FDAI also contains a step-down transformer to convert the 115 VAC input into 5 VAC to power the bulbs, while the Shuttle powered its ADI illumination directly from 5 volts.

The dial of our FDAI was repainted to match the dial of the Shuttle FDAI. The Apollo FDAI had red bands on the left and right of the dial. A close examination of our dial shows that black paint was carefully applied over the red paint, but a few specks of red paint are still visible (below). Moreover, the edges of the lines and the lozenge show slight unevenness from the repainting. Second, the Apollo FDAI had the text "ROLL RATE", "PITCH RATE", and "YAW RATE" in white next to the needle scales. In our FDAI, this text has been hidden by black paint to match the Shuttle display.13 Third, the Apollo LM FDAI had a crosshair in the center of the instrument, while our FDAI has a white U-shaped indicator, the same as the Shuttle (and the Command Module's FDAI). Finally, the ball of the Apollo FDAI has red circular regions at the poles to warn of orientations that can cause gimbal lock. Our FDAI (like the Shuttle) does not have these circles. We couldn't see any evidence that these regions were repainted, so we suspect that our FDAI has Shuttle hemispheres on the ball.

A closeup of the dial on our FDAI shows specks of red paint around the dial markings. The color is probably Switzer DayGlo Rocket Red.

Our FDAI has also been modified electrically. Small green connectors (Micro-D MDB1) have been added between the slip rings and the motors, as well as on the gimbal arm. We think these connectors were added post-Apollo, since they are attached somewhat sloppily with glue and don't look flight-worthy. Perhaps these connectors were added to make disassembly and modification easier. Moreover, our FDAI has an elapsed time indicator, also mounted with glue.

The back of our FDAI is completely different from Apollo. First, the connector's pinout is completely different. Second, each of the six indicator needles has a mechanical adjustment as well as a trimpot (details). Finally, each of the three axes has an adjustment potentiometer.

The Shuttle's ADI (Attitude Director Indicator)

Each Space Shuttle had three ADIs (Attitude Director Indicators), which were very similar to the Apollo FDAI, despite the name change. The photo below shows the two octagonal ADIs in the forward flight deck, one on the left in front of the Commander, and one on the right in front of the Pilot. The aft flight deck station had a third ADI.15

This photo shows Discovery's forward flight deck on STS-063 (1999). The ADIs are indicated with arrows. The photo is from the National Archives.

Our FDAI appears to have been significantly modified for use in the Shuttle simulator, as described above. However, it is much closer to the Apollo FDAI than the ADI used in the Shuttle, as I'll show in this section. My hypothesis is that the simulator was built before the Shuttle's ADI was created, so the Apollo FDAI was pressed into service.

The Shuttle's ADI was much more complicated electrically than the Apollo FDAI and our FDAI, providing improved functionality.16 For instance, while the Apollo FDAI had a simple "OFF" indicator flag to show that the indicator had lost power, the Shuttle's ADI had extensive error detection. It contained voltage level monitors to check its five power supplies. (The Shuttle ADI used three DC power sources and two AC power sources, compared to the single AC supply for Apollo.) The Shuttle's ADI also monitored the ball servos to detect position errors. Finally, it received an external "Data OK" signal. If a fault was detected by any of these monitors, the "OFF" flag was deployed to indicate that the ADI could not be trusted.

The Shuttle's ADI had six needles, the same as Apollo, but the Shuttle used feedback to make the positions more accurate. Specifically, each Shuttle needle had a feedback sensor, a Linear Variable Differential Transformer (LVDT) that generates a voltage based on the needle position. The LVDT output drove a servo feedback loop to ensure that the needle was in the exact desired position. In the Apollo FDAI, on the other hand, the needle input voltage drove a galvanometer, swinging the needle proportionally, but there was no closed loop to ensure accuracy.

I assume that the Shuttle's ADI had integrated circuit electronics to implement this new functionality, considerably more modern than the germanium transistors in the Apollo FDAI. The Shuttle probably used the same mechanical structures to rotate the ball, but I can't confirm that.

Conclusions

The FDAI was a critical instrument in Apollo, indicating the orientation of the spacecraft in three axes. It wasn't obvious to me how the "8-ball" can rotate in three axes while still being securely connected to the instrument. The trick is that most of the mechanism rotates in two axes, while hollow hemispherical shells provide the third rotational axis.

The FDAI has an interesting evolutionary history, from the experimental X-15 rocket plane and the F-4 fighter to the Gemini, Apollo, and Space Shuttle flights. Our FDAI has an unusual position in this history: since it was modified from Apollo to function in a Space Shuttle simulator, it shows aspects of both Apollo and the Space Shuttle indicators. It would be interesting to compare the design of a Shuttle ADI to the Apollo FDAI, but I haven't been able to find interior photos of a Shuttle ADI (or of an unmodified Apollo FDAI).17

You can see a brief video of the FDAI in motion here. For more, follow me on Bluesky (@righto.com), Mastodon (@[email protected]), or RSS. (I've given up on Twitter.) I worked on this project with CuriousMarc, Mike Stewart, and Eric Schlapfer, so expect a video at some point. Thanks to Richard for providing the FDAI. I wrote about the F-4 fighter plane's attitude indicator here.

Inside the FDAI. The amplifier boards have been removed for this photo.

Notes and references

1. There were many Space Shuttle simulators, so it is unclear which simulator was the source of our FDAI. The photo below shows a simulator, with one of the ADIs indicated with an arrow. Presumably, our FDAI became available when a simulator was upgraded from physical instruments to the screens of the Multi-function Electronic Display System (MEDS).

   

   "Forward flight deck of the fixed-base simulator." From Introduction to Shuttle Mission Simulation

   The most complex simulators were the three Shuttle Mission Simulators, one of which could dynamically move to provide motion cues. These simulators were at the simulation facility in Houston—officially the Jake Garn Mission Simulator and Training Facility—which also had a guidance and navigation simulator, a Spacelab simulator, and integration with the WETF (Weightless Environment Training Facility, an underground pool to simulate weightlessness). The simulators were controlled by a computer complex containing dozens of networked computers. The host computers were three UNIVAC 1100/92 mainframes, 36-bit computers that ran the simulation models. These were supported by seventeen Concurrent Computer Corporation 3260 and 3280 super-minicomputers that simulated tracking, telemetry, and communication. The simulators also used real Shuttle computers running the actual flight software; these were IBM AP101S General-Purpose Computers (GPC). For more information, see Introduction to Shuttle Mission Simulation.

   NASA had additional Shuttle training facilities beyond the Shuttle Mission Simulator. The Full Fuselage Trainer was a mockup of the complete Shuttle orbiter (minus the wings). It included full instrument panels (including the ADIs), but did not perform simulations. The Crew Compartment Trainers could be positioned horizontally or vertically (to simulate pre-launch operations). They contained accurate flight decks with non-functional instruments. Three Single System Trainers provided simpler mockups for astronauts to learn each system, both during normal operation and during malfunctions, before using the more complex Shuttle Mission Simulator. A list of Shuttle training facilities is in Table 3.1 of Preparing for the High Frontier. Following the end of the Shuttle program, the trainers were distributed to various museums (details). ↩

2. The Command Module for Apollo used a completely different FDAI (flight director-attitude indicator) that was built by Honeywell. The two designs can be easily distinguished: the Honeywell FDAI is round, while the Lear Siegler FDAI is octagonal. ↩

3. The FDAI's signals are more complicated than I described above. Among other things, the IMU's gimbal angles use a different coordinate system from the FDAI, so an electromechanical unit called GASTA (Gimbal Angle Sequence Transformation Assembly) used resolvers and motors to convert the coordinates. The digital attitude error signals from the computer are converted to analog by the Inertial Measurement Unit's Coupling Data Unit (IMU CDU). For attitude, the IMU is selected with the PGNS (Primary Guidance and Navigation System) switch setting. See the Lunar Module Systems Handbook, Lunar Module System Handbook Rev A, and the Apollo Operations Handbook for more.

   

   The connections to the Apollo FDAIs. Adapted from LM-1 Systems Handbook. I think this diagram predates the ORDEAL system. (Click for a larger version.)

    ↩

4. The roll, pitch, and yaw axes of the Lunar Module are not as obvious as the axes of an airplane. The diagram below defines these axes.

   

   The roll, pitch, and yaw axes of the Lunar Module. Adapted from LM Systems Handbook.

    ↩

5. The amplifier is constructed on a single-sided printed circuit board. Since the components are packed tightly on the board, routing of the board was difficult. However, some of the components have long leads, protected by plastic sleeves. This provides additional flexibility for the board routing since the leads could be positioned as desired, regardless of the geometry of the component. As a result, the style of this board is very different from modern circuit boards, where components are usually arranged in an orderly pattern. ↩

6. In our FDAI, the amplifier boards as well as the needle actuators are connected by pins that plug into sockets. These connections don't seem suitable for flight since they could easily vibrate loose. We suspect that the pin-and-socket connections made the module easier to reconfigure in the simulator, but were not used in flyable units. In particular, in the similar aircraft instruments (ARU/11-A) that we examined, the wires to the amplifier boards were soldered. ↩

7. The board has a 56-volt Zener diode, but the function of the diode is unclear. The board is powered by 28 volts, not enough voltage to activate the Zener. Perhaps the diode filters high-voltage transients, but I don't see how transients could arise in that part of the circuit. (I can imagine transients when the pulse transformer switches, but the Zener isn't connected to the transformer.) ↩

8. In more detail, each motor's control winding is a center-tapped winding, with the center connected to 28 volts DC. The amplifier board's output transistors will ground either side of the winding during alternate half-cycles of the 400 Hz cycle. This causes the motor to spin in one direction or the other. (Usually, control winding are driven 90° out of phase with the motor power, but I'm not sure how this phase shift is applied in the FDAI.) ↩

9. The history of Bill Lear and Lear Siegler is based on Love him or hate him, Bill Lear was a creator and On Course to Tomorrow: A History of Lear Siegler Instrument Division’s Manned Spaceflight Systems 1958-1981. ↩

10. Numerous variants of the Lear Siegler FDAI were built for Apollo, as shown before. Among other things, the length of the unit ("L MAX") varied from 8 inches to 11 inches. (Our FDAI is approximately 8 inches long.)

    

    The Apollo FDAI part number chart from Grumman Specification Control Drawing LSC350-301. (Click for a larger view.)

     ↩

11. We examined a different ARU-11/A where the amplifier boards were not quite identical: the boards had one additional capacitor and some of the PCB traces were routed slightly differently. These boards were labeled "REV C" in the PCB copper, so they may have been later boards with a slight modification. ↩

12. The amplifier scaling resistors were placed on the power supply board rather than the amplifier boards, which may seem strange. The advantage of this approach is that it permitted the three amplifier boards to be identical, since the components that differ between the axes were not part of the amplifier boards. This simplified the manufacture and repair of the amplifier boards. ↩

13. On the front panel of our FDAI, the text "ROLL RATE", "PITCH RATE", and "YAW RATE" has been painted over. However, the text is still faintly visible (reversed) on the inside of the panel, as shown below.

    

    The inside of the FDAI's front cover.

     ↩

14. The diagram below shows the internals of the Apollo LM FDAI at a high level. This diagram shows several differences between the LM FDAI and the FDAI that we examined. First, the roll, pitch, and yaw inputs to the LM FDAI are resolver inputs (i.e. sin and cos), rather than the synchro inputs to our FDAI. Second, the needle signals below are modulated on an 800 Hz carrier and are demodulated inside the FDAI. Our FDAI, however, uses positive or negative voltages to drive the needle galvanometers directly. A minor difference is that the diagram below shows the Power Off Flag wired to +28V internally, while our FDAI has the flag wired to connector pins, probably so the flag could be controlled by the simulator.

    

    The diagram of the FDAI in the LM Systems Handbook. Click for a larger image.

     ↩

15. The Space Shuttle instruments were replaced with color LCD screens in the MEDS (Multifunction Electronic Display System) upgrade. This upgrade is discussed in New Displays for the Space Shuttle Cockpit. The Space Shuttle Systems Handbook shows the ADIs on the forward console (pages 263-264) and the aft console (page 275). The physical ADI is compared to the MEDS ADI display in Displays and Controls, Vol. 1 page 119. ↩

16. The diagram below shows the internals of the Shuttle's ADI at a high level. The Shuttle's ADI is more complicated than the Apollo FDAI, even though they have the same indicator ball and needles.

    

    A diagram of the Space Shuttle's ADI. From Space Shuttle Systems Handbook Vol. 1, 1 G&C DISP 1. (Click for a larger image.)

     ↩

17. Multiple photos of the exterior of the Shuttle ADI are available here, from the National Air and Space Museum. There are interior photos of Apollo FDAIs online, but they all appear to be modified for Shuttle simulators. ↩