Reverse-engineering precision op amps from a 1969 analog computer

We are restoring a vintage1 computer that CuriousMarc recently obtained. Analog computers were formerly popular for fast scientific computation, but pretty much died out in the 1970s. They are interesting, though, as a completely different computing paradigm from digital computers. In this blog post, I'm going to focus on the op amps used in Marc's analog computer, a Simulators Inc. model 240.

The Model 240 analog computer from Simulators Inc. was a "precision general purpose analog computer" for the desk top, with up to 24 op amps. (This one has 20 op amps.)

What's an analog computer?

An analog computer performs computations using physical, continuously changeable values such as voltages. This is in contrast to a digital computer that uses discrete binary values. Analog computers have a long history including gear mechanisms, slide rules, wheel-and-disk integrators, tide computers, and mechanical gun targeting systems. The "classic" analog computers of the 1950s and 1960s, however, used op amps and integrators to solve differential equations. They were typically programmed by plugging cables into a patch panel, yielding a spaghetti-like tangle of wires.

An analog computer was "programmed" by plugging wires into the patch panel. This panel is from an EAI analog computer at the Computer History Museum.

The big advantage of analog computers was their speed. They computed results almost instantaneously with their components operating in parallel, while digital computers needed to chug away performing calculations, often for a long time. This made analog computers especially useful for real-time simulations. A disadvantage of analog computers is they were only as accurate as their components; if you wanted 4 digits of accuracy, you needed expensive 0.01% accurate resistors. (In contrast, digital computers can be made as accurate as desired simply by using more bits of precision.) Unfortunately for analog computers, digital computers became exponentially faster and more powerful, so by the 1970s there was little reason to use analog computers.

Inside the analog computer

The heart of the analog computer was its operational amplifiers or op amps. Op amps could sum and scale their inputs, providing basic mathematics. But more importantly, integrators were constructed by combining an op amp with a precision capacitor (below). An integrator computed the integral of its input over time by charging the capacitor. This allowed analog computers to solve differential equations. (It may seem strange that integration, a mathematically sophisticated operation, was a basic building block of analog computers, but that's the way the hardware worked out.)

The integrators in the analog computer used large precision capacitors. The adjustable capacitor on top is 10 nanofarads, while the large metal box below is an adjustable 10 microfarad capacitor. These capacitors were designed for very low leakage so the integrated value wouldn't leak away. In front are relays to select the capacitors.

Analog computers used multiple potentiometers (below) to set input values and scaling constants. These potentiometers rotated through 10 turns to provide high accuracy. A voltmeter was used to check the potentiometer values. The voltmeter could also be used to display output values, but more often, outputs were displayed on an oscilloscope, strip chart, or X-Y plotter.

At top, the digital section of the analog computer. The potentiometers are below; some were not installed in this model of the computer. The blank panel in the upper left could hold a digital voltmeter.

Some analog computers included digital components such as gates, flip flops, one-shots, and counters. This functionality supported more complex techniques, such as iterating through a solution space. Marc's computer has some digital logic, accessed through the colorful patch panel shown above.

The photo below shows the computer partially disassembled. The computer is more complex inside than I expected, with many circuit boards. The patch panel has been removed, revealing the grid of contacts behind it. When a cable is plugged into the patch panel, the cable connects to these contacts, wiring up the program. The computer has five modules behind the patch panel; the leftmost module has been removed and is sitting in front of the computer.2 The boards visible at the top of the computer support the digital logic and two analog multipliers. The power supply and circuitry for the front panel are at the bottom.

The analog computer with the sides removed to show the internal circuitry. One module has been removed and placed in front of the computer.

A closeup of a module is shown below, with the patch panel contacts in front. The module's eight circuit boards can be seen at the back. From left to right, the boards are four op amps (4 boards), miscellaneous circuitry (1 board), and a multiplier (3 boards). Multiplication was surprisingly difficult to implement in an analog computer; the three boards implement a single circuit to multiply two values.3

One of the modules. The "fingers" on front contact plugs inserted into the patch panel. Square high-precision (0.01%) resistors are visible behind the fingers.

The op amps

In the above photo, each op amp took up a full board of components. Each board includes an op amp integrated circuit, which raises the question of why so many other components are required. The reason is that analog computers placed heavy demands on op amp performance. In particular, the op amps need to work with signals at DC and at low frequencies, and op amps inconveniently perform poorly in this range, operating better at higher frequencies.

In 1949, a solution to op amp problems at low frequencies was developed: the chopper op amp.4 The idea is that a chopper modulates the input at, say 400 Hz. The op amp happily amplifies this 400-Hz AC signal. A second chopper demodulates the AC output back to DC5, providing much better performance than directly amplifying the DC signal.4 The op amp boards in the analog computer add a chopper circuit to the IC op amp to improve its performance.6

The diagram below shows one of the op amp boards.8 The op amp's single input7 is on the right (separated from all the other connections on the left, to avoid noise). The input is split into three paths. The first path is to the DC chopper amplifier. The signal goes through a low-pass filter (i.e. resistor and capacitor) to extract the DC and low-frequency signal. The chopper itself is pretty simple: a JFET transistor alternately grounds the signal as driven by an external 400 Hz oscillator. This modulated 400 Hz signal is fed to the op amp IC, an Amelco 809 high-performance op amp, introduced in 1967.9 The IC is in a round metal can; this packaging was common back then and helped shield the op amp from noise. Finally, the IC's output goes through a second chopper and filter to demodulate it.

An op amp board from the analog computer with functional groups labeled. Even though the board uses an integrated circuit op amp, many additional circuits are necessary to obtain the performance required.

Next, the second input path is combined with the DC amplifier's output. Most op amps are based around a differential pair, and this board is no exception. In a differential pair, two transistors provide high-gain amplification of the difference between two input signals. This differential pair's inputs are the board's input and the signal from the DC chopper amp so it amplifies both the original input and the DC signal. The two transistors in the differential pair need to be exactly balanced for the op amp to function accurately. In particular, the two transistors need to be kept at the same temperature, so they are fastened together with a metal clip (below).

Critical transistors are held together with metal clips to ensure they stay at the same temperature. The differential pair is on the right, while the transistors on the left buffer the inputs.

The third input path goes to the AC amplifier. The input goes through a high-pass filter (resistor and capacitor) and then a simple transistor buffer. This "feedforward" signal is combined with the output from the differential pair to improve the amplifier's frequency response. At this point, the input has been amplified three different ways to yield good low-frequency and high-frequency performance.

The final stage of the op amp board is an output amplifier to provide high-current output for use by the rest of the computer. This amplifier is implemented with a Class AB amplifier circuit. Individual transistors at the time weren't sufficiently powerful, so it uses two NPN transistors and two PNP transistors to drive the output.

Each op amp board has its input and output wired to the patch panel. On the patch panel below, the op amps (A1 through A4) are shaped like pieces of pie; their inputs are green and outputs are red. The op amps used for integrators are also wired to the integration capacitors.

Detail of the patch panel showing the connections for op amps A1, A3, and A4. The inputs are green and the outputs are red. Initial conditions (IC) are in white. The potentiometer connections are above (yellow).

On the patch panel, each op amp has multiple input plugs with different resistor values for scaling; these are the "10" and "100" numbers above. The photo below shows these high-precision resistors (black cylinders) attached directly to the patch panel contacts. Integrator inputs are controlled by relays (below) and electronic switches so the analog computer can initialize the integration capacitors, run the computation, and then hold the result for analysis.

Resistors (black cylinders) are attached directly to the patch panel contacts. The relays in the middle control the computer's different states: initial constants, operate, and hold. The circuit boards plug into the green connectors at the bottom.

Conclusion

Even though op amp integrated circuits existed in the late 1960s, their performance wasn't good enough for analog computers. Instead, a whole board of components was used for a single op amp, combining the IC op amp with a chopper and other circuitry to yield a high-precision op amp. Although improvements in integrated circuits led to exponential increases in digital computer performance, analog computers received much smaller benefits from ICs. As a result, digital computers almost entirely took over and analog computers are now historical artifacts.

The removable patch panel for the analog computer. The computer was programmed by plugging wires into the holes. The panel is removable, so one programmer could use the analog computer while another is wiring up a panel. (Click to enlarge.)

You might wonder why I'm studying the circuitry of this analog computer in such detail. The reason is that we're trying to restore the computer, but we don't have documentation.1011 Thus, I'm reverse-engineering it to determine how to restore it to operating condition and how to program it. While the circuit boards are not too complex, the computer contains many different boards to analyze. The hardest part is figuring out the connectivity of the many tightly-bundled wiring harnesses, mostly by brute-force beeping out connections with a multimeter.

You can expect more analog computer posts as we continue the restoration. Follow me on Twitter @kenshirriff to stay informed of future articles. I also have an RSS feed.

Notes and references

1. The computer's integrated circuits have 1968 and 1969 date codes on them, so I think the computer was manufactured in 1969. ↩

2. When fully populated, the computer has 6 modules behind the patch panel, but the one on the right is missing. At first, we thought the module had been lost at some point, but it appears that this computer was a lower-cost model and was never fully populated. Evidence of this is that 1/4 of the potentiometers above the patch panel are not installed; these potentiometers would be handled by the missing module. ↩

3. Analog computers could implement arbitrary functions using diode-resistor networks. (Each diode turned on at a particular input voltage level, and contributed a ramp to the output.) For multiplication, diode-resistor networks were configured to implement a parabolic function (i.e. squaring). Multiplication was implemented through the identity X×Y = ((X+Y)² - (X-Y)²)/4. The sum and difference were computed using op amps, while squaring was done with the parabolic function generator. ↩

4. Modern chopper op amps use a more complex chopper-stabilizing mechanism, with two op amps. A secondary op amp uses the chopped signal to null out the main op amp. This tutorial discusses the difference between the classic and modern chopper op amps; there's also a discussion here. The point of this footnote is to avoid confusion between the design of chopper op amps used in the analog computer and modern chopper designs. ↩

5. You can sort of think of the chopper as performing amplitude modulation on the signal, like an AM radio signal. However, the demodulation needs to be "phase-sensitive" so it can tell the difference between a positive input and a negative input. This is in contrast to AM-radio demodulation, which can be done with a diode since phase doesn't matter. ↩

6. The diagram below (from the brochure) shows the structure of the op amp board. The basic idea is that part of the input goes through a capacitor (i.e. high-pass filter) into the AC amplifier. The input also goes into the "DC stabilizer amplifier", which has a chopper on its input. The output is demodulated and put through a low-pass filter (resistor/capacitor). The two amplifier outputs are combined and fed into the "DC amplifier", the output amplifier.

   

   Simplified schematic of the op amp.

   Note the circuitry for overload detection and protection. In an analog computer, overload can easily happen if any of the values get higher than expected and exceed the op amp limits (+/- 10 volts). This is bad because it will cause the results to be wrong. The op amp detects overload and illuminates a panel light so the user knows there is a problem. An important part of analog computer programming is how to scale everything so the mathematical values fit within the physical limits of the system. ↩

7. Nowadays, op amps have a positive and negative input. In analog computers, however, op amps usually had just the negative input. Thus, they summed and inverted their inputs. ↩

8. For reference, I've reverse-engineered the pinout of the op amp board. The input is two shorted pins on the right. The pins along the left of the board (with their connector label) are:
   L: balance in
   K: chopper ground
   J: overload signal out
   H: chopper drive in
   F: ground
   E: ground
   D: -15V
   C: +15V
   B: op amp output
   A: unused ↩

9. Although now almost forgotten, Amelco was an important semiconductor company producing high-performance op amps. Among other things, Amelco made the first JFET op amp. It was founded by Hoerni (who invented the "planar process" for ICs at Fairchild). I reverse-engineered a hybrid Amelco op amp and discuss the history of Amelco in this article. The Amelco 809C op amp datasheet can be found here. ↩

10. As far as documentation on this computer, archive.org has a Simulators Inc 240 brochure scanned from "Ted Nelson's Junk Mail". The Analog Computer Museum has a brochure in German for the Dornier 240, an almost identical computer. (I haven't been able to find out the relationship between Simulators Inc and Dornier, but presumably one company licensed it from the other.) ↩

11. If you're looking for books on analog computers, here are my comments on ones I've read recently:
    Analog computer programming is a modern book on analog computers, and a good place to start.
    Introduction to analog computer programming is a reasonable introduction; the PDF is online.
    Analog and analog/hybrid computer programming comprehensively explains how to solve many different types of problems.
    Electronic analog and hybrid computers has a detailed discussion of the hardware implementations of analog computers of this era.
    Analog and hybrid computing provides a basic description of analog computers and their programming.
    Analog computer techniques is hard to follow and from the vacuum tube era, so I don't recommend it. ↩

Risky line printer music on a vintage IBM mainframe

At the Computer History Museum, we recently obtained card decks for a 50-year-old computer music program. Back then, most computers didn't have sound cards but creative programmers found a way to generate music by using the line printer.2 We were a bit concerned that the program might destroy the printer, but we took the risk of running it on the vintage IBM 1401 mainframe. As you might expect, music from a line printer sounds pretty bad, but the tunes are recognizable and the printer survived unscathed.1

The IBM 1401 business computer was announced in 1959 and went on to become the best-selling computer of the mid-1960s, with more than 10,000 systems in use. A key selling point of the IBM 1401 was its high-speed line printer, the IBM 1403. By rapidly rotating a chain of characters (below), the printer produced output at high speed (10 lines per second) with excellent print quality, said to be the best printing until laser printers were introduced in the 1970s.

The type chain from the IBM 1401's printer. The chain has 48 different characters, repeated five times.

Line printers produced a lot of noise, but programmers soon discovered that by printing specific lines of characters, the noise had specific frequencies. It was possible to play a tune by printing the right lines for each note. Around 1970, computer scientist Ron Mak coded up some songs on punch cards using an earlier music program. He recently came across his old programs and gave us the opportunity to try them out.

How the line printer works

To print characters, the printer uses a chain of type slugs that rotates at high speed in front of the paper, with an inked ribbon between the paper and the chain. The printer produces 132-column output so each of the 132 print columns has a hammer and an electromagnet. At the right moment when the desired character passes the hammer, an electromagnet drives the hammer against the back of the paper, causing the paper and ribbon to hit the type slug, printing the character.

Printing mechanism of the IBM 1401 line printer. From 1401 Reference Manual, p11.

The printer required careful timing to make this process work. The chain spins around rapidly at 7.5 feet per second and every 11.1 µs, a print slug lines up with a hammer. The control circuitry has just enough time to read that position's character from core memory, compare it to the character under the hammer, and fire the hammer if there is a match. After 132 time intervals, each hammer has had an opportunity to print one character; this is called a "scan".3 Since there are 48 characters in the character set (no lower case), this process must be repeated 48 times so all the characters can be printed in any column.54 During each scan, the chain moves by just a single character's width6.

A hammer bank in the IBM 1403 printer. At the bottom, the impact points for the 132 hammers (one for each column) are visible. The coils and wiring for 1/4 (33) of the 132 hammers are visible at the top.

The photo below is a closeup of a hammer. The electromagnet coil and wires are on the upper left. We had to replace this hammer after the coil overheated and smoked; you can see a blackened region on the coil. (This problem happened a while ago due to a bad circuit board, and is unrelated to the printer music.)

An individual hammer from the IBM 1403 printer.

Generating music

Now that you see how the printer works, with a hammer potentially firing every 11.1 µs, the strategy to make music should be clearer. By printing carefully-selected text, you can control the times at which hammers fire. By firing hammers at specific intervals, you can create a desired frequency. An A note (440 Hz), for instance, can be produced by printing a line of text that fires the hammers every 1/440th of a second. This can be done by printing a 1 in column 1 (the first hammer to be aligned), followed by a # in column 14 on the next scan, a comma in column 30 the scan after that, and so forth. (There's no real pattern to this; it's just how things line up.3) The full line printed to generate this note is below.7 (It may be a bit surprising that with a character set of just 48 characters, the printer includes unusual characters such as ⌑ and ‡.)

1    ⌑Y     C#    0   Q     3,    ‡F      R T   4 -   ,   I     U     $7        M   V .   *        9N     ⌑        ZE     @     P3

The diagram below shows the timing of the hammers, illustrating the uniform 440 Hz frequency produced by the above print line. The diagram has time on the X-axis, with a red bar when each character is printed. The red bars are spaced evenly with a spacing of 1/440th of a second, generating a 440 Hz note. Each bar is labeled with the associated character and column on the page. Note that characters are printed in a different order from how they appear on the line. There's no simple relationship between the arrangement of characters on the line and their time sequence. There are a few gray lines where you'd expect a hammer to fire, but no character is printed. These correspond to times when the chain is syncing up and can't print.

Timing diagram for the note A4. Each red line indicates a printed character.

By printing a different line, a different note can be produced. Below is the note B5, which is 987 Hz (over an octave higher). As you'd expect, the higher-frequency note has more characters.

1 @EQ4S J   8. N D ‡  S H 7 AM  Y#2   G-  KV . 0 D  Q S J 7&   N D ‡/4  H   AMX0  2 Q G J   W. 0 DP‡  S   7&AM     ‡/4G   *  MX0 D 3

Timing diagram for the note B5. Each red line indicates a printed character.

The printed line for the low note C♯3 (138 Hz) is below. I was puzzled at first why this line (and the other C♯ notes) had all the characters clustered together, rather than scattered across the line like other notes. It turns out that 138 Hz just happens to correspond to hammers that are consecutive on the line. Even though the characters are clumped together on the line, they are spread out uniformly in time.

16#UZKP*E&38                                                                                                                      

Timing diagram for the note C♯3.

Why chain music might be risky

We were concerned that the print chain music program might damage the printer. There are plenty of stories of people destroying line printers by printing a line that fires all the hammers at once. I think these are mostly urban legends (among other things, the hammers on the 1403 fire one at a time, not all at once). Nonetheless, we were somewhat concerned about chain music overstressing the print chain and breaking it. The photo below shows a print chain that broke during normal use; you can see the broken wires and the individual type slugs.

A broken 1403 print chain. It broke during normal use, not from line printer music. (Photo from TechWorks.)

Print chain were manufactured by winding a thin wire into a band, with type blocks attached. Up until recently, print chains were rare and irreplaceable; if the wire broke, there was no way to fix it. However, the Techworks! museum in Binghamton, NY recently developed a technique to rebuild print chains. Because of this, Frank King (our IBM 1401 guru) approved the use of a rebuilt chain for line printer music, with some trepidation. Fortunately, the chain survived the music generation just fine. (After studying the music program carefully, I think it puts less stress on the chain than the average program, unless there's some really unfortunate resonance.)

Closeup of the type chain (upside down) for an IBM 1403 line printer.

The program

Card decks to play a variety of songs, courtesy of Ron Mak.

The source code to the program is long gone, so I disassembled the machine code on the cards to determine how the program works (listing here). First, it reads "frequency cards" that define what line to print for each note. It builds up an array of print lines in memory, along with a table of note names and addresses of the print lines. Next, the program reads the notes of the song, one note per card. (As you can see above, some songs require many cards.) For each note, it looks up the appropriate print line in the note table. Based on the note's duration, it prints the line the appropriate number of times (using a jump table, not a loop). A rest is implemented by looping 200 to 2000 times to provide silence for the appropriate delay.

A closeup of cards with the machine code for the music program. For some reason, the contents of each card are printed twice on the card.

Machine code for the 1401 is very different from modern machines. One difference is that self-modifying code was very common, while nowadays it is usually frowned upon. For instance, the table of print lines is created by actually modifying load instructions, replacing the address field in the instruction. Even subroutine returns use self-modifying code, putting the return address into a jump instruction at the end of the subroutine. To handle a note, the program generated on-the-fly a sequence of three instructions to load the print line, jump to the print code, and then jump back to the main loop. Self-modifying code made it more challenging for me to understand the program since the disassembled code isn't what actually gets run.

The program cards are followed by frequency cards, defining the print line for each note. The code supported up to 20 different notes, so the frequency cards were selected according to the song's need. Each 132-column line is split across two cards, with the first card defining the right half of the line. Each card is punched at the right with the note name and frequency.

Frequency cards. Each pair of cards defines the 132-character print line that generates the specified note. At the right, the card is punched with the note name (e.g. E4) and frequency (e.g. 329 Hz). The notation F/C labels the first card in the deck.

The final set of cards creates the tune, with one card per note (or rest). Each card is punched with a note and duration. A long song may use hundreds of cards. It is straightforward to create a new song, just a matter of punching the tune onto cards. The notes are specified in Standard Pitch Notation with the note name followed by an octave number. For example, C4 is middle C. Since only some print chains had the # symbol, sharps were indicated with an "S", e.g. CS for C♯.

Closeup of the cards for the song Silver Bells. Each card has the note and octave, followed by its duration. The first card is (confusingly) "END", indicating the end of the frequency cards.

Conclusion

We succeeded in generating music on the IBM 1403 printer, running programs that hadn't been run in almost 50 years. Although the music quality isn't very good, we were happy that the printer didn't self-destruct. Ron Mak last ran these programs in 1970; this link has some songs from then, such as Raindrops keep fallin' on my head. The video below shows an excerpt of La Marseillaise; in this video you can see each line being printed.

I announce my latest blog posts on Twitter, so follow me at @kenshirriff for future articles. I also have an RSS feed. The Computer History Museum in Mountain View runs demonstrations of the IBM 1401 on Wednesdays and Saturdays so if you're in the area you should definitely check it out (schedule). Thanks to Ron Mak for supplying the vintage programs, Carl Claunch for reading the cards, and the 1401 restoration team for running the program, in particular, Robert Garner and Frank King.

Notes and references

1. In case you're wondering why nothing shows up on the printer in the video, the printer's line feed was disabled to save paper. You can see the lines being printed in the video at the end of the article. ↩

2. Programmers also used the 1401 to generate music on an AM radio via RF interference. Running the right instruction sequence generated a particular tone. We hope to try this in the future. ↩

3. I've created an animation of the print chain here that shows exactly how it works; it's more complex than you'd expect. ↩

4. The print chain and hammer alignment scheme may seem excessively complicated. But what makes it clever is that the 11.1 µs between hammer times is just enough time to read a character from core memory to see if it matches the chain slug under the hammer, and thus should be printed. In other words, the system is designed to match the mechanical speed of the chain to the electronic speed of core memory. ↩

5. The printer's operation is explained in detail in the Field Engineering Manual of Instruction. The section starting on page 37 discusses the chain timing in detail. Each scan is broken down into 3 subscans, but I won't get into that here. Note that while a line is 132 characters, printing a line takes about 150 time intervals (1665 µs); the extra time is used to sync the chain position. (This explains why some notes have "missing" characters in the timing plots.) ↩

6. The chain only moves 1/1000 of an inch during the 11.1 µs time., but that is enough to line up the next character and hammer. The trick that makes this work is that the hammer spacing and the chain spacing are very slightly different (a vernier mechanism), so a tiny chain movement causes a much larger change in the alignment position. ↩

7. I've archived the code and full set of frequency cards here for future reference. ↩

Reliable after 50 years: The Apollo Guidance Computer's switching power supplies

We recently restored an Apollo Guidance Computer, the revolutionary computer that helped navigate to the Moon and land on its surface.1 At a time when most computers filled rooms, the Apollo Guidance Computer (AGC) took up just a cubic foot. This blog post discusses the small but complex switching power supplies that helped make the AGC compact enough to fit onboard the spacecraft.

Inside the Apollo Guidance Computer. The power supplies are the tangles of wires on the far left.

The photo above shows the Apollo Guidance Computer after separating its two trays. Tray A on the left holds the logic and interface modules, while Tray B on the right has the memory circuitry. The AGC has two power supplies in Tray A on the far left: a +4V power supply and a +14V power supply; the power supplies look like a tangle of wires in the photo. The logic circuitry, entirely built from NOR gates, was powered by 4 volts. The interface circuitry and memory used the 14 volt supply.

The spacecraft generated 28 volts from fuel cells, which combined hydrogen and oxygen to produce both water and electricity.3 The task of the power supplies was to convert the spacecraft's 28 volts into the 4 and 14 volts required by the computer.2 The 4-volt power supply could output about 10 amps (i.e. 40 watts) while the 14-volt power supply could output about 5 amps (i.e. 70 watts).4 Thus, the power supplies are roughly equivalent to laptop chargers (although a laptop charger deals with more challenging AC line voltages).

The power supply module in front of the AGC. The module in position A30 provides +14 volts, while the (identical) module in position A31 provides +4 volts.

Cordwood construction

The power supplies, like the AGC's other non-logic modules, were built with cordwood construction. In this high-density technique, cylindrical components were inserted into holes in the module, passing through the module, with their leads exiting on either side.6 The left side of the photo below contains resistors, capacitors, and diodes. Because of the cordwood construction, the components are not visible except for the ends of their leads poking through holes. Point-to-point wiring connected the components with welded connections. (The other side of the module is similar, connecting the other ends of the components.) The shiny rectangle on the right is a relay, used to shut off power for standby operation. The ends of large filter capacitors are visible below the relay.

Cordwood construction in the power supply. On the left, components are mounted vertically through the module, with welded wiring on both sides. The metallic box on the right is a relay. Underneath the relay, the ends of filter capacitors are visible.

Cordwood construction was used for high density in applications from aerospace to Cray's CDC 6600 computer. For flight, the AGC's cordwood wiring was encased (potted) in epoxy, protecting it from vibration.

How the power supplies worked

Because the power supplies needed to be lightweight and efficient, they were switching power supplies, an unusual technology for the time. Most computers back then used linear power supplies, which were simpler but much too inefficient for the AGC since excess voltage is turned into waste heat.5 A switching power supply, on the other hand, switches the input voltage on and off at a high frequency. This yields the desired output voltage with very little wasted energy.

The AGC's power supplies used a common switching circuit called a buck converter, which converts an input voltage to a lower voltage. The diagram below shows the key components: a switch (transistor), inductor, diode, and capacitor. The key idea is that if the switch is closed for more time, more of the input voltage will appear across the load. Thus, the output voltage is controlled by the switch timing. The inductor stores energy and releases it when the switch is open, producing a relatively stable output.

A buck converter rapidly switches between the on state and the off state. When on, current flows from the voltage source (V) through the switch and inductor to the load (right). When the switch is open, stored energy in the inductor continues to provide current to the load, through the diode. (Source: Cyril Buttay, CC BY-SA 2.5).

A switching power supply requires a complex control mechanism to switch on and off at the right time. The AGC used a technique called PWM (pulse-width modulation), where power is switched on and off at a fixed frequency (e.g. 20 kilohertz), but changing the fraction of the time the power is on to regulate the voltage.

The schematic below shows the AGC's power supply. (Don't worry about reading the details; click for a larger version.) The buck converter itself (outlined in the lower right) has the expected switching transistor, diode, inductor, and capacitors. However, the power supply has many more components to implement the PWM control circuitry.

Schematic of the AGC's power supply. The main signals are highlighted: 28-volt input (red), 4-volt output (orange), reference voltage (green), comparator output to control the PWM (purple), and PWM output (brown). (source)

To summarize the power supply's operation, 28 volts (red) is supplied at the upper left and filtered. The buck converter in the output circuit (right) reduces the voltage to 4 volts (orange) On the control side (left), the output voltage is used for feedback. A two-transistor comparator (lower left) compares the output voltage with a reference voltage (green) set by a Zener diode and resistor network. The output of the comparator (purple) goes through the PWM control circuit where it modifies the width of the pulses (brown) produced by the PWM circuit. These pulses drive the switching transistor in the buck converter, closing the feedback loop. The computer's clock signal providing timing for the PWM circuit.7

Astronauts interacted with the AGC through the Display/Keyboard (DSKY). The STBY button (lower right) put the computer in standby mode, which was indicated via the STBY light (left). Photo from Virtual AGC.

The power supply also included a standby circuit. By pressing the STBY key on the display/keyboard (DSKY), a relay would disconnect most of the computer's power. This reduced energy consumption when the computer wasn't needed.8

The diagram below shows the top of the power supply module with the major components labeled. Note the large size of the transistors, inductors and filter capacitors compared to the tightly-packed cordwood circuitry on the left. The switching transistor for the buck converter is almost an inch in diameter.

The major components of the AGC's power supply. The components for the buck converter are much larger than the control circuitry.

The transistors of the 1960s were barely able to support switching power supplies since they required a power transistor that could operate at both high speed and high current, which was difficult at the time. (Modern transistors (MOSFETs) are cheap and can handle much higher voltages, leading to ubiquitous low-cost phone and laptop chargers that run off an AC outlet.) The switching transistor required a high-current control signal, which was provided by three drive transistors (in a "complementary Darlington" configuration).

Closeup of transistors in the power supply. The large transistor on the right is the high-current switching transistor. Driving it required the three transistors on the left.

Testing the power supply

We extensively tested the AGC's components before powering up the system. For the power supply, we first checked all the tantalum capacitors since tantalum capacitors are prone to shorting out. We found that the capacitors were all in good shape with the proper capacitances. This is in contrast to modern capacitors, which often leak or fail after a few years. NASA used expensive aerospace-grade capacitors and X-rayed each one to test for faults, and this made a large difference.

Wiring up each power supply for testing (below) was more complex than you might expect. The AGC used two identical power supplies that supplied 4 or 14 volts. The output voltage was selected by backplane wiring that connected different resistors in the feedback resistor network. We reproduced these connections on a breadboard, as well as connecting up the input and output. Some high-wattage resistors (lower right) served as the load.

The setup we used to test the power supply. Connections were made to the pins on the bottom of the module. These pins connected the module to the rest of the AGC. In this view you can see the white wires on the side of the module that connected the circuitry on top of the module to the pins on the bottom.

We powered-up the AGC modules with 28 volts using a current-limited supply to limit potential damage from any faults. We took measurements and found that 4V power supply produced 4.09 volts while the 14V power supply produced 14.02 volts. The quality of the power was good, with about 30mV of ripple. We were somewhat surprised that both power supplies worked flawlessly after 50 years.

Conclusion

The Apollo Guidance Computer used advanced switching power supplies that were lightweight and efficient. While switching power supplies were exotic in the 1960s, improved semiconductors have made them cheap and ubiquitous. Now the switching transistor, a high-precision voltage reference, and the control logic can be combined on a single chip. The modern equivalent of the AGC's power supply is a tiny 5A buck converter for $1.50 on eBay (below). While I wouldn't trust this converter to get to the moon, let alone still work 50 years from now, it illustrates the dramatic improvements in switching power supply technology. (I've written more about the history of switching power supplies.)

A modern 5A buck converter is compact and costs $1.50.

To learn more about our AGC restoration, see Marc's series of AGC videos; the video below shows us testing the power supplies. I announce my latest blog posts on Twitter, so follow me @kenshirriff for future articles. I also have an RSS feed. Thanks to Mike Stewart for photos.

Notes and references

1. The AGC restoration team consists of Mike Stewart (creator of FPGA AGC), Carl Claunch, Marc Verdiell (CuriousMarc on YouTube) and myself. The AGC that we're restoring belongs to a private owner who picked it up at a scrapyard in the 1970s after NASA scrapped it. For simplicity, I refer to the AGC we're restoring as "our AGC". ↩

2. The first version of the Apollo Guidance Computer was known as Block I. The AGC was extensively redesigned to produce the Block II version that was flown. The Block I used +3V and +13V power supplies, while the Block II used +4V and +14V. The Block I power supply is documented here in section 4-8.7. The Block II power supply is documented here in section 4-5.9. ↩

3. The power systems were different between the command/service module and the lunar module. On the command/service module the 28 volts was fed to the different parts of the spacecraft using two buses (Main A and Main B) for redundancy. Main A bus was connected to the A31 power supply module, while the Main B bus was connected to the A30 power supply module (schematic). The two buses were tied together inside the AGC after passing through power rectifiers, so either bus could power the AGC.

   (You may recall from Apollo 13: "Houston, we've had a problem. We've had a Main B Bus undervolt". When the oxygen tank exploded, the voltage from the fuel cells dropped, triggering the low voltage alarm.)

   The lunar module used batteries for its 28 volt supply, rather than fuel cells. Instead of Main Bus A and Main Bus B, the lunar module had a Commander bus (CDR BUS) and a Lunar Module Pilot bus (LMP BUS). The AGC on the lunar module was only connected to the CDR BUS, so there wasn't redundancy. ↩

4. I estimated the wattage of the power supplies by looking at the current-limit feature. The power supplies have two 0.12Ω current-sense resistors. The voltage drop across these resistors will turn on transistor Q13, which will reduce the PWM output and thus the power supply's output voltage. The 4V power supply has the two resistors in parallel (connected by external wiring). Assuming the transistor turns on at 0.6V, this corresponds to a current of 0.6V / 0.06Ω = 10 A. The 14V power supply uses one current-sense resistor, so it will be limited to around 0.6V / 0.12Ω = 5A. ↩

5. Some calculations show the problem with using a linear power supply. The AGC's power supply produced 4 volts at 10 amps, which is 40 watts. A linear power supply would dissipate 24 volts (of the 28 volts) at 10 amps, i.e. 240 watts. The linear power supply would be 14% efficient, wasting 86% of the energy. When you need tanks of liquid hydrogen and oxygen to provide the energy, wasting 86% is unacceptable. In addition, disposing of waste heat on a spacecraft is difficult, so an additional 240 watts would be a problem. ↩

6. In the power supplies, the cordwood components are mounted differently from the other cordwood modules. Most AGC modules had components running from one side to the other as shown below, while components in the power supply went from top to bottom, parallel to the pins. This allowed the use of longer components, in particular, the large filter capacitors.

   ↩

   Most of the cordwood modules, such as this interface module, had components running from side-to-side through the module.

7. One interesting thing about the power supply is that the PWM circuit was driven by the computer's oscillator. But the oscillator was powered by the power supply, raising a chicken-and-egg problem of how the system started up. The solution was that the PWM circuit would self-oscillate at 20 kilohertz if there was no external clock signal, so it would still produce the correct output voltage. Once it provided power to the oscillator module and the oscillator produced a clock signal, the power supply synchronized to this clock signal (50 kilohertz for the 4V supply and 100 kilohertz for the 14V supply). ↩

8. The standby (STBY) key on the DSKY was changed to PRO (proceed) on later versions of the DSKY and the functionality was changed somewhat. ↩

Looking inside a 1970s PROM chip that stores data in microscopic fuses

The MMI 5300 was a memory chip from the early 1970s, storing 1024 bits in tiny fuses.1 Unlike regular RAM chips, this was a PROM (Programmable Read-Only Memory); you programmed it once by blowing fuses and then it held that data permanently. The chip I examined originally cost $70 and was built by MMI (Monolithic Memories Incorporated), a leading PROM manufacturer at the time.

The highly magnified photo below shows the chip's silicon die. The metal layer on top of the silicon is most visible in this photo; the transistors and resistors fabricated from silicon are underneath. The wires around the edges are the 16 bond wires between the silicon die and the external pins. In the upper left, the 1024 bits of data are stored in a 33×33 array of diodes and fuses. (I'll explain the extra row and column below.) This chip is built from NPN transistors, unlike the MOS transistors used in most modern chips.

Die of the MMI 5300 PROM chip, holding 1024 bits of information. Click image for a larger version.

To produce the die photo, I started with the chips below, in their 16-pin ceramic packages; the 5300 and 6300 chips are essentially the same.2 Since the chips were in ceramic packages, I could decap the chip simply by knocking the metal lid off with a chisel, revealing the silicon die.

The MMI 5300 and 6300 PROM chips are in ceramic packages. The chips have 1974 and 1973 date codes.

In the photo below, the silicon die is mounted very off-center in the package. It's unclear if that is intentional or sloppy manufacturing. Tiny bond wires connect the die to the metal contacts of the package.

The MMI 5300 PROM with the lid removed, exposing the die.

Inside the chip

The diagram below shows the main parts of the chip, with the pins labeled. The chip stores 1024 bits as 256 4-bit words. The 8 address lines A0-A7 select one of the 256 words, and the bits are output on pins Out1-Out4. The Program pin is used to store data in the chip by blowing fuses. The Vcc and ground pins power the chip.

Die of the 5300 PROM with components labeled.

The 1024 bits of data are stored in a 33×33 array of diodes and fuses. Note that the data array only takes up about a quarter of the chip; the rest of the chip holds the supporting circuitry. Below the data array, address decode circuitry used the address lines to select one of 32 columns in the array. To the right, DTL multiplexers4 reduced the 32 rows of output to the 4 desired outputs. The output drivers amplified these signals and sent them to the output pins.

The fuses

The chip stored data in tiny fuses. An intact fuse represented a 1, while a blown fuse represented a 0. Thus, the chip was shipped containing all 1's, and the user programmed the chip by blowing fuses where a 0 bit was required. The fuses were fabricated from tiny regions of Nichrome metal that heat up and melt under high voltage. (Nichrome is a nickel-chromium alloy that has much higher resistance than typical metals, causing it to heat up. It is commonly used in applications such as toasters.)

A closeup of the fuses (purple) that store data. Inset circle shows a magnified fuse, showing the tiny horizontal crack indicating the fuse was blown.

The fuses are visible in the die photo above; they are the purple regions between the metal wiring. The fuses are very small, about 8µm long. I expected a blown fuse would vaporize entirely, but instead a blown fuse contains a tiny crack roughly 700 nm wide. (This is the wavelength of red light, so the crack is just barely visible under the microscope.)

Address decoding

The PROM stored 1024 bits as 256 words of 4 bits. However, the bits are physically arranged in a 33×33 grid since a square memory grid is more efficient than a highly-rectangular one. To access the memory, address bits A3-A7 select one of the 32 columns. The selected 32 bits in the column go through the multiplexers at the right, which select one bit out of each group of eight, based on address bits A0-A2. The four selected bits become the four outputs. Thus, addressing has two parts—one to select the column and one to select the four output bits—and they have separate circuitry,

Column selection uses 32 NAND gates, implemented with multiple-emitter transistors.3 (The N and P silicon regions of the transistors are visible as rectangular boxes, with the brownish metal layer on top connecting the regions together.) Each NAND gate has a different combination of address bits A3-A7 either inverted or uninverted, so each address activates a different NAND gate, selecting the associated column. You can see the binary counting in the emitters. The A7' and A7 lines alternate connections every column. The A6'/A6 connections alternate every two columns, while A5'/A5 alternate every four, and so on.

Part of the column address decoder. Each (vertical) transistor decoded a particular address, determined by which address lines are connected to the emitters. There were 32 columns in total.

The data array consists of a diode and fuse for each bit. (Without diodes, the bits would all be shorted together.) If the fuse is present, a low signal on the selected column will pull the corresponding row low through the diode (indicating a 1). If the fuse is blown, the row will remain high (indicating a 0). Thus, by blowing fuses, bits are programmed into the memory array. The diodes themselves are mostly hidden under the metal layer below. The column select lines run vertically in the silicon under the metal layer; the metal stripes on top reduce the resistance.

12 bits in the memory array. Each bit has a diode and fuse between a column select line and a row line.

Programming

To program the PROM, the user melted the necessary fuses one at a time using carefully-controlled high voltage pulses. After selecting the desired address, 27 volts was applied to the programming pin (a much higher voltage than the typical 5V TTL level). After a carefully-timed interval, the desired output was brought to 20 volts for a few microseconds to blow the fuse. (The timing and voltages needed to be precise so the fuse would blow without damaging other parts of the chip.) The process was repeated for each desired 0 bit. Customers used a PROM programmer such as shown below to perform these operations automatically.

The Data I/O PROM programmer. It had sockets of various sizes (turquoise) to hold different sizes of chips. This programmer cost about $6000. Photo: Michael Holley.

For the most part, reading a fuse and blowing a fuse used the same circuitry; the difference was the output pin circuit, which could either output a bit or sink a large current to blow the fuse, depending on the programming mode.5 The transistors along the programming path were larger than regular transistors to handle the larger current. For example, the diagram below compares the multiplexer NPN transistors to a typical NPN transistor on the chip. Also note that the 8 transistors in a multiplexer share a single collector (the multiplexer output).

Eight transistors in a multiplexer (left) compared to a typical NPN transistor (right). The metal layer was removed for this photo. The remaining oxide gives the transistors a colored appearance. N-doped silicon appears darker.

Testing

Before examining this chip, I didn't consider how difficult it was for the manufacturer to test a PROM. Most chips can be extensively tested before shipping to the customer, but you can't test the PROM fuses without irreversibly programming the chip. Even testing the address decode logic is difficult; since the chip is manufactured with all 1's, you'd read the same result even if the address circuitry is broken.

The chip included an extra row and column of fuses for testing.

To support testing, the chip contains an extra row and column of fuses.6 As a result, the storage grid was 33×33, rather than the 32×32 grid you'd expect. Half of the test fuses were missing, so reading the correct pattern of 0's and 1's tested the address decoders. By blowing test fuses and reading the results, the programming characteristics could be tested. The datasheet says that the chips should have a programming yield of over 95%. Compared to most integrated circuits, this is a high failure rate, but it's understandable given the difficulties of testing the chip.

Unused circuitry

One interesting feature of the 5300 die is that it has some transistors that aren't connected to anything else in the chip. The diagram below shows some of the unused transistors in the output circuitry. There are no metal connections from these transistors to the rest of the chip, making them apparently useless.

The output driver circuitry has some unused transistors, marked with arrows.

The reason for these unused transistors is that the same silicon layout could be used for slightly different PROM chips, depending on how the metal layer was wired up. The 5300 PROM had open-collector outputs, but a tri-state version (6301) was also available.7 Both chips used the same silicon die, but had small modifications to the metal layer to wire the transistors differently. Since the 5300 didn't need output transistors to pull the output to 1, these were left unconnected, yielding the unused circuitry seen above.

Conclusion

PROM chips were an important part of early computer systems, often holding the boot code, but their popularity peaked in the early 1980s as they were replaced by erasable PROM (EPROM) chips. EPROM chips had a distinctive quartz window over the die; shining ultraviolet light on the chip erased it, so it could be reused. EPROMs in turn were replaced by electrically-erasable PROMs (EEPROMs), similar to flash memory. To see how much technology has improved, consider that the 6300 PROM chip cost $70 in 1971 and stored 128 bytes. Now you can get a 128 GB flash drive for under $20: a billion times the storage, plus you can write it more than once.

Die with the metal stripped off to expose the silicon. This makes the structure of the transistors visible. The thin remaining oxide layer produces a rainbow effect in some areas.

I announce my latest blog posts on Twitter, so follow me @kenshirriff for future articles. I also have an RSS feed. Thanks to Marc Verdiell for supplying the chip.

Notes and references

1. For more information on the chip, see the MMI Bipolar LSI Databook chapter 2. ↩

2. The 6300 was the commercial 1K PROM, while the 5300 was the same chip specified for the military temperature range (-55 °C to +125 °C). The 6200/5200 was a compatible ROM chip, so you could design your system using the 6300 PROM and then drop in a mask-programmed ROM when everything was working. (A mask-programmed ROM was cheaper, but needed to be manufactured in large quantities.) ↩

3. Multiple-emitter transistors may seem strange, but they are used in the common 7400-series TTL chips. ↩

4. I suspect that diode-transistor logic was used instead of TTL for the output multiplexer because the simpler DTL circuit worked better with the high programming voltages. The DTL circuitry uses very large resistors biased to the programming voltage. They are visible near the right side of the die. ↩

5. The current path for reading and programming bits is as follows. When reading a bit, the output driver circuit was pulled to ground through a multiplexer transistor, the (intact) fuse, the associated diode, and finally the column select transistor. To blow a fuse, the programming pulse disabled the output driver circuits and transistors connected the output pins directly to the multiplexers. Then the high voltage pulse on the output pin followed the same path as before (multiplexer transistor, fuse, diode, column select transistor and ground), but the much higher current blew the fuse, permanently switching the bit to 0. ↩

6. One interesting question is how the test fuses are accessed. There are no test points on the chip; from the circuitry it appears that these fuses are accessed by applying unusual voltages to some of the pins. To access the row of test fuses at the top of the chip, the programming pin is raised to a higher voltage. To read or write the column of test fuses at the left of the chip, address pin A6 is pulled higher. This allows the row select logic, output drivers and programming circuitry to be tested. (These actions are undocumented and based on my reverse engineering.) ↩

7. The motivation for using tri-state or open-collector outputs is that often multiple memory chips were connected together to create a larger memory. The problem was how to combine the outputs. One solution was "open collector" outputs: essentially an output is either 0 or "nothing", and an external resistor pulls the output to 1 if it's not 0. A second solution was tri-state outputs: an output can be 0, 1, or "nothing" if not enabled. Since only one output was enabled at a time, the outputs could be tied together. ↩