Inside a 20-Watt Traveling Wave Tube Amplifier from Apollo

How did the Apollo astronauts communicate on their trip to the Moon, 240,000 miles back to Earth? They used a 32-pound amplifier, built around a special kind of vacuum tube called a traveling-wave tube. In this blog post, I look inside this amplifier and explain how the traveling-wave tube works.

The Collins Radio traveling-wave tube amplifier. The label says "Not for flight" so this amplifier was only used on the ground. Click this photo (or any other) for a larger version.

Surprisingly, this amplifier only produced 20 watts of power, not much more than a handheld walkie-talkie.1 You might wonder how a 20-watt signal could be received all the way from the Moon. To pick up the weak signal, NASA built a network of 26-meter (85-foot) dish antennas that spanned the globe, with ground stations in Spain, Australia, and California. For the signal to the spacecraft, the ground stations broadcast a strong, focused 10,000-watt signal that could be picked up by the spacecraft's small antennas. Additional ground stations with smaller 9-meter (30 foot) antennas filled in coverage gaps, along with tracking ships and airplanes.2

NASA's 26-meter antenna at Honeysuckle Creek, Australia. Photo from NASA.

The communication system on Apollo was very complex, as shown in the diagram below. The amplifier, highlighted in yellow, was just one component of this system (which I'm not going to try to explain here). Most communication went over the "Unified S-Band", which sent voice, data, telemetry, TV, control, and ranging through one unified system. In comparison, the Gemini missions used separate systems for different purposes. (S-band refers to the microwave frequency band used by this system.)

Diagram of the Apollo Block II Telecommunications System. (Click for a larger version.) From "Apollo Logistics Training", courtesy of Spaceaholic.

Inside the amplifier

The amplifier was built by Collins Radio, a company that had a large role in the space program.3 (Collins claims that from Mercury and Gemini to Apollo, every American voice transmitted from space was via Collins Radio equipment.) The photo below shows the amplifier with the cover removed, showing the circuitry inside. Note the tangles of coaxial cables for the high-frequency RF signals. The "Danger High Voltage" warning is due to the thousands of volts required by the traveling-wave tubes.

Inside the amplifier, many coaxial cables connect the RF circuitry.

The block diagram below shows the structure of the amplifier,4 centered on the two traveling-wave tubes that perform the amplification. The amplifier takes two inputs: voice/data and the TV signal. In normal use, one tube amplifies the voice/data signal and the other amplifies the TV signal. An important feature is that either signal can be sent to either tube, or the amplifier can be bypassed entirely. This allows communication if a tube fails, or even if the amplifier entirely fails. The signals are directed by RF relays, electrically-controlled switches. The triplexer sends the two amplified signals to the antenna, and directs the signal from the antenna to the receiver.)

Simplified block diagram of the amplifier. From CSM Functional Integrated System Schematics.

The photo below shows the amplifier with the case removed. (We were unable to disassemble the amplifier completely so this photo is from the documentation.5) The traveling-wave tube is the black cylinder at bottom right, about 10 inches long. The second tube is in the same position on the back of the amplifier.

Photo of the traveling-wave tube amplifier used in Apollo. Photo from Collins S-Band Power Amplifier.

How a traveling-wave tube works

The traveling-wave tube (TWT) is the heart of the amplifier. TWT systems have been popular for satellites because they are compact and provide high amplification with very wide bandwidth.7 They are still widely used in satellites, radar, and other systems.

A traveling-wave tube uses an interesting technique to amplify the input RF signal, different from typical vacuum tubes. It creates a beam of electrons and transfers energy from this beam to the RF signal. In more detail, an electron gun shoots electrons down the tube, (a bit like a CRT). As these electrons travel through the tube, they interact with the RF signal and bunch together, transferring energy to the RF signal.6

The problem is that the electron beam and the RF signal need to travel at approximately the same speed in order to interact, but the electron beam travels at about 10% the speed of light,8 while the RF signal travels at the speed of light. The trick is to put the RF signal through a helix, wrapped around the beam. Because the RF signal travels through the long helix rather than in a straight line, its path through the tube is slowed. With the proper helix design, the RF signal and the electron beam travel at approximately the same net speed down the tube, allowing them to interact.

Diagram of the TWT amplifier. From "Apollo Logistics Training", courtesy of Spaceaholic.

The diagram above shows the components of the traveling wave tube in detail. The heart of the TWT is the drift tube that holds the electron beam, wrapped in the helix for the RF signal. At the left, the electron beam is created by the components of the electron gun (heater, cathode, and electrodes). The RF input and output provide the connections to the helix for the signal that is being amplified. The collector absorbs the weakened electron beam after it has passed through the tube. Finally, the permanent magnets keep the electron beam focused through the tube.

It's hard to see the traveling-wave tube inside the amplifier, since it is mounted at the bottom under a bunch of coaxial cables; the photo below is the best I could do. The tube looks like a black cylinder, but you can see the coaxial cables attached at the left and right.

The traveling-wave tube inside the amplifier.

Other parts of the amplifier

Next, I'll briefly describe the other circuitry inside the amplifier. A traveling wave tube requires high voltage to accelerate the electron beam. The photo below shows two of the power supply transformers. The amplifier was powered with 115 volts AC, 3-phase at 400 cycles per second. It also used 28 volts DC for the control circuitry. Note the circuitry encased in plastic at the bottom of the photo.

The amplifier uses high-voltage transformers to power the traveling-wave tubes.

As described earlier, the RF relays switch signals between the two tubes to provide redundancy. The relays (below) are the fairly large square units with coaxial cables attached. These relays are more complex than typical relays because they must transfer gigahertz RF signals. The internal wiring is constructed from metal strips between double ground planes along with waveguides.

The relays with coaxial cables attached.

Another interesting component of the amplifier is the triplexer, a special RF component that connects the antenna to the amplifier. The idea of the triplexer is that it has three ports, each for a different frequency, and keeps the signals on each port separate from each other. Specifically, it combines the main 2287.5 megahertz signal with the TV's 2272.5 megahertz signal and sends these to the antenna. The signal from the ground is at 2106.4 megahertz; the triplexer directs this signal from the antenna to the receiver. Internally, the triplexer has band-pass filters for each frequency, providing a large amount of isolation (60 dB) between its three ports.

The triplexer.

The triplexer is the metal box in the photo above. Note the coax connections with the antenna connection labeled. Although the triplexer says "Danger High Voltage" on top, this refers to the surrounding power supply circuitry, not the triplexer itself.

Controlling the amplifier

The astronauts had control switches in the Command Module to turn the power amplifier on and off, and switch between the primary and secondary tubes. The diagram below shows the location of these switches, marked PWR AMPL. The PRIM/SEC selects which tube was used for the main signal and which was used for the TV signal. The HIGH/OFF/LOW switch selected the power output level for the amplifier. When the amplifier was off, the input signal was connected directly to the antenna, bypassing the amplifier.

Astronauts controlled the amplifier through switches on the console. Diagram from Command/Service Module Systems Handbook p208.

Conclusion

This power amplifier illustrates the complexity of the communication systems for Apollo.9 Even though the amplifier is complex internally with redundant traveling-wave tubes, it is just one of many pieces of hardware. The diagram below shows the Command Module's equipment bay, with the amplifier highlighted in yellow. (The Apollo Guidance Computer was directly above the amplifier, two rows up.)

Diagram of the Apollo Command Module's equipment bay with the S-band power amplifier highlighted. From Command/Service Module Systems Handbook p212.

We are currently investigating the possibility of powering up this amplifier to see if it still operates. I announce my latest blog posts on Twitter, so follow me @kenshirriff for updates. I also have an RSS feed. Thanks to Steve Jurvetson for loaning me this amplifier. Thanks to Spaceaholic and Mike Stewart for providing diagrams and the Collins Aerospace Museum for additional information.

Notes and references

1. Walkie-talkies typically use 0.5 to 5 watts of power, with some models providing 8 watts, mostly limited by FCC regulations. There are a few 20-watt or even 25-watt handheld radios. ↩

2. This photo shows the Vanguard tracking ship. This ship was a surplus tanker from World War II that was repurposed as a missile tracking ship by covering it with antennas. NASA used the ship for communication with Apollo, and the ship was scrapped in 2013.

   

   The Vanguard ship, from Wikimedia.

    ↩

3. The document Collins S-Band Power Amplifier has technical specifications for the amplifier. The presentation Collins Role in Space Communications describes the Collins equipment used in Mercury, Gemini, and Apollo. Collins built equipment for the spacecraft and transmitting and receiving equipment on the ground. ↩

4. Here's a more detailed diagram of the power amplifier circuitry. This diagram shows the power supply and control circuitry in more detail. In particular, the circuitry lets the tubes heat up for 90 seconds before use. Circuitry also shuts down the power if there is a fault or loss of a power phase.

   

   The power amplifier diagram. From "Apollo Logistics Training", courtesy of Spaceaholic.

    ↩

5. The Collins photo makes it look like you can simply remove the case from the amplifier. However, the photo is misleading since the amplifier doesn't come apart like that. We attempted to remove the amplifier from the case, but it is fastened with many inaccessible screws and some components are glued down. We suspect that the amplifier was assembled inside the case, making it very difficult to perform any pre-launch maintenance. We gave up on disassembling the amplifier completely, which is why all our photos show the view from the top. ↩

6. The interaction between the electron beam and the RF signal in the helix is complex, but the net result is that energy is transferred from the beam to the signal. Specifically, the electric field from the RF signal produces positive and negative waves. These accelerate and decelerate the electrons, causing them to bunch together. (On the whole, the electron beam decelerates more than it accelerates, so it loses energy.) The moving bunches of electrons induce more current in the helix, strengthening the RF signal. The result is a feedback loop, causing the RF signal to grow exponentially as it travels through the tube.

   For more information on how traveling-wave tubes work, see Traveling Wave Tube, Recent theory of traveling-wave tubes, or this long presentation. ↩

7. A traveling-wave tube can amplify a large range of frequencies (i.e. it has a high bandwidth) because it doesn't have any resonant elements (unlike a klystron, for instance). Thus, it doesn't need to be tuned to a particular frequency. ↩

8. Ignoring relativistic effects, the speed of an electron beam accelerated by a voltage is given by

   

   Equation for electron beam speed.

   where v₀ is the velocity, V_b is the voltage, e is the charge of an electron, and m_e is the mass of an electron. For example, applying 6000 volts yields an electron speed of 46,000 km/second, about 15% the speed of light.

   This equation is a rearrangement of the kinetic energy from the velocity and the energy from the voltage potential difference.

   

   Kinetic energy equation.

   In the traveling-wave tube, the electron beam must be slightly faster than the (net) RF signal speed so the beam will transfer energy to the RF signal as the beam is slowed.

    ↩

9. For more information on Apollo communication, see Apollo Experience Report - S-Band System Signal Design and Analysis. See also CSM Functional Integrated System Schematics and Command/Service Module Systems Handbook. ↩

"Space age electronics": Inside a GE thin-film paperweight from the 1960s

In the early 1960s, General Electric developed a technology called thin-film electronics.1 These circuits were built from thin films of material, much more compact than individual components. For weight-sensitive applications such as satellites and military equipment, thin-film electronics could potentially be revolutionary.

The GE paperweight consists of circuitry and a satellite model encased in thick clear plastic. It is labeled "Light Military Electronics Department, Defense Electronics Division, General Electric. Space Age Electronics, thin film circuits."

GE's Light Military Electronics department1 built the paperweight above to showcase their "Space Age Electronics". In the center is a thin-film circuit, next to a model of an early satellite. However, the paperweight contained a surprise: when picked up, the paperweight emitted a beep-beep-beep noise, sounding just like a satellite.2 In this blog post, I reverse-engineer the "Space Age Electronics" inside this paperweight and explain how it works. In brief, the visible thin-film circuit implements a flip flop. The remaining circuitry is hidden in the compartment on the left: two oscillators that produce the beeps. These oscillators are implemented in another unusual 1960s technique called "cordwood'.

The thin-film module

The most visible part of the paperweight is the thin-film module. The idea behind thin film is to build resistors and capacitors as thin layers on a substrate, rather than using individual components. Resistors are formed from thin strips of resistive material, the vertical reddish-brown lines on the module's surface. For higher resistance, these lines zig-zag back and forth.3 Capacitors are formed from two thin layers of metal (the plates), separated by an insulating dielectric material.

This angle view shows how the semiconductor components are mounted above the thin film circuitry.

Thin-film transistors were not commercially practical in the 1960s, so the module has tiny discrete transistors and diodes mounted on top, connected by golden wires. (This must have been expensive to manufacture.) In the photo above, the shadows show that the semiconductor components (black blobs) are slightly above the surface. You can distinguish the diodes by their green dots. At the left, five metallic strips provide power and signal connections to the module, with golden contacts connecting these strips to the thin-film circuitry.

A closeup of the thin-film module.

Interest in thin-film technology declined in the mid-1960s as integrated circuits became commercially available. Integrated circuits were cheaper, could fit more components into a chip, and could be mass-produced. For these reasons, integrated circuits took over the electronics market. Thin-film circuits are still used, but only for specialized applications.

I traced out the paperweight's thin-film circuit and found that it implements a toggle flip flop, a standard electronic circuit. The flip flop stores either a 1 state or a 0 state, like a single bit of memory. When it gets a negative pulse on the trigger input, it flips to the opposite state. Thus, as it receives input pulses, it goes "on", "off", "on", "off", etc. In the paperweight, the flip flop creates the separate beeps. The paperweight generates a beep while the flip flop is on, and is silent when the flip flop is off.

Schematic of the circuit in the thin-film module.

You can match up the components in the schematic with the components in the photo: two transistors, two diodes, four capacitors, and multiple resistors. Note that the two sides of the circuit are symmetrical, both in the schematic and in the photo. One side of the circuit is on and one side is off. Depending on which side is on, the circuit holds a 0 or a 1. See the footnote4 for more details.

Inside the paperweight

The left side of the paperweight has a compartment with some interesting circuitry inside. The paperweight was powered by a 22½ V battery, which was relatively common back then but is now obsolete. It looks a bit like a 9-volt battery, except it has one contact at each end. Next to the battery is a vintage earphone, the round pink component. It acts as the speaker in this device.

Looking inside the paperweight's compartment reveals more circuitry.

Another unusual component is the tilt switch in the lower right, which turns the paperweight on and off. (I don't know if this tilt switch contains mercury or has a rolling ball inside.) When the paperweight is horizontal, the tilt switch is open. But if the paperweight is picked up, the tilt switch closes. This probably added to the "drama" of the paperweight, since someone will think it is just a decoration until they pick it up and it starts beeping.

The tilt switch turns the paperweight on and off.

In the upper right of the compartment, a block of plastic encases the oscillator circuitry. The module is built with "cordwood" construction, a way of building high-density circuits that was popular in the 1960s. Instead of mounting components flat on a circuit board, cordwood puts components between two boards. (They are stacked together like logs, giving cordwood its name.) The photo below shows the components; it isn't as clear as I'd like because the components are embedded in yellowing plastic.

This view of the module shows three resistors (striped) and two capacitors (silver).

On each side of the module, components are wired with point-to-point wiring, as shown below. This photo also shows how the insulated connection wires are also embedded in the module. The large dark circles are the two transistors.

Closeup of the cordwood module, showing the wiring. The transistors and the ends of the resistors and capacitors are visible.

The oscillators use unijunction transistors, a somewhat unusual type of transistor, different from standard NPN and PNP transistors. Oscillators could be easily created from unijunction transistors due to their nonlinear characteristics. The unijunction transistor was invented by General Electric in 1953, so it's not surprising that General Electric made use of them in this paperweight. The GE logo is visible on top of the transistors.

In this view of the module, the script "GE" logo is visible on top of the transistors. These transistors are part number 2N491

The cordwood block holds two oscillators, to control the duration of each beep and to generate the beep sound itself. The first oscillator generates five pulses per second. These pulses go to the thin-film flip-flop circuit, which will change its state between off and on with each pulse. That is, the flip flop is off for 200 milliseconds, on for 200 milliseconds, and so forth. The output from the flip flop powers the second transistor oscillator, which generates a 3.5-kilohertz tone. The result is the repeating beep-beep-beep output from the paperweight.

Schematic of the unijunction transistor oscillators.

The schematic above shows the two oscillators. The idea behind a unijunction transistor oscillator is that the capacitor slowly charges through the resistor. As the capacitor charges, the voltage on the emitter (symbolized by the arrow) increases. When it reaches the trigger voltage, the transistor turns on and the capacitor discharges to ground. The cycle repeats, generating a sequence of pulses on the output.

Conclusion

I think the paperweight is from approximately 1962, based on GE's thin-film research at the time and the appearance of the paperweight's model satellite.6 The paperweight was produced in the midst of the space race; John Glenn became the first American in orbit in 1962. Satellites were still a new "space-age" thing at the time, so the paperweight was a symbol of General Electric's advanced technology.5 The beeps from the paperweight are similar to those produced by Sputnik (1957). At the time, the paperweight must have been an impressive object, a vision of the future.

Thanks to Peter B. Newman, technology collector and educator for sending me the paperweight for analysis. Thanks to Mikes Electric Stuff for identifying the tilt switch for me.

I announce my latest blog posts on Twitter, so follow me @kenshirriff. I also have an RSS feed.

Notes and references

1. The paperweight was built by GE's Light Military Electronics department. In the early 1960s, this department produced aerospace electronics such as digital guidance computers, flight-control systems, and satellite sensors. These were used in weapons including the F-105 fighter-bomber, Sidewinder missile, and Polaris and Atlas ICBMs. ↩↩

2. Sputnik's beeps were approximately 150-300 ms long at 1.5 kilohertz. (The frequency isn't well-defined because the transmission was just a carrier switched on and off, but this is the frequency in typical recordings.) The paperweight's beeps were approximately 200 ms long at 3.47 kilohertz. The point is that the paperweight's beeps were designed to resemble the beeps from a satellite such as Sputnik, and people would have recognized this at the time. You can hear the beeps of the paperweight here; I had to edit the audio a bit because I discovered too late that the doorbell rang in the middle of the recording. ↩

3. In the module, some of the resistors are connected to the metal layer through structures that have teeth kind of like a comb. I'm not sure what the purpose of these structures is. My hypothesis is that by changing the number of "teeth", the active length of the resistor can be changed, adjusting the resistor. (Modifying the metal layer is easier than modifying the thin-film layers.) ↩

4. The two transistors are cross-connected, so when one transistor is on, it forces the other one off. The trigger capacitors are pre-charged through the corresponding output. The result is that the transistor that is currently on (output low) will be pulled lower than the transistor that is currently off (output high). This turns off the first transistor, flipping the state of the circuit. It's a fairly standard flip-flop circuit; more details are here. ↩

5. In 1960, GE hoped to build a commercial communications satellite network, and formed a subsidiary "Communication Satellites Inc" in 1960. However, GE abandoned that goal in 1961 (probably due to antitrust issues) to focus on manufacturing equipment for space vehicles. ↩

6. The satellite in the paperweight resembles the Ariel 1 (1962) and Ariel 2 (1964) satellites, with its paddle-like solar cells. It's not an exact match, so I don't know if the satellite is an artist's conception, or is a different satellite. If you recognize the satellite, please let me know. ↩

Inside a transistorized shift register box, built in 1965 for Apollo testing

One of the under-appreciated aspects of the Apollo launches to the Moon is how much testing was required. I recently came across an item that was part of this testing: the Computer Buffer Unit. It is essentially a 16-bit shift register that interfaced test equipment to the Apollo Guidance Computer. While a shift register is a trivial circuit nowadays, back then it took a box full of transistors that weighed about 5 pounds. In this blog post, I look inside this unit, describe its unusual packaging and circuitry, and explain how it works.

The Computer Buffer Unit is a 4"×6"×6" box. The three electrical connectors on the left are covered by protective covers. It has a humidity indicator and pressurization valve at the bottom.

Testing for the Apollo missions

The Apollo spacecraft required extensive testing even while it was sitting on the launch pad. Thousands of different spacecraft components needed to be activated and analyzed for various tests. Since the control room was miles away from the launch pad, it wasn't practical to run separate wires to each component. Instead, NASA invented (and patented) a complex digital test system that communicated efficiently between the control room and the rocket. This test system sent digital commands to the launch site, where racks of control and interface units were wired to the spacecraft components. These units decoded the commands and performed the specified operation. Massive quantities of measurement data from the spacecraft were encoded digitally and serialized for communication back to the control room.

The complexity of testing is illustrated by the control room below.2 This is not Mission Control, but a separate control room specifically for testing, called ACE-S/C (Acceptance Checkout Equipment-Spacecraft). These consoles were crammed with control switches, tape readers, CRT displays, chart recorders, and status panels for conducting tests and recording results. The ACE-S/C system supported manual, semiautomatic, and automatic testing, driven by two minicomputers1.

ACE control room. From Applicability of Apollo Checkout Equipment.

All parts of the spacecraft were tested, including the fuel cells, cryogenic fuel storage, communications, and environmental control. For this blog post, the relevant subsystem is "Guidance and Navigation", responsible for determining the Apollo spacecraft's position in space using inertial navigation and guiding it on the proper trajectory including the landing on the Moon's surface. The key to Guidance and Navigation was the Apollo Guidance Computer, 70-pound computers onboard the Lunar Module and the Command Module.

The Apollo Guidance Computer that we restored, next to a replica DSKY.

In space, astronauts operated the Apollo Guidance Computer through the Display/Keyboard (DSKY), a box (above) with keys, indicator lights, and numeric displays. But for ground testing, there needed to be a way to feed commands into the Apollo Guidance Computer from the testing system. The solution was the Computer Buffer Unit, the box that I'm examining. To operate the Apollo Guidance Computer remotely, the ACE test system encoded each DSKY keypress as a 16-bit command3 and sent it to the Buffer Unit. The Buffer Unit converted the message to serial, transferring one bit at a time to the Apollo Guidance Computer, which then processed the desired keypress.4 Thus, the Apollo Guidance Computer could be controlled remotely for testing, providing control over the Guidance and Navigation system, and the Computer Buffer Unit was the interface with the Apollo Guidance Computer.

Inside the Computer Buffer Unit

Next, I'll discuss the physical construction of the Computer Buffer Unit. Removing the lid reveals the components inside.5 The main circuitry consists of six horizontal circuit boards wired into a vertical backplane board; the top board is visible below. One unusual feature is the bag of desiccant inside the unit, zip-tied to the right side of the case. The designers of the unit were worried about Florida humidity and the risk of corrosion.6 To guard against damp air, the unit has a valve on the front so it can be pressurized with dry nitrogen. On the front of the unit, you can see a humidity sensor that changes color to indicate 10%, 20%, and 30% humidity. If the internal humidity exceeded 30%, the desiccant needed to be replaced, as described by the warning label.

The Buffer Unit with the lid removed.

I removed the circuit boards with some difficulty, as they fit tightly. The photo below shows the stack of six printed circuit boards wired into the vertical backplane. The wires from the connectors are soldered directly to the backplane.

With the circuit boards pulled out of the unit, the wiring to the backplane is visible.

The circuit boards can be opened up like a book to provide access to the inner boards. The boards are not soldered directly to the backplane, but are connected by short, flexible wires, allowing them to swing apart. To prevent short circuits between the boards, they are separated by white sheets of (probably) silicone.

After removing six screws, the boards can be unfolded like a book.

The circuitry is constructed in a very unusual way that I haven't seen before. Instead of mounting components directly on the circuit boards, components are mounted on small boards, each forming a module with a logic gate or two. These smaller modules are then soldered on pins above the main circuit boards, forming two-layer boards. Essentially they built pseudo-integrated-circuits on small boards, and then constructed the circuitry from these modules.

Closeup of logic modules mounted on the circuit board. A blue resistor is visible on the underside of the module.

It is difficult to see the components sandwiched between the main board and the smaller modules, but the side view below shows some of the components. The two boards are connected by the vertical pins. A tiny glass diode is visible towards the left. The longer components are resistors. The shiny metal-can transistors are in the middle of the module and harder to see.

This side view shows a latch module (bottom) attached to the circuit board (top). The diodes, resistors, and transistors of the latch module are visible.

One question is why the circuitry is implemented with small circuit boards attached to the larger circuit board, instead of mounting the components directly on the circuit board. This approach seems overly complex and makes the boards twice as thick. One advantage, though, is that the separate logic modules could be manufactured, testing, and repaired separately, important in an era when semiconductors were less reliable. Second, the main boards and the logic modules are different types of printed circuit boards: four-layer circuit boards with widely-spaced traces versus single-sided but dense boards.

Logic gates

The circuitry is implemented with a logic family called Diode-Transistor Logic (DTL). This type of logic was used in the early 1960s as it only required one (expensive) transistor per gate, using cheaper diodes where possible. As transistor prices dropped, Transistor-Transistor Logic (TTL) became more popular because of its better performance. Nowadays fast, low-power CMOS logic is used in most integrated circuits.

I reverse-engineered the schematic below, which shows a NOR gate from this unit. This gate has two inputs, as well as two outputs (for reasons that will be explained below). If both inputs are low (0), the transistor will be turned off. As a result, the resistors pull the outputs high, producing 1 outputs.

The NOR gate with both inputs low, outputs high.

If an input is high, the circuit behaves as shown below. Current flows from the input pull-up resistor through the diodes and the transistor's base, turning the transistor on. As a result, current flows from the outputs, through the transistor to ground, pulling the outputs low. Thus, the circuit implements a NOR gate: the output is 1 if all inputs are low, and 0 otherwise.

The NOR gate with a high input, outputs high.

The reason for multiple outputs is clever. If you connect the outputs from multiple gates together, this combined output will be pulled low if any output is low (i.e. the transistor is turned on), and otherwise will be pulled high by the resistor.7 This logic is equivalent to an AND gate. Note that the AND gate is implemented "for free" by wiring outputs together, without requiring additional logic; this is called wired-AND. However, you can't use a gate's output in two different wired-AND gates, since everything will be shorted together. Instead, a gate provides multiple outputs that can be wired independently; the diodes keep the outputs isolated from each other.

The board I examined has 5 different types of logic module8, from an inverter with 1 input and 8 outputs to a module with two 2-input, 5-output NOR gates. These modules follow the circuit above, but with different numbers of inputs and outputs.

Implementation of the shift register

The idea behind a shift register is to store multiple bits in a row. Each time a clock signal is activated, the bits are shifted by one position. Shift registers can be used to store data, convert parallel data to serial, or convert serial data to parallel. In this Buffer Unit, the shift register converted a 16-bit parallel value from the test equipment into a serial stream of bits for the Apollo Guidance Computer.9

The board implements four bits of the 16-bit shift register. The schematic below shows the circuitry for a one-bit stage of the shift register. There's a lot going on, but I'll try to explain it. The heart of the stage consists of two latches, which store one bit. A bit is stored by first updating the primary latch, and then the secondary latch. (Each latch consists of two cross-coupled NOR gates, and can hold either a 0 or a 1.) The shift out lines are the outputs from the shift register stage, a regular output and an inverted output.

One stage of the shift register. It can read the bits in parallel, or shift a bit from one stage to the next.

Each shift out line is fed to the shift in lines of the next stage, allowing the bits to be transferred from stage to stage through the shift register. The shift and load control lines, along with the AND gates, select the input to each stage. With shift high, the input will be the shift out from the previous stage. With load high, the input reads the external bit in lines. This allows a 16-bit data word to be read into the shift register in parallel. (I'm not sure what the clear bit function is used for.) After a bit has been loaded into the primary latch, the clock line is activated to load the bit into the secondary latch, completing the shift or load cycle.

An interesting function of the unit is that after loading, the value in the latch is compared to the input value, to make sure that the circuit is operating correctly. If there is a mismatch, a compare AND gate will activate, clearing the match line. (A compare AND gate will activate if the input bit is 1 and the latch bit is 0, or vice versa.) This circuit also detects a fault in the bit input wires. Each bit is provided over two wires: one with the bit value and one with the inverted bit value. If a wire is broken or affected by noise, the comparison will fail.10

This diagram shows the functions of the gates. Note that the circular golden transistors are faintly visible through the circuit boards.

The board above11 contains four of these shift-register stages. The photo above shows how these stages map onto the hardware. The external signals (4 pairs of bit lines) enter at the bottom of the board, and pass through the input inverters. The 8 primary latch NOR gates implement four primary latches. Four secondary latch modules implement the four secondary latches, since each module contains two NOR gates. The clock driver, load driver, and shift driver provide 8 copies of the clock, load, and shift signals for the circuitry. Finally, the two match NOR gates combine the 8 match signals. (Note that since the AND gates are implemented with wired-AND, they don't use additional circuitry and do not appear in this diagram.)

I/O and power

I'll wrap up with a few comments on the I/O and power supply for the Buffer Unit. The unit has three military-style connectors on the front. At the top is a 61-pin connector for receiving the parallel data and control signals from ground equipment. (The pin count is larger than you might expect because each bit uses two wires as discussed earlier. Also, many of the 61 pins are unused.)

The unit has three connectors. The unit receives parallel data from ground equipment through the 61-pin connector at the top. The middle connector communicates the serial data to the Apollo Guidance Computer. The unit is powered with 28 volts through the bottom connector, which has larger pins for the high-current supply.

The middle connector has four pins that provide the serial data stream to the Apollo Guidance Computer. The wiring is a bit unusual. Instead of transmitting data over one serial line, the unit uses two pairs of lines: one to transmit "0" bits and one to transmit "1" bits. To provide electrical isolation between the unit and the Apollo Guidance Computer, these signals are transmitted via two small pulse transformers, shown below. When a pulse is fed into a pulse transformer, a similar pulse is produced on the output. (In modern equipment, an optoisolator provides similar functionality.)

Two pulse transformers on the top circuit board. Each small transformer is about 1 cm in diameter.

The bottom connector on the unit has two thick pins to provide 28 volts to the unit. This view inside the unit shows the power converter, a sealed black box. I believe this is a switching power supply module that converted the 28-volt input into the lower voltage required by the logic circuitry. It also provided electrical isolation from the power supply. The smaller black box on the right is an EMI filter on the power input; the Apollo ground test equipment encountered faults from voltage transients and electrical noise, so they added filtering.

The power supply components are sealed in black plastic.

Conclusion

This Computer Buffer Unit was built in 1965, a time when the industry was shifting from transistors to integrated circuits. This may explain the Unit's unusual construction technique, small circuit-board modules that are like integrated circuits built from discrete components.12 Interestingly, Motorola built a similar Buffer Unit for NASA that used integrated circuits (but was just as large),13 illustrating that transistors and integrated circuits were both viable approaches in 1965.

This box also illustrates the rapid pace of integrated circuit technology since the 1960s. The first commercial MOS integrated circuit was a 20-bit shift register introduced in 1964 and by 1970, Intel was producing a 512-bit shift register. In 1971, Western Digital was selling a UART chip, putting a complete parallel-to-serial and serial-to-parallel communication system onto a chip. Thus, it took 6 years to shrink the complex shift-register box down to a single chip (more or less). Nowadays, this functionality forms a tiny part of a complex chip. Coincidentally, Moore's Law, describing the exponential growth of integrated circuits, was published in 1965, the same year this box was manufactured.

I announce my latest blog posts on Twitter, so follow me @kenshirriff. (The Twitter thread corresponding to this blog post is here.) I also have an RSS feed. Thanks to Steve Jurvetson for letting me examine this artifact. A video tour of his space museum is here. Thanks to Mike Stewart for providing documents and extensive information on this box.

Notes and references

1. The photo below shows the ACE computer room that supported ACE testing. The system was controlled by two 13-bit CDC 160-G minicomputers. Strangely, the CDC 160-G minicomputers were 13-bit computers, with 13-bit addresses, 13-bit registers, and 13-bit arithmetic. The earlier CDC 160 computer was 12 bits, and CDC improved the 160-G model by adding one more bit. The CDC 160 was designed by Seymour Cray, reportedly over a weekend.

   

   "An ACE Station with twin Control Data computers." From Computers in Spaceflight.

    ↩

2. There were about 10 ACE installations for testing at various sites. ACE testing was performed at contractor sites, as well as at the launch pad. ↩

3. To send a DSKY keypress through the testing system, each keypress was encoded as 5 bits as shown below. The 16-bit message consisted of a 1 bit followed by three copies of the 5-bit keypress, with the middle copy inverted. (Sending the keypress in triplicate detected communication errors.)

   

   The encoding of keys when communicating with the Apollo Guidance Computer. From ACE-S/C Operator's Manual.

    ↩

4. The serial protocol used by the Apollo Guidance Computer is a bit unusual compared to modern serial protocols. Instead of a single serial line, it used two pairs of wires: one to receive a 1 bit and one to receive a 0 bit. This worked well with the Apollo Guidance Computer hardware, which included a feature for incrementing and decrementing counters in response to interrupts. In particular, a serial input 0 triggers a SHINC instruction (shift left), while a serial input 1 triggers a SHANC (shift and increment by 1) instruction.

   (The interrupt-triggered counter mechanism worked well except during the Apollo 11 landing, when the power supply for the Apollo Guidance Computer and the power supply for the rendezvous radar had a phase difference. For complex reasons, this resulted in a high rate of interrupts, overloading the Apollo Guidance Computer and causing restarts. This was indicated by the famous 1201 and 1202 program alarms during the landing.)

   

   The K-START (Keyboard - Selections To Actuate Random Testing) panel is used to send commands to the Apollo Guidance Computer. From ACE-S/C Operator's Manual.

   In the ACE testing control room, DSKY keypresses were entered on a panel called K-START (Keyboard - Selections To Actuate Random Testing), shown above. The keyboard corresponds to the keyboard on the DSKY, while it has other switches specific to testing. These key entries could also be recorded on perforated tape and played back at high speed. ↩

5. Another interesting feature of the unit is how it is mounted on a rack. The back of the unit has two Teflon-lined holes. Two "dagger pins" from the rack fit into these holes. On the front, the unit has two small hold-down hooks; a knob on the rack engages with the hook to hold the unit in place. The mounting hooks are type NAS 622, an aerospace standard. The hold-down mechanism is described here.

   

   Back of the Buffer Unit with identifying label and two holes for dagger pins. The labels say "Unit, Computer Buffer Guidance & Navigation. NAA/S & ID Control No. ME901-0271-0002. Stock No. Contract No. M5H3XA-450001. NAA/S & ID Inspection Serial No. Control Data Corporation MFGR Part No. 106068-0002. Mfgr Serial No. 10136SA08185. US Nov 19 1965.

    ↩

6. The document Acceptance Checkout Equipment for the Apollo Spacecraft discusses the corrosion problems encountered by the test equipment due to humidity and insufficient air conditioning. The specifications don't discuss pressurization of the unit, but I'm assuming they used nitrogen based on other items I've studied. ↩

7. One subtlety with the wired-AND gate is that connecting multiple outputs together will result in multiple pull-up resistors in parallel, which may provide too much pull-up current. The solution is that some gates have outputs without pull-up resistors, so each wired-AND output has a single pull-up. The wired-AND isn't entirely free, since the multiple outputs require multiple diodes, but diodes are inexpensive compared to transistors. I should admit that I'm not 100% sure of the circuitry. Since the components are all hidden underneath the module, I had to deduce the circuitry by probing it from above. There were a few inputs that didn't seem to have connectivity; perhaps there are capacitors to make these inputs pulse-based. ↩

8. The board I examined uses the following types of modules:
   2304: 1-in, 8-out inverter
   2309: 3-in, 4 out NOR
   2311: 4-in, 2-out NOR
   2319: 1-in, 4-out inverter
   2314: dual 2-in, 5-out NOR (larger than the other modules) ↩

9. The specifications for the Buffer Unit describe its purpose: "This specification covers the requirements for a Guidance and Navigation Computer Buffer Unit, hereinafter referred to as the G&N buffer. The G&N buffer shall form a part of the Digital Test Command System (DTCS) which is the up-link portion of the Automatic Checkout Equipment (ACE). The ACE will be used as ground support equipment for the Apollo space craft. The G&N buffer shall receive remotely generated digital test commands from the control room via the DTCS and shall store, verify, and shift out G&N data in appropriate format to the G&N on-board computer."

   

   Functional diagram of the Buffer Unit. Image from Specification MC 901-0666 courtesy of Mike Stewart.

   The specifications for the Computer Buffer Unit can be viewed online: MC901-0666, ME901-0666, ME901-0271, ME476-0070.

   The unit includes more functionality than just a shift register (but not much more). As shown in the functional diagram above, the unit also includes the clock oscillator that controls the timing of the serial pulses. Second, it contains a control circuit to handle loading the bits in parallel and then shifting them out serially. Third, for reliability reasons, it has a comparator circuit to check that the bits loaded into the shift register match the input bits. ↩

10. Modern systems often use differential signaling, using two complementary signals for a bit. Looking at the difference between the two signals provides noise immunity, since electrical noise will often affect both signals equally, and thus will be canceled out. Although the Buffer Unit uses two complementary signals, it doesn't provide this noise immunity, since the two signals are processed independently rather than differentially. ↩

11. I only reverse-engineered one of the boards, since I didn't want to risk more disassembly, and one board is enough to understand the basic logic. I studied board 6 of the unit, which implements bits 15 through 18 of the shift register. Board 3 implements bits 3-6, board 4 implements bits 7-10, as well as mode bits 1 and 2, board 5 implements bits 11 through 14, and board 6 (the one I examined) implements bits 15 through 18. Boards 2 and 4 implement control logic, while board 1 has the output driver transformers.

    

    With board 6 folded down, board 5 is visible.

    The photo above shows board 5. Note that the circuit layout is entirely different from board 6. I thought that the unit might consist of four identical 4-bit shift register boards, but it turns out that the boards are optimized for particular roles. ↩

12. In the context of "not-quite-integrated circuits", I should mention IBM's use of hybrid modules (called SLT) for the System/360 mainframes. These small aluminum-cased modules contained a few transistor or diodes as silicon dies, mounted on a ceramic substrate along with thick-film resistors. These modules were not quite integrated circuits, since they were built from discrete (but unpackaged) components. But they were closer to integrated circuits than the modules in the Buffer Unit, which used packaged transistors, resistors, and diodes on a printed circuit board. ↩

13. Motorola made a similar Buffer Unit, but they used integrated circuits, specifically Motorola's line of high-speed ECL chips, introduced in 1962. Since each chip is a few gates, it still took multiple boards to build the unit. Apollo Guidance Computer expert Mike Stewart has photos of the Motorola box here, as well as reverse-engineered schematics. The functionality of the Motorola box is nearly identical, except it has separate inputs for the 16-bit compare value. It is built with chips such as the MC308 flip flop and MC 309 dual NOR gate, described here.

    

    A board from the Motorola version of the Buffer Unit. Each metal can is an integrated circuit. Photo courtesy of Mike Stewart.

     ↩

A circuit board from the Saturn V rocket, reverse-engineered and explained

In the Apollo Moon missions, the Saturn V rocket was guided by an advanced onboard computer system built by IBM. This system was built from hybrid modules, similar to integrated circuits but containing individual components. I reverse-engineered a circuit board from this system and determined its function: Inside the computer's I/O unit, the board selected different data sources for the computer.

A circuit board from the Saturn V LVDA. (Click this image (or any others) for a larger version.) This board was partially disassembled when I received it and some chips are missing.

This post explains how the board worked, from the tiny silicon dies inside its hybrid modules to the board's circuitry and its wiring in the rocket. This board was first studied by Fran Blanch in The Apollo Saturn V LVDC Project. Then EEVblog made a video about it. Now it's my turn to analyze the board.

The Launch Vehicle Digital Computer (LVDC) and Launch Vehicle Data Adapter (LVDA)

The race to the Moon started on May 25, 1961, when President Kennedy stated that America would land a man on the Moon before the end of the decade. This mission required the three-stage Saturn V rocket, the most powerful rocket ever built. The Saturn V was guided and controlled by the Launch Vehicle Digital Computer (below), from liftoff into Earth orbit, and then on a trajectory towards the Moon.1 In an era when most computers ranged from refrigerator-sized to room-filling, the LVDC was very compact and weighed just 80 pounds since it was mounted inside the rocket. The downside was that it was very slow, performing 12,000 instructions a second.

The LVDC mounted in a support frame for testing. Behind the operator is a test system called ACME (Aerospace Computer Manual Exerciser). The ACME paper tape reader is visible at the back. Photo from IBM.

The LVDC worked in conjunction with the Launch Vehicle Data Adapter (LVDA, below), which provided the input/output functions for the computer. All communication between the computer and the rocket went through the LVDA, which converted the rocket's analog signals and 28-volt control signals to the serial binary data the computer required. The LVDA contained buffers (implemented with glass delay lines) and control registers for its various functions. The LVDA had analog-to-digital converters to read data from the inertial measurement unit's gyroscopes and digital-to-analog converters to provide control signals to the rockets. It also processed telemetry signals that were sent to the ground and received ground-based commands for the computer. Finally, power to the LVDC was provided by redundant switching power supplies in the LVDA.

The Saturn V LVDA was a 176-pound box that provided I/O for the LVDA. It had 21 round connectors for cables to other parts of the rocket. From System Description and Component Data.

Because the LVDA had so many different functions, it was almost twice the size of the LVDC computer. The diagram below shows the circuitry crammed into the 176-pound LVDA.2 It had two sections filled with circuit boards called "pages": the front logic section and the back logic section. (The board I examined was from the front logic section.) The power supplies and filters were in the central section. A methanol coolant solution flowed through channels in the LVDA to keep it cool. The LVDA was wired to the LVDC and other parts of the rocket through the 21 round connectors on the ends.

Exploded diagram of the LVDA, from NASA.

Diode-Transistor Logic

There are many different ways to build logic gates. The LVDC and LVDA used a technique called Diode-Transistor Logic (DTL) that builds a gate from diodes and a transistor. This was more advanced than the Resistor-Transistor Logic (RTL) used by the Apollo Guidance Computer, but inferior to Transistor-Transistor Logic (TTL), which became very popular in the 1970s.

The standard logic gate in the LVDC was an AND-OR-INVERT gate3 that implements a logic function such as (A·B + C·D)'. It gets its name because it ANDs together sets of inputs, ORs them, and finally inverts the results. The AND-OR-INVERT gate was powerful because it could be built with many inputs, e.g. (A·B + C·D·E + F·G·H)'. While the AND-OR-INVERT gate may seem complex, it only required one transistor which was important in an era when every transistor counted.

If you want to understand how the gate works internally, look at the diagram below. It shows a four-input AND-OR-INVERT gate with two AND terms. First consider inputs A and B, which are both set to 1 (high). The pull-up resistor4 pulls the AND value high (red, 1). In comparison, in the lower AND gate, input C is 0, so current flows through input C, pulling the AND value low (blue, 0). Thus, the diodes and the pull-up resistor implement an AND gate. Next, look at the OR stage. Current from the top AND (red) pulls the OR stage high (1). Finally, this current turns the transistor on, pulling the output low (blue, 0) and providing the inversion. If both AND stages were 0, the OR stage wouldn't be pulled high. Instead, the pull-down resistor would pull the OR value low (0), turning off the transistor and causing the output to be pulled high (1).

An AND-OR-INVERT gate computing (A·B + C·D)'. Since inputs A and B are both high, the output is pulled low.

An AND-OR-INVERT gate could be built with more resistors or diodes to provide as many inputs as required, potentially many inputs to each AND, and many blocks ORed together. You might expect that AND-OR-INVERT gate would be implemented on a single chip, but the LVDC used multiple chips for each gate, as will be shown below. Different chips had various combinations of diodes, resistors, and transistors that were wired up in flexible ways to form the desired logic gate.

Unit Logic Devices (ULD)

The LVDC and LVDA were built with an interesting hybrid technology called ULD (Unit Logic Devices).5 Although they superficially resembled integrated circuits, ULD modules contained multiple components. They used simple silicon dies, each implementing just one transistor or two diodes. These dies, along with thick-film printed resistors, were mounted on a .3-inch-square ceramic wafer. These modules were a variant of the SLT (Solid Logic Technology) modules used in IBM's popular S/360 series of computers. IBM started developing SLT modules in 1961, before integrated circuits were commercially viable, and by 1966 IBM produced over 100 million SLT modules a year.

ULD modules were considerably smaller than SLT modules, as shown in the photo below, making them more suitable for a compact space computer. ULD modules used flat-pack ceramic packages instead of SLT's metal cans, and had metal contacts on the upper surface instead of pins. Clips on the circuit board held the ULD module in place and connected with these contacts. The LVDC and LVDA used more than 50 different types of ULDs.

ULD modules (right) are smaller than SLT modules or more modern DIP integrated circuits (left). An SLT module was about 0.5" on a side, while a ULD module was 0.3" on a side and much thinner.

Internally, a ULD module contained up to four tiny square silicon dies. Each die implemented either two diodes or one transistor. The photo below shows the internal components of a ULD module, next to an intact ULD module. On the left, the circuit traces are visible on the ceramic wafer, connected to four tiny square silicon dies. While this looks like a printed circuit board, keep in mind that it is much smaller than a fingernail. Thick-film resistors were printed on the underside of the module, so they are not visible.

A ULD of type "INV" opened to show the four silicon dies inside. The upper-right die is a transistor, while the other three dies are dual diodes. The module was protected by pink silicone, which has been removed to show the circuitry. Photo courtesy of Fran Blanche.

The microscope photo below shows a silicon die from a ULD module that implements two diodes. The die is very small; for comparison, grains of sugar are displayed next to the die. The die had three external connections through copper balls soldered to the three circles. The two lower circles were doped (darker regions) to form the anodes of the two diodes, while the upper circle was the cathode, connected to the substrate. Note that this die is much less complex than even a basic integrated circuit.

Photo of a two-diode silicon die next to sugar crystals. This photo is a composite of top-lighting to show the die details, with back-lighting to show the sugar.

The schematic below shows the circuitry inside the "INV" module shown earlier.7 The left side forms an AND-OR-INVERT gate with a single input. A gate with a single input may seem pointless, but additional AND inputs can be attached to pin 1 and additional OR gates can be attached to pin 3. The right side of the schematic provides components that can be used as additional inputs.

Schematic of the "INV" inverter module. Based on Saturn V Guidance Computer, Semiannual Progress Report, page 2-37. Pins 7 and 14 switched from original, which didn't match the actual circuitry.

The board also uses AND gate modules (types "AA" and "AB"), shown below. Keep in mind that these aren't independent gates, but components that can be wired to an INV chip to provide more AND or OR inputs.6 These modules can be wired up in many flexible ways; there are no specific inputs and outputs. One common configuration is to use half of an AA chip as a three-input AND gate. Part of an AB chip can provide two more inputs if needed.

Internal schematics of the type "AA" and type "AB" AND gates. From Laboratory Maintenance Instructions for LVDA, Vol 1.

The photo below shows the semiconductors (dual diodes) inside an AA gate. You can match up the components with the schematic above if you wish; pins 1 and 5, the common pins, are most interesting. Note that the pin numbering does not match the standard IC scheme.

A ULD of type "AA" opened to show the four silicon dies inside. The four dies are dual diodes with the cathodes connected. Original photo courtesy of Fran Blanche.

The board's circuitry

To determine what the board did, I tediously beeped out the connections between chips with a multimeter to create wiring diagrams. (Shortly after I finished, LVDA manuals with schematics turned up8 making my reverse-engineering effort unnecessary.) The board forms a 7-input multiplexer, selecting one of 7 input lines and storing the value in a latch. With 1960s technology, this simple function required a whole board of chips.

The schematic below is a simplified diagram of the board. At the left, the board receives 7 inputs; six of them are 28-volt signals that need to be buffered to generate logic signals, while the seventh is already a 6-volt logic signal. One of the seven select lines is energized to select the corresponding input, which is then stored in the latch.9 (The main simplification is that there are multiple select lines for each input. The full schematic is in the footnotes.10) When the "reset multiplexer" signal and the "multiplexer address" are energized, the latch is reset.

Simplified schematic of the board. It is a multiplexer that selects one of the six inputs and stores the value in the latch.

While the schematic shows many logic gates, it is implemented with just two AND-OR-INVERT gates. The yellow gates form one large AND-OR-INVERT gate, while the blue gates form a second. (The two yellow OR gates merge into one.) The two gates are implemented across eight chips: two chips of type INV, four AA, and two AB. This illustrates the flexibility and expandability of the AND-OR-INVERT logic model, but it also shows that circuits use many chips. Note that there are only two transistors in the logic circuit (one in each INV chip); almost all of the logic is implemented with diodes.

The buffer circuitry

Of the 26 chips on the board, 18 of them were analog chips that buffered and processed the input signals. The inputs were 28-volt signals, while the logic requires 6-volt signals. Each input (except #7) passes through a "Discrete Interface Circuit" that converts the input to a logic signal. The diagram below shows the circuit, built from chips of types 321, 322, and 323.11 The photos show the contents of each chip. Since the 321 chip only consists of resistors (on the underside), the chip appears empty from the top. The 322 chip contains a single diode, while the 323 chip contains two transistors. (The dies are missing from the 323 photo; they are small squares as in the 322.)

Discrete Input Circuit, type A (DIA). The published "322" pinout is wrong, showing two pins 5. From Laboratory Maintenance Instructions for LVDA, Vol 1, Figure A-15. 321 and 322 photos courtesy of Fran Blanche.

The diagram below summarizes the structure of the board. The eight logic chips in the middle are outlined in green. Each of the six input buffers consists of three chips (321, 322, and 323). The signal flow through these chips is shown with the blue arrows. The board has 35 spots for chips, of which 26 were used. By putting chips in the empty locations, the same circuit board could be reused for slightly different functions.13

The circuit board with input paths in blue and logic circuitry in green. Original photo courtesy of Fran Blanche.

The board's role in the LVDA

This board was part of the multiplexer in an LVDA subsystem called the "System Data Sampler" that selects signals and sends them either to the computer or to the ground for telemetry. The System Data Sampler consists of a multiplexer that selects one of eight signals, and the Serializer-Selector that converts the 14-bit data to serial form. The multiplexer has several data sources: the RCA-110 ground computer that was connected to the rocket before launch;14 the "command receiver" that received computer commands from the ground after the rocket had launched; the "control distributor" box that provided various discrete signals;12 "spare discrete inputs"; feedback from the "switch selector", a relay box that the computer used to control the rocket; telemetry from the Digital Data Acquisition System (DDAS); and real-time data.

Physically, many of these data sources were large boxes in the Instrument Unit. For instance, the "control distributor" was a 35-pound box next to the LVDA, connected by a thick cable. The LVDA's "command receiver" input came from the "command decoder", a 7.5-pound box connected to other boxes that provided radio input and output. Because the LVDA was cabled to many different devices in the Instrumentation Unit, it required 21 connectors.

The locations of the LVDA, LVDC, Command Decoder, and Control Distributor in the Instrument Unit. Also shows the electronic assembly (ST-124-M3) that interfaces the inertial measurement unit to the LVDA. From the Saturn V Flight Manual page 7-8.

The board's physical structure

The circuit boards in the LVDA and LVDC used interesting construction techniques to withstand the high accelerations and vibrations of the rocket and to keep the circuitry cool. The board I examined was damaged and missing its mounting frame but the photo below shows an intact unit called a "page". The page's frame is made from a magnesium-lithium alloy that combines light weight, strength, and good heat transfer properties. Heat from a board flowed through the frame to the LVDA or LVDC's chassis, which was liquid-cooled via methanol flowing through channels drilled in the chassis.

A page including the metal frame. This board implemented voting circuitry in the LDVC. Photo from Dmitris Vitoris via Virtual AGC.

Each page could hold two circuit boards, one on the front and one on the back. The printed circuit board has 12 layers, which is a remarkably high number for the 1960s. (Even in the 1970s, commercial PCBs typically had just two layers.) The page has a 98-pin connector, with 49 connections to each PCB. The two boards were connected by 30 "thru pins" at the top of the board. The top of each board also has 18 test connections; these allowed signals to be probed while the boards were installed. (IBM reused this page construction in its System/4 Pi aerospace computers.15)

The board I examined had been forcibly separated from the other board in the page. The photo below shows the back of the board. The thru-pins are visible at the top; they would have been connected to the other board. At the bottom, the 49 connections from the connector to the missing board are visible. Some of the board's insulation has been removed, showing the 12 vias at each ULD module position. These provide a connection from a chip pin to any of the 12 layers of the circuit board.

Back of the LVDA board. A second board was mounted on this side originally, but has been removed.

Conclusion

This small circuit board illustrates several stories about computing in the 1960s.

The board used hybrid modules rather than still-new integrated circuits. While this technology may seem backward, it was a key to IBM's success with the IBM System/360 line. Introduced almost exactly 56 years ago (April 7, 1964), these computers used hybrid SLT modules with AND-OR-INVERT logic. These computers dominated the market for years, and the System/360 architecture is still supported by IBM's mainframes.

The LVDC and LVDA also led to IBM's System/4 Pi line of aerospace computers, announced in 1967. These computers used the same "page" design and connectors as this board, even though they abandoned ULD modules for flat-pack TTL integrated circuits. The System/4 Pi line of computers evolved into the AP-101S computers used on the Space Shuttle.

Finally, the board shows the remarkable improvements in technology since the 1960s. Each ULD module contained up to 4 transistors, so even a basic circuit like a multiplexer took a whole board of modules. Now, an iPhone processor has over 8 billion transistors. It's amazing that such simple technology was enough to get to the Moon.

I announce my latest blog posts on Twitter, so follow me @kenshirriff for future articles. I also have an RSS feed. This work builds on Fran Blanche's Apollo Saturn V LVDC Project. Thanks to Fran for providing photos, Ben Krasnow for passing the board along to me, and Mike Stewart for documentation. For more information on the LVDC, see the Virtual AGC project's LVDC page. I recently wrote about the core memory stack in the Saturn V LVDC.

Notes and references

1. The LVDC was one of several computers onboard the Apollo mission. The better-known Apollo Guidance Computer (AGC) guided the spacecraft to the Moon's surface. (I recently helped restore an Apollo Guidance Computer to running condition.) The Command Module had an AGC while the Lunar Module had a second AGC. The Lunar Module also contained the backup Abort Guidance System computer. The LVDC/LVDA was connected to the Flight Control Computer, a 100-pound analog computer mounted in the Instrument Unit.

   

   Multiple computers were onboard an Apollo mission. The Launch Vehicle Data Adapter (LVDA) is discussed in this blog post.

   The LVDA and LVDC were mounted in the rocket's Instrument Unit, a ring between the rocket stages and the payload, the Apollo spacecraft. The Instrument Unit contained the guidance and control systems for the Saturn V rocket as well as extensive telemetry systems sending hundreds of parameters to the ground.

   

   The Saturn V Instrument Unit under construction. The LVDC (Launch Vehicle Digital Computer) and LVDA (Launch Vehicle Data Adapter) are silver boxes. For scale, note the engineer sitting on the left. Photo from NASA.

    ↩

2. The detailed block diagram of the LVDA below is from the IBM Study Report. (Click the image for a larger version.) This diagram shows that the LVDA has many different functions, registers, and circuits, with many connections to the LVDC (left) and the Instrument Unit (top and bottom). The board I examined is part of the "Digital Input Multiplexer", highlighted in yellow. Note the various data sources feeding into the multiplexer.

   

   Block diagram from IBM Study Report.

    ↩

3. IBM's use of diode-based AND-OR logic goes back to vacuum tube computers from the 1950s. The large 700-series computers primarily used AND-OR diode networks for their logic, with vacuum tubes for amplification instead of transistors. The photo below shows an 8-tube module. Note the large number of diodes (black components with white stripes) in the module below. I think the role of semiconductor diodes is largely ignored in the era of vacuum tube computers. The IBM 709, for instance, used 2000 vacuum tubes and 14,500 diodes in its arithmetic unit.

   

   Tube module from an IBM 700-series computer in the 1950s. Note the many diodes, especially in the lower left.

    ↩

4. One unusual feature of the LVDC's gates is that the pull-up resistor often isn't connected to the positive voltage source, as you'd expect. Instead, it is connected to a clock signal. When the clock is high, the AND gate functions normally, but when the clock is low, the AND gate is disabled. This has two benefits. First, the pull-up acts as an additional input, ANDing the clock into the result. Second, this reduces power consumption, since there is no current through the pull-up resistor when the clock is low. ↩

5. Dr. Wernher von Braun wrote an interesting article about the use of ULD modules for Apollo: Tiny Computers Steer Mightiest Rockets (Popular Science, Oct 1965). ↩

6. The ULD logic chips exist in a liminal space, a transition between individual components and integrated circuits. They are not arbitrary components, but neither are they logic gates with defined functions. Instead, they are sets of components that can be pieced together into gates in flexible ways. ↩

7. While the ULD chips have 14 pins, the numbering doesn't match normal 14-pin integrated circuits. The top contacts are numbered 1 through 7 (left to right), and the bottom contacts are 8 through 14 (left to right). (Note that The Apollo Saturn V LVDC Project does not use the IBM numbering.) In addition, the circuit board can only use 12 of the pins because of the 12 vias at each position; contacts 4 and 11 (the middle ones) are not connected. ↩

8. There is very little documentation available for the LVDC and even less for the LVDA. The Virtual AGC document library is the best source that I found. In particular, the strangely-named "Laboratory Maintenance Instructions for LVDC" volume 1 and volume 2 provide detailed explanations and schematics. The recently-uncovered "Laboratory Maintenance Instructions for LVDA" volume 1 and volume 2 provide similar detail for the LVDA. The System Description and Component Data has photos of the Instrument Unit components and brief descriptions. The Saturn V Flight Manual discusses the LVDC and LVDA at a high level. The IBM Apollo Study Report has more high-level information on the LVDC and LVDA and some nice diagrams. To get more information the LVDC and LVDA, I'll need to visit the US Space and Rocket Center in Huntsville, Alabama, but currently travel is off the table. ↩

9. The latch is a circuit to store a single bit; it is a standard SR NOR latch, built by cross-coupling two NOR gates. ↩

10. The schematic for the board is below. (Click for full-size.) Each box corresponds to a logic element, part of a chip. The top line "A", "I" shows the element type (AND, INVERT) while the bottom line ("A31") shows the chip position on the board. ("NU" indicates "Not Used"; the board is wired with the circuitry but the chip is not installed.) The left side of the schematic is the input buffers, while the right side is the logic.

    

    Schematic of the board. From Laboratory Maintenance Instructions for LVDA, Volume II, page 10-114.

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11. Most of the chips in the LVDA/LVDC have descriptive alphabetic codes such as INV (invert), DLD (delay line driver), or ED (error detector). However, the analog chips on the board have numbers instead: 321, 322, 323, and 324. It looks like instead of coming up with descriptive names for these chips, they just took the last three digits of the part number, e.g. "323" has part number "6000323". I also noticed that on the 6000322 parts, the last "2" has been retouched on the chips; I'm not sure what significance that has. ↩

12. The "discretes", the binary inputs to the LVDA/LVDC, consisted of high-level signals such as "Liftoff", "S-IB Outboard Engine Out", "S-IVB Engine Manual Cutoff", or "S-IB Stage Separation". I was surprised that the hundreds of measurements throughout the rocket are ignored by the computer; it only cares about the major state transitions such as the engine stopping and a stage separating. (As well as the inertial guidance data, which was key to the computer's navigation.) ↩

13. The board has nine empty positions where modules aren't installed, but these positions are wired into the circuitry. The purpose of this is that the same circuit board can be used for multiple functions based on which chips are installed. Specifically, the multiplexer used 13 boards of which 4 were identical to the one I examined, 8 had a few different chips, and 1 was entirely different. The reason for this is that the multiplexer was 14 bits wide, while the inputs were of varying widths. For instance, there were 8 Discrete Input Spares and 10 Telemetry Scanner bits. Thus, some of the boards didn't use some of the inputs and those chips could be omitted, saving a small amount of weight and cost. The diagram below shows the missing chips that can be added.13

    

    The circuit board with the missing chips filled in. The chip with an X could be replaced by the 321 below it. Original photo courtesy of Fran Blanche.

    The board had two unused inputs; to use these, additional 321/322/323 chips were installed. The board also had one input wired up so it could use either a 324 input chip (as in the board I examined) or a 321 input chip. The 321 chip was used for a discrete input that used standard 28-volt signaling, while the 324 chip was used for a signal that was either grounded or floating. The 324 chip included a diode and pull-up resistors. By putting the necessary chip in the appropriate spot, the same PCB could be used for either type of input.

    Two of the boards included an extra logic gate separate from the multiplexer (the INV and AA chips). These gates generated the signals to switch the command input between the RCA-110 mainframe when on the ground, and the radio command decoder after liftoff. In other words, when the umbilical cable pulled out of the Instrument Unit during launch, the signal ("ICS") from the ground computer was lost. Through these two gates, the multiplexer switched the command input from the ground computer to the command decoder, enabling radio commands for the LVDC. ↩↩

14. The RCA-110A computer that communicated with the rocket was in the mobile launch platform, complete with card reader, keypunch, and line printer. In other words, they were moving a whole computer room on the crawler out to the launch pad, with the rocket mounted on top. (In the photo below, the computer room is at the front left of the blue launch platform, under the launcher-umbilical tower.) It communicated with a second RCA-110A computer in the firing room. For details on the mobile launcher and swing arms, see Apollo Maniacs or the book Rocket Ranch. To summarize the wiring, cables went from the RCA-110A computer room near the rocket nozzles, up the tower and across swing arm 7, through the umbilical panel, and to the LVDA. One bit of these signals went to the multiplexer board I examined.

    

    Apollo 11 Saturn V on the mobile platform, July 1, 1969. Swing arm #7 (marked with arrow) is connected to the Instrument Unit and the top of the S-IVB stage. Photo from NASA.

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15. IBM's 4 Pi series aerospace computers in the 1960s used the same mechanical board structure as the LVDC, with two multi-layer boards mounted on a "page" mounted in a metal frame. The 4 Pi boards were also double-width or triple-wide compared to the LVDC boards, using two or three of the same 98-pin connections. (Compare the board below with the board that I examined.) The circuitry was entirely different though; the 4 Pi boards used flat-pack TTL integrated circuits instead of ULD modules. The 4 Pi architectures and instruction sets were also entirely different from the LVDC. These early 4 Pi systems were used in aircraft such as the A-7E, F-111 and space missions such as Skylab. The 4 Pi series led to the AP-101 computer used on the Space Shuttle.

    

    An IBM 4 Pi page. From Technical Description of IBM System 4 Pi Computers (1967).

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