👩🏻‍🍳 👨🏻‍🚒 ❓ PCB from the Saturn-5 rocket - reverse engineering with explanations 🎠 🚭 👩🏽‍💼

Translation of an article from Ken Shirrif's blog

In Apollo's lunar missions, the Saturn-5 rocket was controlled by an advanced on-board computer developed by IBM. The system was assembled from hybrid modules, similar to integrated circuits, but containing separate components. I carried out the reverse development of the printed circuit board from this system and figured out its purpose: in the input / output module of the computer, this board selected the desired data source.

When this board with Saturn 5 came to me, it was partially disassembled and it lacked chips.

In this article I will explain how the board worked - from tiny silicon crystals inside hybrid modules to the circuit board and its connection to the rocket. The first to study itFran Planch at Apollo Saturn V LVDC. A video was made about her on the EEVblog blog . Now it's my turn.

Booster Digital Launch Computer (LVDC) and Booster Data Adapter (LVDA)

Lunar RaceIt began on May 25, 1961, when President Kennedy announced that the United States would send a man to the moon before the end of the decade [Kennedy did not like the lag behind the USSR, and he initially offered Khrushchev a joint mission to the moon, but he refused because of secrecy / approx. transl.]. The mission required a three-stage Saturn-5 rocket, the most powerful of all built at that time. The rocket was directed and controlled by the Launch Vehicle Digital Computer (LVDC) digital launch computer, which put it into orbit around the Earth, and then into the trajectory towards the moon. In an era when most computers ranged from a refrigerator to a room, the LVDC was very compact and weighed only about 40 kg. Its minus was a very low speed - it performed only 12,000 instructions per second.

LVDC «». , . LVDC LVDA , 50- , .

LVDA LVDC , .

LVDC . – ACME ( ). .

LVDC worked in conjunction with the Launch Vehicle Data Adapter (LVDA), which provided input / output for the computer. All communication between the computer and the rocket went through LVDA, which converted the analogue rocket signals and 28 V control signals to serial binary data required by the computer. LVDA had buffers (on glass delay lines ) and control registers for its various functions. LVDA had analog-to-digital converters for reading data from an inertial module with its gyroscopes, and digital-to-analog converters for supplying control signals to missiles. He also processed telemetry signals sent to the Earth, and received commands from the Earth that were destined for the computer. And finally, LVDC was powered by switching power supplies with redundancy from LVDA.

Saturn 5's LVDA was an 80-pound box providing LVDA input / output. He had 21 round connectors for cables to other parts of the rocket.

Since LVDA had so many different functions, it was almost twice as large as LVDC. Below is a diagram of all the schemes squeezed into 80 kg LVDA. It is divided into 2 sections filled with printed circuit boards, or “pages”: the front logic section and the rear logic section (the board from the front section fell into my hands). Filters and power supplies were in the central section. Methanol-based coolant was pumped through the LVDA channels. LVDA was connected to LVDC and other parts of the rocket through 21 round connectors.

LVDA Detailed scheme of work

Diode transistor logic

Logic gates can be created in many ways. For LVDC and LVDA, they used technology such as " diode-transistor logic " (DTL), which allows you to make a gate of diodes and a transistor. This was a more advanced technology compared to the resistor-transistor logic (RTL) used on the Apollo on-board control computer, but it was inferior to the transistor-transistor logic (TTL), which became very popular in the 1970s.

The standard logic gate in LVDC was AND-OR-INVERT (AOI), which implements a logic function such as (A • B + C • D) '. It is called so because it applies the logical function AND to the input data set, then OR, and then changes the result to the opposite. The AOI valve was functional, since it was possible to form elements from it with a different number of inputs, for example, (A • B + C • D • E + F • G • H) '. And although the AOI valve may seem complicated to you, it took only one transistor to implement it, which was important in an era in which you had to save on their quantity.

To understand how the valve works, refer to the following diagram. It shows an AOI valve with four inputs and two AND members. The first is responsible for inputs A and B, the value of which at the moment is 1 (high voltage). A pull-up resistor pulls up the AND value (red, 1). In the bottom AND gate, input C is 0, so current flows through input C, pulling the AND value down (blue, 0). In this way, diodes and a pull-up resistor implement the AND gate. Now let's look at the OR step. The current from the top AND (red) pulls the OR step up (1). Finally, this current turns on the transistor, pulling the output down (blue, 0) and providing inversion. If both AND steps are 0, then the OR step will not be pulled up. Instead, a pull-up resistor will pull the OR value down (0), turning off the transistor,as a result, the output will be pulled up (1).

An AOI gate can be made of more resistors or diodes, providing as many inputs as needed. It could be expected that this valve is implemented on a single chip, however, LVDC used several chips for each valve. Different chips have different combinations of diodes, resistors and transistors, flexibly connected to form the necessary logic gates.

Modular Logic Devices

LVDC and LVDA are created using an interesting hybrid technology called Unit Logic Devices (ULD). Although they looked like integrated circuits, ULD modules contained several components. They used simple silicon crystals, each of which sold only one transistor or two diodes. These crystals, together with thick-film resistors, were mounted on a ceramic substrate with an area of 2 cm2. These modules were a variation of Solid Logic Technology (SLT) used in the popular IBM S / 360 computers. IBM began developing SLT modules in 1961, before integrated circuits were commercially viable, and by 1966 produced 100 million SLT modules per year.

ULD modules were significantly smaller than SLT modules, as seen in the photo, and as a result were better suited for a compact space computer. ULD modules used flat ceramic bags instead of SLT metal cans, and had metal contacts on the top instead of pins. The clips on the circuit boards held the ULD modules and connected to these pins. LVDC and LVDA used more than 50 different types of ULD.

On the right are ULD modules, significantly smaller than SLT modules or more modern DIP ICs (left). The SLT module was 13 mm long, and the ULD module was 8 mm, and was much thinner.

The ULD module contained up to four tiny square silicon crystals. Each of them sold either two diodes or one transistor. The photo below shows the internal components of the module, next to the untouched module. On the left, the paths of the circuit on a ceramic substrate are visible, connected to four tiny square silicon crystals. It looks like a printed circuit board, but keep in mind that the device is actually much smaller than a nail. Thick film resistors were printed on the bottom of the module, so they are not visible.

The INV type ULD is open so that four silicon crystals are visible. The upper right one is a transistor, the other three are double diodes. The module was protected by pink silicone.

The microscopic photo below shows a silicon crystal from the ULD module that implements two diodes. The crystal is very small - sugar grains are shown on the photo for scale. The crystal has three external contacts - copper balls soldered to three circles. Impurities (dark areas) were added to the two lower circles to form the anodes of the two diodes, and the upper circle was a cathode connected to the substrate. Note that this crystal is much simpler than even the simplest integrated circuit.

Composite photo of a diode silicon crystal next to sugar grains

The following diagram shows a diagram inside an INV module. The left side forms an AOI gate with one input. A single-input valve may seem pointless, however, additional inputs AND can be connected to leg 1, and additional OR valves can be connected to leg 3. The right side forms components that can be used as additional inputs.

Inverter module circuit

The board also uses AND gate modules (types AA and AB). Note that these are not independent gates, but only components that can be connected to the INV chip to provide more inputs AND and OR. These modules are connected flexibly, in various ways, there are no special inputs and outputs. One common option is to use half the AA chip as an AND gate with three inputs. Part of the AB chip can, if necessary, provide two more inputs.

Diagram of AND gates of types AA and AB.

The photo below shows semiconductors (double diodes) inside the valve AA. You can match the components with the circuit above; The most interesting are contacts 1 and 5. Note that the numbering of the contacts does not coincide with the standard circuit for the IC.

AA type ULD opened to reveal four silicon crystals. These are dual diodes with connected cathodes.

PCB circuit diagram

To understand the functions of the board, I spent the tedious job of ringing with a multimeter all the connections between the chips to draw a wiring diagram. However, shortly afterwards, we got into the hands of the LVDA instructions with all the schemes, which is why my attempts to reverse-engineer were redundant. The board forms a multiplexer with 7 inputs, selects one of 7 inputs and saves the received value in a trigger . And for the technology of the 1960s, such a simple action required the creation of an entire board with several chips.

The diagram below shows a simplified diagram of the board. On the left, the board has 7 inputs; six of them are 28 V signals that need to be buffered to receive logical signals, and the seventh is 6 V logical signal. A current is applied to one of the seven lines to select the corresponding input, and then the data is stored in the trigger. When current is applied to “reset multiplexer” and “multiplexer address”, the trigger is reset.

Simplified circuit board operation Full circuit board. Rectangles denote logical elements. NU indicates unused inputs - there are tracks on the board, but the chip is not connected.

Although many logic gates are drawn on the diagram, everything is implemented with just two AOI gates. The yellow valves form one large AOI valve, and the blue ones form the second. Two yellow ORs merge into one. Two gates are implemented on eight chips - two INV chips, four AA and two AB. This demonstrates the flexibility and extensibility of the AOI logic model, as well as the use of a large number of chips by the circuit. In the entire circuit, only two transistors are used - almost all the logic is implemented on diodes.

Buffer scheme

Of the 26 chips on the board, 18 were analog, and were engaged in buffering and processing input signals. 28 V signals were input, and the logic required 6 V. Each input (except No. 7) passed through a “discrete interface circuit” (DIA), which turned the input into a logical signal. The following diagram shows a circuit assembled from chips 321, 322 and 323 (for most of the chips on the board, the designations are in alphabetic code, such as INV, DLD and ED; however, for analog chips, the designations are digital, and, apparently, just the last three digits of spare part number). The photo shows the contents of each of the chips. Since the 321 chip only consists of resistors (bottom), it looks empty from above. Chip 322 consists of one diode, and chip 323 consists of two transistors (there are no crystals in photo 323; these are the same small squares as on 322).

Discrete input circuit type A (DIA). The given connection diagram 322 has an error - two contacts No. 5.

The following diagram gives the general structure of the board. The eight logic chips in the middle are circled in green. Each of the six input buffers consists of three chips (321, 322, and 323). The path of the signal passing through them is shown by blue arrows. There are 35 places for chips on the board, and 26 are used. If you place additional chips in free places, the same board can be used for other purposes.

Board role in LVDA

This board was part of a multiplexer in the LVDA subsystem called the “System Data Sampler”, which selects signals and sends them either to a computer or to Earth for telemetry. SDS consists of a multiplexer that selects one of eight signals, and a serializer-selector that converts 14-bit data into a serial form. The multiplexer has several data sources - the RCA-110 ground computer, which before launch was connected to the rocket; “Command receiving device”, which received computer commands from the ground after launching a rocket; feedback from the "selector", a set of relays 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 located in a tool module. For example, the “control distributor” was a 17-kg box mounted next to the LVDA and connected to it by a thick cable. The signals received from the “command decoder”, a 4-kg box connected to other boxes involved in the reception and transmission of radio signals, were input to the LVDA “command receiving device”. Since the LVDA was connected by cables to many different instrument module devices, it required 21 connectors.

Where in the instrument module were located LVDA, LVDC, command decoder and control distributor.

Board physical structure

The boards in LVDA and LVDC used interesting manufacturing techniques in order to withstand the great acceleration and vibration of the rocket, as well as to cool the elements. The board that fell into my hands was damaged, it did not have fasteners, but the photo below shows a whole module called the “page”. The page frame is made of an alloy of magnesium with lithium - a durable, lightweight material that conducts heat well. The heat from the board went through the frame to the LVDA and LVDC chassis, which was cooled by liquid methanol through the channels drilled into the chassis.

Page with a metal frame.

Each page can accommodate two printed circuit boards, front and back. A printed circuit board has 12 layers - quite a lot for the 1960s (even in the 1970s there were usually 2 layers on commercial printed circuit boards). The page has a connector for 98 contacts - 49 for each of the boards. The boards are connected by 30 legs passing through, at the top of the boards. There are also 18 test contacts at the top of each board — they made it possible to check the boards when they were already installed. IBM then reused this design with “pages” in System / 4 Pi aerospace computers.

The board that came to me was torn from the other board on the page with force. The following photo shows its inverse. Through contacts are visible at the top - they must be connected to another board. Below are visible 49 contacts of the missing board. Part of the insulation is removed from the board, and 12 vias are visible for each ULD module in place. Thanks to them, the chip contact can be connected to any of the 12 layers of the printed circuit board.

Conclusion

This small circuit board illustrates several things related to 1960s computers.

The board does not use integrated circuits, which only appeared at that time, but the technology of hybrid modules. Although it may seem backward, it has become the key to the success of the IBM System / 360 line. It was introduced 56 years ago (April 7, 1964), and it used hybrid SLT modules with AOI logic. Such computers have dominated the market for many years, and the System / 360 architecture is still supported on IBM mainframes.

LVDC and LVDA also served to create the IBM System / 4 line of aerospace computers introduced in 1967. These computers also used the same “pages” and connectors as this board, although they abandoned ULD modules in favor of TTL flat ICs. The System / 4 Pi line then evolved to the space shuttle AP-101S computers.

Finally, the board shows how much technology has improved since the 1960s. Each ULD module contained up to 4 transistors, so even for such a simple circuit as a multiplexer, it was necessary to make a whole board of modules. Today's iPhone processor contains over 8 billion transistors. Surprisingly, such primitive technology was able to bring the rocket to the moon.

PCB from the Saturn-5 rocket - reverse engineering with explanations