Monday, May 27, 2019

Readying the DSKY substitute and preparing to interface to the Apollo Guidance Computer


Reviewing and documenting signals

I spent about a dozen hours poring over different documents and schematics to be sure that I had a complete map of the interfaces between the Apollo Guidance Computer and the rest of the spacecraft. There are 360 pins on connector A51 that provide these connections between the interface modules A25, A26, A27, A28 and A29 inside the computer and the external systems.

Some of the pins are used for power connections, some are spare but the majority are important input and output signals that we may need to connect with in order to create a realistic reenactment of some portion of the Apollo 11 mission. In addition to the signals that operate the DSKY, there are 264 pins that implement 176 input/output signals.

One document I built lists all the pins, which interface module and circuit it connects, and the external system purpose of that signal. Another document lists each interface circuit in the five modules and cross references it to the A51 pin and purpose.

I used those to look for holes in the lists - interface circuits that aren't listed in some document or pins that aren't understood. I am down to two pins on the A51 connector, one of which I know has a thermistor wired to it for analog temperature sensing; likely the other has a similar analog monitoring role.

From these documents I moved on to look for discrepancies. As an example, the schematic for module A26 has different interface circuits hooked to the wires in the spacecraft used to turn the LM Ascent or Descent engines on and off, compared to the other documents I have been using.

Fortunately, the interface circuits are associated with known software locations thus I can look to the Luminary code to determine which set of interface circuits is correct. The engine is either controlled by channel 12, bit 7 or the on and off lines are controlled separately by bits 13 and 14 of channel 11. In fact, this was a late change and the proper signals are controlled by bits 13 and 14 of channel 11.

The control signals for the landing and rendezvous radars were misdocumented in some places but the schematics and LM wiring showed the correct assignments, which I rolled into my master spreadsheets.

Listing needed interface circuits for AGC signals

I also built a spreadsheet listing the AGC pins that get routed over my cable through the 85 pin connector to make use of the DSKY substitute when running the real AGC.


I installed the connectors for the AGC and modern power cables into the acrylic panels and glued together the base upon which the aluminum enclosure will sit. This gives me plenty of room inside for the Electroluminescent wire boost converter and driver circuits, plus room for the cabling between the PCB up inside the aluminum enclosure and the external connectors down in the acrylic section.

New acrylic base and external connectors
I installed wiring for the signals between the AGC and the DSKY that are relevent to me. Some provide 115V 400Hz and other power feeds which aren't necessary with my replica. Others activate relays fit inside the DSKY whose contacts are routed back out to external spacecraft circuits but never affect the DSKY operation. These I didn't implement; if necessary we can wire them externally to the AGC and house the relays elsewhere. 

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