Wednesday, November 28, 2018

PCBs sent to fab house and other work started for continued Apollo Guidance Computer restoration


Building realistic test modules to practice techniques

Mike has ordered the same potting materials, magnet wire and fiberglass boards that are used in the erasable memory module. He is casting sections that match the actual module as accurately as we can, including breaks in the wire at the points we suspect.

This will let us practice milling, dissolving and laser welding before we work on the irreplaceable artifact. We want to figure out tools, speeds, pressures and directions with these test modules, plus practice to be sure we do the real work as well as possible.

We have time scheduled on a 3D tomographic X-ray machine in mid December, which will give us micron level resolution of the actual module interior and help us find the place where the wire has broken. Once we have that information, we can do a couple more practice runs on test modules configured the same way.


Verification of circuits before PCB fabrication

I set up a MAX7219 and some 7-segment displays in order to validate the exact circuit on the PCB, just in case I got a pin wrong or made some other error that would cause the DSKY to malfunction. Basically I wired up a few of my new green 7segment displays (the components to sit on the PCB) and four discrete LEDs.

An Arduino UNO drove it to prove that the digits display properly and the signs work. I cycled through all ten digits on each display and cycled through plus, minus and blank for the sign.  This worked well, plus I saw that my high output (90 mcd) LEDs were moderated while in circuit with the MAX by some 5K resistors. I suspect the MAX chip would maintain the current regardless but this is a safety feature.

Next up, I temporarily wired up four of the small surface mount LEDs and placed them in their approximate relationship, to verify that it looks like a plus or minus sign. These LEDs are very tiny, 0603 size, which makes tack soldering very difficult. I did manage to light one up and felt comfortable with the light it gave off and its shape, so I will consider this test completed.

The last circuit to prove out are the chips driving the various LED lights that are under AGC control, mainly caution and warning lights but also the computer activity light. These sit on an I2C serial chain and sink the current through up to eight channels of LEDs.

I had used a module with these installed in my breadboard DSKY, but out of caution I wired up the same circuit today to test it again, since I was wiring this directly on the PCB instead of on a module and thus could have the pins wrong. This worked fine when tested.

Final check before sending out the design is to step through the schematic and PCB view, ensuring that every net is correctly attached to the right pins of all components:
  • Twenty-three input circuits with six components apiece, plus input from the AGC IN connector and output to the Arduino connector. 
  • Eight relay circuits with five components each, plus input from Keypad connector and output to AGC Out connector
  • Standby power sense relay circuit with four components, input from AGC EXTRA connector and output to Arduino connector
  • Two loop circuits on AGC EXTRA connector
  • CompActy light circuit with eight components and output from Arduino connector
  • Legends and lines lighting circuit, with 36 resistors and 36 surface mount LEDs
  • Caution and warning lights on I2C chain, with 72 components including 28 LEDs
  • First MAX7219 chain with five 7-segment digits for R1, two digits for Noun, and four discrete LEDs for the plus//minus sign, with 18 components but many nets/traces
  • Second MAX7219 chain for R2, Verb and plus/minus sign, another 18 components
  • Third MAX7219 chain for R3, Prog and plus/minus sign, another 18 components
Almost 1200 pins/pads, 400 components and quite a few connectors comprise this board, so checkout was a long careful process. By the end of Wednesday I had this completed.

Submission to fab house to create the PCB

I transmitted the GERBER files and drill list to the PCB fabrication site once I was satisfied it was (probably) correct. It will take about 10 days to produce, plus shipping time from China to here.


Ken finished his design of the driver to hook to the two Core Rope Simulator modules that came with the AGC. It makes use of a Beaglebone controller and a PCB. The PCB was submitted to the fab house yesterday. He is still developing testing plans for proving this out as well as possible before we cable it up to the real AGC next year.

1 comment:

  1. Your attention to detail and methodologies are fantastic - this is great reading material for curious folks with engineering inclinations. Keep up the good work!