Monday, October 29, 2018

Major progress wiring and testing the DSKY substitute to use with the Apollo Guidance Computer restoration


Building and testing DSKY substitute

I got back to work on the Register 3 chain (five digits for R3, two digits for Program, and the R3 sign digit). I had some problems including failure of the top segment on all the displays which undoubtedly were due to wiring errors.

It turned out that the problems were solder faults on the new headers. The conductive barrels that run through the driver board PCB had been pulled out during extraction of the old small display modules. The fix was to solder the pins on to both the top and bottom of the PCB. With that restored, the display worked perfectly.

I had some code changes to make to align the digits properly with the displays, as I had assumed the eight digits were addressed from left to right, 0 to 7, but the order ran from least significant digit on the right, as 0, leftward to the high digit as 7. My register thus use 7, 6, 5, 4, and 3 from left to right and my special two digit associated displays, in this case Program, used 1 and 0 from left to right.

Next up was wiring for Register 2 and its Noun display. I learned to drop the decimal point from the wiring as it is not used in the DSKY. I continued on and wired up Register 1 and the Verb display. All that was missing were the two sign displays.

I decided to relocate the driver boards for the 7 segment displays (total of three boards) and make use of a solder version of a breadboard to ensure good connection of the display boards to the wiring. These three small boards will sit beneath the main breadboard. I have all the wires for the seven active segments and the seven display selection lines routed out to the right so they can reach the new driver boards I am soldering up.

After soldering the first 7 segment driver board to a breadboard PCB and soldering all the wires from the displays, I discovered that one of the pins that selects a display is open. I can't repair it where the driver board sits, so I had to make an ugly hack to my wiring and code to substitute the unused display position on the driver for the dead one.

I moved the wire, hacked the code, and had the everything working well. I shall have to proceed cautiously for the next two driver boards, ensuring that I have full connectivity before I solder on the wires from registers 1, 2, Verb and Noun.

Test of Reg 3 and Prog displays after repair on YouTube

I installed a vertical breadboard strip to the left of my existing displays, which will hold the ten caution and warning LEDs. I bought another large breadboard panel onto which I will install the timers and relay modules. The driver boards for the three signs (14 segment displays placed in front of registers 1, 2 and 3) will sit on the existing main breadboard.

Some quick prototyping proved the flashing design. When the KeyRel caution light is on, it must flash at 1.5Hz. When the OprErr caution light is on, it must flash at 1.5Hz. In addition, the AGC can request that the Verb and Noun displays flash at 1.5Hz (these are each 2 digit numbers).

I set up a timer module producing 1.5Hz square waves and routed that to three NAND gates which are controlled by the KeyRel, OprErr and flash-verb-noun signals from the AGC. When the signal from the AGC goes high, it allows the 1.5Hz flashing signal to pass through the gate, setting that gate low 1.5 times per second.

The RelKey and OprErr outputs are connected to one relay apiece, those relays interrupting the cathode connection to the connected LED. The flash-verb-noun output drives four relays, each of them interrupts the select signal (common cathode) for one of the four 7-segment displays that comprise the Verb and Noun output. These relays will activate 1.5 times per second when the flasher square wave is passing through the control NAND gate.

I still have to choose a couple of structural or cosmetic approaches to finish this off. First, I need a mounting mechanism for the twin 4x4 membrane keyboards that serve as the DSKY keyboard. These will sit below the display panel (main breadboard panel). Second, I want to create an overlay panel with the legends for the Caution and Warning lights plus the Comp Activity light. I imagine a clear plastic sheet upon which I will print the legends.

My last action of the night was to install the ten LEDs, in two vertical columns of five signals each, that represent the Caution/Warning panel of the DSKY. The left column has white LEDs, for Caution signals, while the right column has yellow LEDs for Warning alarms.

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