Thursday, September 10, 2015

SAC enhancement to push 1130 buttons working, reworking SAC driver board to eliminate the unreliable wire connections


Once my button wiring modifications to the 1130 were working properly, I was ready to load the typewriter diagnostics and test out the 1053. Alas, the damned bit 12 went missing again, due to the ratty quality of the connections on my driver board. Fortunately, my new connectors arrived today, thus I swung into action updating the SAC box to use these new more reliable connections. I don't ever want to be stopped by erratic wire contacts again.


I spent a bit of time in the workshop in the early morning while it was cool, trying to sort out the wiring issue with the Prog Start key relays. The pair of relays are wired so they will flip or reverse the connections of the N/C and N/O connections from the Prog Start button. However, I have an open connection somewhere, either in one of the two vampire taps or in the pin insertion onto the resistor panel.

Relay circuit for the Prog Start button
I validated that my vampire taps are working for the two Prog Start lines, although when I measure the lines they have a 1K resistance in series with the +12V, unlike what I see in the machine schematic. Since there had been a local modification by the prior owner to wire additional Prog Start buttons on the peripherals right across the common and NC of the button, I may have to figure out what wiring really exists here before I continue.

An additional issue arose after I ensured I had solid connections to the two vampire taps - the machine is now in permanent Reset mode, which might be a poor tap on the third line. I will do some tracing of this at the same time as I sort out what is going on with the Prog Start button wiring. To do this, I will have to open and tilt up the keyboard/main control plate that holds the various buttons.

The wiring of the Prog Start button appears normal, thus I must have some other issue causing the problems I am facing. Sadly, the temperature is already up over 90 at lunchtime, meaning no more power-on work until late tonight. After it cooled, I came out and got everything straightened out.

I put the processor in a one instruction loop, then requested the Program Load function from the PC program. I watched the processor stop, reset, wait long enough for me to have stuffed in eighty words of instructions into core, then started executing at 0x0000.

I decided to add another daughter card with a hex inverter chip, since the current design will trigger interrupts on levels 0 and 1 if the SAC Interface box is disconnected or unpowered. With inverted logic for those inputs, the behavior will be more appropriate. I began wiring that up with quick trips into the workshop-oven.

I also decided after looking at the existing driver board in the SAC Interface Box, that I wanted to build a new board with the solid connectors, rather than wrestle desoldering and cleaning up the existing board. The connectors I chose will give me 30 driven signals per board, so I need a second but smaller board to support another 12 - I get six signals per chip and comfortable room for just five chips on a board using the new connectors.

Since I used IC sockets, I won't have to repurchase the seven DIP chips, just move them over once I solder up the new sockets and connectors on the new board. When the board is done, I need to relocate all the input and output wires from the old board to the ribbon cables hung off the new female connectors. These can then plug into the males on the new board I just built.

I think it will take me into the weekend before this replacement is done, when I can get back to the 1053 debugging with its diagnostics, then run the 1132 printer diagnostics, and return to the virtual or real devices such as the 2501 card reader and paper tape reader/punch devices. 

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