Tuesday, April 2, 2019

RC 2019/03 finale

With just one weekend left of RetroChallenge 2019, it was becoming clear that I was running out of time. Only extraordinary luck would allow me to complete my KIM-1 simulator for the Apple IIe by the end of the month. Unfortunately, that luck never came. I finished most of the soldering and installed my KIM-2e card (without the 6532 or adapter) into my un-enhanced Apple IIe, only to have the computer not boot. It was clearly related to the card, and after a bit of trouble shooting (removing ICs), I realized that I had pulled the outputs of the KIM's 75LS145 secondary address decoder low instead of high. As a result, the card was always asserting /INHIBIT, leaving the Apple IIe without access to ROM or RAM. Some jumper rearranging fixed the problem, but now I could not get the card to ever inhibit.

The small BASIC program listed below should be putting some data into memory, then activating the card and checking the memory. The retrieved value should be different from the value that was set since there should be nothing on the data bus when my card is activated (because the 6532 is not installed).

Unfortunately, the values are the same. I broke out my cheap USB oscilloscope to see what was going on.

The initial flip-flop is being set properly, and all of the addresses leading to the 74LS138 address decoder are giving good signals, but the decoder is never activating the output to enable the 74LS145 secondary encoder. I tried different ICs and bench tested the chips to make sure they were good, but no luck when installed. Basically, I think either:

  1. Using A12 as a negative enable signal for the decoder was a bad idea, and I am trying to operate out of spec.
  2. The signal is actually correct, but so brief that my oscilloscope is not picking it up, and something else is wrong.

Either way, I had too much going on this weekend to give this my best effort and had to be content with the card not causing my computer to catch on fire. I suppose it is a good start, considering this is my first Apple II card, but I think I have a lot more work to do to get this operating successfully.

No comments:

Post a Comment