Museum restoration of 8800

[philrrtx]

I finally got the sense switch read problem sorted out. It turned out to be the 74L30 chip that detected all ONEs on A15-A8 (the I/O device address). I didn't have a 74L30 ready at hand, so currently have a 7430 in the socket. The only disadvantage of that I can think of is it presents a higher loading on the address bus. I'll replace it with the low-power chip as soon as we acquire one.

So, everything seemed to be working correctly. I was able to poke in the test routines for the 3P+S I/O card and the program was responding to the sense switches properly. I put the front panel back on and everything went to hell in a handbasket. The 4K memory card at 0000 disappeared, and one started showing up at 16K, which should have been impossible. It seemed almost like the address bus was screwed up, but there have been no changes to any of that wiring, and the LEDs are giving the correct indications.

After some experimenting, I managed to get three of the 4K memory boards working in the 0–12K range, but the fourth card would not come up at 12K. From what I have read in a number of forum postings (other forums, not here), these 88-S4K cards had a lot of problems, and were sensitive to timing problems, so that may be what's causing the behavior I see. Puzzling, however, is that the whole set of four 4K cards used to work and even passed an intensive memory test. I reached my frustration threshold today, and put the mess aside to work on another project for a while. Grrrrrr...!