AltairClone.com

I am trying to get the front panel to work in my 8800c with a JADE Big-Z Z80.

Run, Stop, and Deposit appear to work. When I press Examine, /WAIT goes low at the Z80 and PWAIT goes high at the bus but /WAIT never goes high again, so the CPU continues to run. The only way to get the CPU to stop is to press RUN then STOP.

I've captured the signals I think are involved. I'm not 100% sure how examine works with the front panel other than having the CPU execute 3 instructions and placing a JMP instruction followed by sense switch addresses on the data bus.
Any assistance on where to look would be appreciated.

Another interesting note is if I put the Big Z in a real 8800, the behavior is different. Examine causes random address to appear on the address LEDs, but the CPU goes back into WAIT, unlike the 8800c where it doesn't go back into WAIT.

The front panel circuitry on the Altair 8800 (and 8800c and IMSAI) ensures the processor always waits (stops) on an M1 cycle. However, instead of looking at the sM1 status bit on the bus, these boards look at DO5 while pSYNC is asserted. DO5 carries the M1 status bit during pSYNC that gets latched into the status register on the CPU board, which, in turn, drives the sM1 bus signal. However, by looking at DO5 directly, the M1 status can be detected sooner. It doesn’t appear to me that the your Jade board puts the status bits on the data out bus during pSYNC like these front panels expect.

Mike

It there a Z80 CPU board you can recommend that will work with the Altair front panel?

I got a TDL ZPU board fixed and working in an Altair not long ago, but even when working, it had a documented quirk or two specific to operation in an Altair/IMSAI. I’d have to look at my notes to recall more.