BigDumbDinosaur wrote:
The trouble was coming from an unsuspected place...The [ZIF] socket turned out to have an intermittent connection in it.
The old ZIF socket did have an intermittent on one pin, but replacing it with a brand new socket didn't cure the problem.
The random crashes are being caused by something else.
After I had replaced the ZIF socket, I discovered I hadn't fixed anything, which sent me into a period of deep thought about what might be going on. Visions of obscure timing issues, excessive bus loading, a flaky 65C816, etc., were dancing in my head. So what I did was remove the ZIF socket, plug the ROM directly into its socket and powered up. Much to my amazement, the unit WOULD NOT RUN, even though it had run before in this configuration. At this point is was clear that there was a hardware issue lurking within.
The next step was to removed the Ø2 oscillator and substitute a much slower one, taking the unit's Ø2 rate from 12.5 MHz down to 4 MHz. Now the unit would boot and
/IRQ and
RDY activity were back to normal.
Reinstalling the faster oscillator would result in the unit booting but then crashing. So it now appeared to be clock speed related.
At this point, I'm stuck in trying to troubleshoot the unit, as none of the parts I used should have any problems with high clock rates. The slowest item is the 55ns EPROM, which I also used in POC V1.1, which can boot at 15 MHz without the SCSI host adapter in place. Grrrrr!
P.S. The thought just occurred to me that this could be a problem being caused by signal voltage levels. I may have a basic design problem related to TTL vs. CMOS voltage differences.