27 Aug
2014
27 Aug
'14
10:58 a.m.
On Tue, Aug 26, 2014 at 11:58 PM, Brent Hilpert <hilpert at cs.ubc.ca> wrote:
This is all addressed in the previous messages, but in short:
- the CPU card is required to be able to 'set' the address LEDs,
- with no memory board you can expect the D0-D7 LEDS to be all 1s
(on), not 0s.
Thanks for condensing that info, very helpful.
So in that case, the DRAM card is clearly not doing what one would expect -
the data LEDs are perma-lit irrespective of DRAM presence.
I have also nailed-down some of the crazy behavior wrt the address
switches.. it all seems to be related to A5. If A5 is on, I can stop/start
the cpu and perform semi-sane stepping operations with ex/ex next. But set
A5 off, and the machine goes out of control.. and so far, it seems that A5
and only A5 has this odd control effect. This smells like a good hint, so
next step is to check my address line wiring work, and A5 in specific, as
well as whatever ICs / components play a role in that signal chain,
FYI, I have tried two different CPUs (well, actually three) and also a
second 8212 on the CPU board. These changes had no effect on behavior that
I could notice.
Another possible clue - the prot/unprot function is totally dead. I don't
know how the prot scheme / circuit works, but it does seem to signal a lack
of communication & control between the DRAM and the CPU/front panel. Any
info on how that circuit works would be helpful - meantime, I'll see what I
can figure out on my own from the schematic.
Thanks again for the assistance, I'm learning this stuff as we go along.