On 1/10/2015 4:53 PM, Michael Thompson wrote:
We did more debugging on the PDP-8/I today. The
individual CLA and CMA
instructions work OK, but the combined CLA CMA instruction does not
set all
of the AC bits to 1s. Tracing with a 'scope found that the AC ENABLE
signal
was not active during the combined CLA CMA instruction. Replacing the
M160
in slot E30 fixed the missing AC ENABLE signals and fixed the combined
CLA
CMA instruction. The Instruction Test 1 and Instruction Test 2
diagnostics
ran OK, so the processor is probably working fine. The TC01 DECtape diags
did not run as expected so we need to read the manual before we try it
again next week
Stab in the dark from discussion with Sherman Foy last night. When
instructions don't play nice together, the timing may be off. Apparently
timing on the E at least worked in many cases because the speed of some
of the IC's at the time they were new caused the timing to be different
on old systems.
Another client who was (and is) servicing old systems had him looking at
the problem in systems which were remote and ran for years at a time.
When turned off they frequently would not come up.
Anyway the client actually found that the problem could also be fixed by
the fact that the bypass caps were all old. He replaced the bypass caps
on the timing board, and joy.
I don't know if this will translate to any problem on the I you are
looking for, but he found it by looking at the signals on a logic
analyzer and noticing they were not the same with different timing
boards. In the I the logic is probably on more boards, but older
logic might have more problems if you replaced problem IC's with newer
faster and less delay parts. Using older parts was not a full solution
(to try to get similar timing to the older circuits.)
Just tossing it out as a place to look
Re-read the post. They fixed the CPU. Their problem now is a DECtape
controller.
Johnny