PDP-8/I at the RICM

jwsmobile jws at jwsss.com
Sun Jan 11 03:41:29 CST 2015

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


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