PDP-8/I at the RICM

Johnny Billquist bqt at update.uu.se
Sun Jan 11 04:09:37 CST 2015

On 2015-01-11 10:41, jwsmobile wrote:
> 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 


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