PDP-8/I at the RICM

jwsmobile jws at jwsss.com
Sun Jan 11 04:52:21 CST 2015

On 1/11/2015 2:09 AM, Johnny Billquist wrote:
> 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 
> controller.
>     Johnny
I see that now.

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