PDP-8/I at the RICM

Paul Anderson useddec at gmail.com
Sun Jan 11 04:59:22 CST 2015

Hi Jim,

That's still good info to know.

Thanks, Paul

On Sun, Jan 11, 2015 at 4:52 AM, jwsmobile <jws at jwsss.com> wrote:

> 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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