23 Mar
2008
23 Mar
'08
5:10 a.m.
Hi
I recently aquired a complete PDP-8/e. As it had not run for a
while I took the usual precautions of reforming the electrolytics in the
PSU as well as dismantling the whole system and cleaning everything.
I was rewarded with a working system. I started to run some simple
toggle in programs and got to the stage of driving a VT 320 with a
display of whole the character set at 110baud.
I was about to toggle in the next test which would have been a keyboard
echo routine when I discovered I could no longer load anything into
memory.
Load address works but setting the data to load into memory in the
switch register and raising DEP no longer writes the data to memory.
.
RUN works so it looks like the timing plus the RUN/STOP flipflop are
running ok.
However one shot operations like DEP and EXAM are difficult to diagnose
as is hard to see a one off 500nS pulse.
I don't think its the memory itself ( 4 x 4K of core) as I have tried
each 4K bank on its own with the same result.
I do have the CPU manual and a decent Tektronix scope.
Has anybody any suggestions?
Rod Smallwood
23 Mar
23 Mar
11:55 a.m.
Rod,
I use both Tek scopes here, a 465 and a TDS410A. Most Tek scopes have
the facility to perform a single sweep trigger to detect pulses such as
that. However old your Tek scope is, there's supposed to be a button to
select between 'Auto' and 'Single Swp' triggering. Also, there is a
trigger source selector, this can be set for Chan.1, since your most
likely to have one scope probe hooked up already. Then in the same area
of the trigger panel there is an arming indicator light. This will let
you know the scope is ready to detect a pulse after you set the trigger
level--adjusting this to the middle should be good enough. You can also
set the Horz sweep to somewhere between 100nsec and 500nsec to get a
visual with the brightness turned up alittle more since it will be
quick. If the pulse is detected the arming indicator will clear and the
trigger indicator will blink just once. You'll have to arm the trigger
again by selecting the "single sweep' button to check each signal.
hope that helps,
=Dan
[ Pittsburgh --- http://www2.applegate.org/~ragooman/ ]
Rod Smallwood wrote:
Hi
I recently aquired a complete PDP-8/e. As it had not run for a
while I took the usual precautions of reforming the electrolytics in the
PSU as well as dismantling the whole system and cleaning everything.
I was rewarded with a working system. I started to run some simple
toggle in programs and got to the stage of driving a VT 320 with a
display of whole the character set at 110baud.
I was about to toggle in the next test which would have been a keyboard
echo routine when I discovered I could no longer load anything into
memory.
Load address works but setting the data to load into memory in the
switch register and raising DEP no longer writes the data to memory.
.
RUN works so it looks like the timing plus the RUN/STOP flipflop are
running ok.
However one shot operations like DEP and EXAM are difficult to diagnose
as is hard to see a one off 500nS pulse.
I don't think its the memory itself ( 4 x 4K of core) as I have tried
each 4K bank on its own with the same result.
I do have the CPU manual and a decent Tektronix scope.
Has anybody any suggestions?
Rod Smallwood
6070
days inactive
6070
days old
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