M7264 Troubleshooting

Noel Chiappa jnc at mercury.lcs.mit.edu
Mon May 20 11:51:36 CDT 2019

    > From: Mister PDP

    > After a day of confusing and mixed up signals (I don't really use
    > this type of equipment very often) .. I switched over to the
    > oscilloscope 

Don't feel bad, I too prefer to rely on an oscilloscope by default; not
only does it let you see what's really happening (intermediate voltages,
noise, etc) but it's simpler, and there are less ways to get incorrent

    > to confirm that the four clock signals on the MCP-1600 chipset were
    > working properly .. and was able to see that the very basics of the
    > CPU were working.

It's not necessarily good if the 4 clocks (I take it that's what the
latter refers to) are working, because if one or more of those were broken,
it's a relatively easy/simple fix, whereas if they are working, and
the CPU's still not running, it could be a failed CPU chip, and the only
fix there is to replace it.

    > the SRUN signal coming off of the backplane.

Note that SRUN isn't crucial to the machine's operation, it's just user
info. If SRUN is somehow broken on it's own (i.e. failed component
somewhere between where the CPU generates it, and the display LED) finding
and fixing that issue won't help.

Far more useful, in terms of getting the thing running, would be to know
if BSYNC on the QBUS is hopping around (as ODT tries to talk to the
console - an easy thing to check into, too). If not, that's a show-stopper
that needs to be looked into.

    > From: Glen Slick

    > the SRUN L signal is driven on to the backplane bus line AF1.

The '78-'78 "microcomputer processors" says (pg. 3-15) that on the LSI-11
it's also on CH1; just to complete the complexity, it also says (pg. 3-32)
that on the LSI-11/2 it's also on AH1!

    > From: Mister PDP

    > I hooked the oscilloscope up to SRUN off of E68, and found that it
    > oscillates low at 58.68KHz. .. Hitting the Run/Halt switch does not
    > have any effect on the period or amplitude of the oscillation.

So maybe the CPU is actually running after all? Although turning
RUN/HALT to 'HALT' should stop it - the Run light would I think go
out when ODT is running. (It certainly does on the /23.)

Can you check that BHALT on the QBUS is actually asserted (i.e. 0V)
when the switch is in HALT?


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