M7264 Troubleshooting

Mister PDP vintagecomputersmn at gmail.com
Mon May 20 16:07:41 CDT 2019

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

I checked the BHALT signal going into the backplane, and it seems to be in
good working order. I took a picture of the readouts for SRUN, which you
can see here:

Checking the BSYNC, it looks like there is life. It oscillates at 58.605
KHz, and has wider peaks than the SRUN signal. This signal does not respond
to the Run/Halt switch being toggled, but I would assume that to be normal
as I have the board jumpered to run at ODT.

On Mon, May 20, 2019 at 11:51 AM Noel Chiappa via cctalk <
cctalk at classiccmp.org> wrote:

>     > 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
> info.
>     > 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?
>         Noel

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