PDP 11/24 - A Step Backwards

Noel Chiappa jnc at mercury.lcs.mit.edu
Thu Mar 31 21:44:54 CDT 2022

    > From: Brent Hilpert

    > DCLO & ACLO behave as power-on-reset signals to the system.

Minor nit: actually, I think it's DCLO which performs that function in a lot
of places; see e.g. the latches on pg. K2 (pg. 153 of the PDF) and K7. (INIT,
usually in buffered form, is used more widely for this function, but I doubly
digress in that observation.)

As I explained, ACLO is only used to trigger a 'power-failing' interrupt; CPU
operation is otherwise un-affected by ACLO (so the CPU can get ready). DEC P/S's
carefully sequence ACLO and DCLO such that on power-down, ACLO is asserted
first (to allow the CPU to get ready); on power-up, DCLO is de-asserted first
(the later de-assertion of ACLO is the signal for the CPU to start running).

However, you make a good point with:

    > If they are allowed to just float up as the power supply comes up you
    > have no guarantees as to the end result ('end' meaning the state of
    > things after the power supply has come up)

DEC specs state that DC power has to be up and stable 5 usec before DCLO can
be de-asserted ("pdp bus handbook", pg 53). This is precisly so that
everything is in a known state when operation commences.

So I guess I'll go back to my original suggestion: disconnect the ACLO from
the P/S (with its bogus -15V), leaving DCLO, so that it can properly set
everything to a known state on power-on, and then you can see see if E70 has
been fried, or is still working.

    > Manually connecting/disconnecting bus-ACLO to GND after power-up will
    > ... disable the clock.

I can't see anything in the clock circuitry on pg. K1 (pg. 152 of the PDF),
where all the clocks are generated, that looks at ACLO, or its inverted form
POWER OK, or its latched form, PFAIL (both generated in the bottom RH corner
of K2)? Did I miss something? All I can see is DCLO.

I'm too burned out right now to check for uses of ACLO/POWER-OK/PFAIL, to see
definitively what it does do; tomorrow.


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