PDP 11/24 - A Step Backwards

[Noel Chiappa]

    > The 'unused' gate in E52 is the one that the added wires from the ACLO
    > ECO went to; I wonder if it was damaged by the -15V, somehow?

So, I checked, and the wire that goes from the plated-through hole next to the
etch cut on E70p1 winds up at E52p4 (the bus line on that transceiver),
thereby connecting the latter directly to UNIBUS ACLO (on pin BF1). So that's
almost certainly what caused other gate(s) in E52 to fail.

I think I have managed to trace where all the other two wires to that 'new'
gate in E52 went to/from, to see exactly what the ECO is. Given that E52 is a
transceiver, it was likely substituted for E70 so that the KDF11-U could also
_drive_ BUS ACLO.

I discovered that E52p5 (the new transceiver's input) is connected to E73p13
(an DS8640 quad NOR used as a bus reciver); that's in the upper left corner of
K2. It's BUS PB L NOR BUS PA H - i.e. a parity error has been detected in
memory - so it apparently then power-fails the system!

The incoming BUF ACLO (E52p6) goes to a PTH next to E70p8. On the bottom,
there's a trace from that PTH that goes to E66p13 - which is the inverter
shown on K2 which converts BUF ACLO H to POWER OK H. So that probably the
answer to my plaint about E70p1 being left floating: perhaps theres an etch
cut somewhere that disconnected E66p13 from E70p2, so the former can now be
driven by E52p6.  I can't see one, but there's a trace from that latter PTH
that dives under E70, and I can't see it well, but it looks like it goes to
E70p2. It would be interesting to know what they did about the E70p2 -> E66p13
connection.

	Noel