PDP8/e power controller connections

Tony Duell ard.p850ug1 at gmail.com
Sat Feb 4 11:12:25 CST 2017

I am currently rack-mounting my PDP8/e and its peripherals. And of course I want
to have the peripherals power up when I turn on the CPU. I have an 861 power
controller in the rack, but you can't just link that to the power
control sockets
on the CPU, DEC changed the wiring at some point...

Let me explain.

The 3 pin power control sockets on the 861 and just about every other power
controller and all my PDP11s carry the following 3 signals : Ground, On/ (ground
to turn the unit on) and Off/ (ground to force the unit off, e.g. for
an overheat

The 3 pin sockets on the PDP8/e CPU are not wired quite in parallel. One pin
is ground. Another pin is On/ (as above). But the middle pins are linked via the
frontpanel switch and overheat thermostat. The normal thing to do is to put
a jumper in each socket so that one side of the switch is grounded, the other
goes to On/. If you have more overheat thermostats in peripheral boxes, they
can be linked into the chain. There is a mains output on the PDP8/e PSU that
was (according to the printset) used to operate a contactor directly to power
up the peripherals.

A moment's thought made me realise you could use a normal power controller
with the PDP8/e. The only disadvantage is that the overheat switch in the
power controller would not shut down the CPU. Since I don't run my machines
unattended that is no great loss.

What I did was to cut a normal DEC power control cable (with the 3 pin plug
one each end) in half. Call the 2 halves 'CPU' and 'Pwr'

Then wire as follows :

CPU/Green (Gnd) - Pwr/Green (Gnd)

CPU/Red (On/) --->|--- CPU/Black (Switch)

Pwr/Red (On/) --->|--- CPU/Black (Switch)

Pwr/Black (Off/) : Not connected

Now plug the ends into the power controller and one of the CPU
power control sockets (make sure you have them the right way
round). In the other CPU power control socket fit the jumper plug
that links the middle pin to ground.

The idea is that when the CPU switch turns on, both the CPU On/
line and the power controller On/ line are pulled to ground via the
diodes. The diodes prevent the voltage from one power switching
circuit ending up in the other.

The diodes can be just about anything that will carry the power
relay coil current. I used 1N4007's as I happen to have them to
hand. I built it in a spare telephone junction box with 6 pairs of
terminals. One set of terminals carries the cables. The other
set carries the diodes and a wire between the 2 ground wires.

Needless to say construction is not critical. It's very low speed,
it's about 24V (and isolated from the mains) at < 1A.


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