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 shutdown). 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. -tony
