FWIW, the SuperMite SM24 LH Research PSU of the 1980s had a 9-pin (3x3) Molex connector that incorporated the following signals: - on/off (input) - AC power fail (output) - DC power OK (output) - "I" monitor (output) - hi margin - lo margin - +sense - -sense The on/off pin was switched to ground to shut down the PSU. The fan ran continuously. To get the SM24 to regulate properly, you needed to connect the sense wires to their respective load points, or jumper them directly to the supply's outputs. The hi/lo margin pins could be left open. I'm guessing that the on/off pin would be pulled up to a standby voltage via a resistor.