The H742 Power Supply is a modular power supply system from DEC, used in PDP-11/40, PDP-11/45 and PDP-11/70 systems mounted in a BA11-F Mounting Box.
It runs off standard wall AC; it can be jumpered to use either 110V or 220V supply.
The H742 supplies a modest amount of +15V DC power to various backplanes mounted in the BA11-F, along with a line time clock signal, and AC and DC 'power OK' signals.
It also supplies 20-30V AC to several DEC standard modular regulators (up to five, in spaces numbered 'A' through 'E') mounted in the H742, which provide other voltages such as +5V to the backplanes in the mounting box.
Generally the power harness supports only limited options as to which regulator can be in which spaces; e.g. the later -11/40 harness mandates that A, B and C hold H744 +5V Regulators, D holds an H745 -15V Regulator, and E may hold either an H745 or H754 +20V, -5V Regulator.
The harness used DEC power distribution connectors to provide power to the backplanes; it normally used the DEC standard power wire colour coding.
The H742 did not support the Remote Switching Control Bus; the H742 was generally plugged into an 861 Power Controller, which did have the remote control capability.
Later versions of the basic H742 are the H742A and H7420A; the differences are not yet known.