It's firmware for a companion product to give soft power capability to a Macintosh that doesn't have it. It imitates the PowerKey Classic (model PK-1) by Sophisticated Circuits and can power the Macintosh on by a press of a connected ADB keyboard's power key and power it off when it shuts down as well as powering the Macintosh on and off on a user-defined schedule.
It also features a PS/2 keyboard interface that allows an independently powered PS/2 keyboard to power the Macintosh on and later makes it appear to the Macintosh as an ADB keyboard.
.--------.
Supply -|01 \/ 08|- Ground
ADB <--> RA5 -|02 07|- RA0 <--- PS/2 Data
ADB Power Key ---> RA4 -|03 06|- RA1 <--- PS/2 Clock
Line Clock ---> RA3 -|04 05|- RA2 ---> Relay
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The relay control line is active low.
External resistors should be used for the PS/2 lines (10 kΩ to 1 kΩ).
The ADB power key and line clock inputs have internal weak pullup resistors.
Building the firmware requires Microchip MPASM, which is included with their development environment, MPLAB. Note that you must use MPLAB X version 5.35 or earlier or MPLAB 8 as later versions of MPLAB X have removed MPASM.