This is the front most board in the stack and contains all the printer specific circuitry. The large arrays of transistors on a shared heat sink are for driving the very power hungry servo motors in the printer assembly. This board also features a Z80 PIO chip, similar to the Processor Board.

These are the printer specific memory addresses that can be used to control the printer.

These are the official Wang schematics, provided by the awesome Rich.

All schematics are available here in PDF format.