Macintosh Prototype Logic Board

I toured the storage vault of the Computer History Museum today. This cool item will be in their future Macintosh exhibit. It’s a Macintosh prototype logic board #4 with Motorola 68000 CPU, wire-wrapped by hand on the backside. Built in 1981.

Summary from builder Daniel Kottke: “The basic Mac architecture was very spare: about 32 ICs not counting the 68000 and the 16 RAM chips. There was the Timing State Machine (TSM), made out of a PAL and some flipflops; the Linear Address Generator (LAG) made out of a PAL and a couple of counters; the Bus Mgmt Unit (BMU) PAL, 4 multiplexors for the RAM addressing, 2 EPROMs, some bus drivers, a video output shift register, and the 6522 Peripheral Interface Adapter (PIA) which had 16 programmable I/O lines, which handled the keyboard and mouse interfaces and some other tasks. That was the core architecture, to which was added the dual serial ports, the internal and external floppy drive ports, the real-time clock chip, and the sound output. The PALs were the Programmable Array Logic parts made by MMI, which allowed one to write output logic equations to define each of the 8 outputs. They were fairly power-hungry (by today’s standards at least) but were cheap and flexible”