As mentioned, the Macintosh 128K, the Macintosh 512K, and the Macintosh Plus were based on the Motorola 68000, which is a 32-bit microprocessor using a 16-bit data bus and a 24-bit address bus, allowing it to access up to 16 MB of memory.
The motherboard of the Macintosh Plus was clearly based on the motherboard of the Macintosh 512K, but two major changes were made. First, was the addition of SIMM-30 memory sockets, originally coming with four 256 KB memory modules installed. Because the computer now used memory modules instead of having the memory chips soldered on the motherboard, you could replace the four 256 KB memory modules with two 1 MB memory modules to have a computer with 2 MB of RAM or with four 1 MB memory modules to have a computer with 4 MB of RAM (since the 68000 CPU uses a 16-bit data bus and each SIMM-30 memory module is an eight-bit entity, you need two or four memory modules installed; you can’t install one or three memory modules). As mentioned before, in January 1988, Apple started offering the Macintosh Plus with 2 MB or 4 MB of RAM.
In order to install memory modules with more than 256 kB, you need to cut one of the legs of the R8 resistor (labeled “256K BIT”). See Figure 14.
The second main addition to the motherboard of the Macintosh Plus was a SCSI controller for the external SCSI port. This was accomplished by using an NCR 5380 SCSI controller.
Other chips used on the Macintosh Plus were the same as the Macintosh 128K and the Macintosh 512K, such as the 6522 “Versatile Interface Adapter,” in charge of mouse and keyboard communications; the Z8530 serial communications controller, in charge of the two serial ports; and the custom-made IWM (Integrated Woz Machine), in charge of controlling the floppy disk drive. It also used the same six PAL (Programmable Array Logic) chips, named LAG (Linear Address Generator), TSM (Timing State Machine), BMU0 and BMU1 (Bus Management Unit), TSG (Timing Signal Generator), and ASG (Analog Sound Generator).
Other features that were inherited from the Macintosh 128K and the Macintosh 512K were the reset and interrupt buttons (seen at the top right corner in Figure 12 and in Figure 14), which were targeted to programmers. These buttons were normally not accessible from outside the computer. However, as these buttons were located in front of the side ventilation slits of the computer, programmers could buy a special “programmer’s switch” that could be attached to this vent (located on the left-side of the computer) and therefore access them.
The Macintosh Plus motherboard’s part number was “630-4122.”
In Figure 13, you can see the four 256 KB SIMM-30 memory modules that came with the Macintosh Plus.