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The Motherboard

As mentioned, the Macintosh SE was based on the Motorola 68000, which was 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 Macintosh SE/30 was based on a different processor, the Motorola 68030. Because of that, we will discuss the motherboard used on this computer later. 

Macintosh SE TutorialFigure 20: Motherboard of the Macintosh SE

The motherboard of the Macintosh SE, part number 820-0176 or 630-4125, had four SIMM-30 memory sockets, originally coming with four 256 KB memory modules installed. The four 256 KB memory modules could have been replaced with two 1 MB memory modules to have a computer with 2 MB of RAM. The alternative would have been to have four 1 MB memory modules to have a computer with 4 MB of RAM. Since the 68000 CPU used a 16-bit data bus, and each SIMM-30 memory module is an eight-bit entity, you would need two or four memory modules installed; you can’t install one or three memory modules.

In order to install memory modules with more than 256 KB, you need to cut one of the legs of the R35 resistor (labeled “256K BIT”). See Figure 21. On some revised motherboards you need to simply move the position of a jumper, instead of having to cut a resistor. See Figure 22. In this case, you must move the jumper to the “2/4M” position to enable 2 MB of memory. However, in order to enable 4 MB of memory, you must remove the jumper from the motherboard (and not place it at the “2/4M” position, as it would be logical to assume).

Macintosh SE TutorialFigure 21: Resistor you must cut in order to have more than 1 MB of RAM

Macintosh SE TutorialFigure 22: Jumper to enable more than 1 MB of RAM (you must remove the jumper for 4 MB)

The Macintosh SE used the same NCR 5380 SCSI controller as the Macintosh Plus. It also used a 65C22, which was an upgraded version of the 6522 “Versatile Interface Adapter” used on the previous Macintosh models and in charge of mouse and keyboard communications. It also used 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 drives.

On the FDHD and on the SuperDrive models, the IWM chip was replaced with the new SWIM (Super Woz Integrated Machine) chip, to support 1.44 MB floppy disk drives. For this reason, you couldn’t install 1.44 MB floppy disk drives into the standard Macintosh SE and the dual floppy Macintosh SE, as their control circuits didn’t support this kind of drive.

The Macintosh SE had two floppy disk drive ports on the motherboard instead of only one.

Two new custom-made chips were added to the Macintosh SE, replacing the six PAL (Programmable Array Logic) chips available in previous Macintosh models. These chips were called BBU (Bob Bailey Unit, a big chip manufactured by VLSI) and GLU (General Logic Unit).

Other features that were inherited from previous Macintosh models were the reset and interrupt buttons (seen at the top right corner in Figure 20), 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.

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Gabriel Torres is a Brazilian best-selling ICT expert, with 24 books published. He started his online career in 1996, when he launched Clube do Hardware, which is one of the oldest and largest websites about technology in Brazil. He created Hardware Secrets in 1999 to expand his knowledge outside his home country.