Inside the Macintosh 128K
By Gabriel Torres on December 17, 2012
Today we are going to take an in-depth look at one of the most iconic computers of all times, the very first Macintosh, released in 1984. It was renamed to Macintosh 128K in September 1984, as a second version of the Macintosh with 512 kB was to be released in 1985.
Its historical importance comes from the fact that it was the first computer targeted to end-users to come with an operating system that had a graphical interface, a mouse, and a 3.5” 400 kB floppy disk drive (at the time, most computers used 5.25” 360 kB floppy disk drives). It had a 9-inch black-and-white video monitor with a resolution of 512 x 342. It had 128 kB of RAM and was based on the Motorola 68000 microprocessor, which was one of the most powerful CPUs available at the time.
The computer didn’t come with a hard drive, so the operating system and programs had to be loaded through floppy disks. All the time, we see people listing old Macs on eBay, saying that it is “defective” because the operating system is not loading and the computer is showing an icon with a floppy disk and a question mark. (The person selling the computer does not realize that old computers didn’t come with a hard drive.) This is the normal behavior of the computer when it doesn’t find a floppy containing the operating system, and it means the computer is working as expected.
The Macintosh 128K was yellowish in color, the same tone as the Apple IIe.
Nowadays, the first thing you will notice looking at the original Macintosh is how small it was. In Figure 2 we compare it to a 21-inch LCD monitor.
Differently from the Apple II and Apple III, the keyboard was not part of the body of the computer; it was connected to the computer using a spiraled cable similar to the ones used by telephones. The keyboard was mechanical and almost identical to the one used with the Apple IIe, except that the old Open Apple and Solid Apple keys were replaced by the Command and the Option keys, respectively.
The mouse was rectangular with a single button. To this day, Apple mice still have only one button. It was connected to the computer through a DE-9 connector, identical to the one used on the Macintosh’s serial ports, but the mouse port used a proprietary format.
In Figures 5 and 7 you have an overall look of the Macintosh 128K. The Macintosh 128K had a brightness adjustment button on its front panel, below the Apple logo.
The easiest way to detect that this is the original Macintosh from 1984 is that it only has “Macintosh” written on its back or “Macintosh 128K” on models manufactured after September of 1984. The Macintosh 512K had “512K” written on its back, and the successor to the 512K, the Macintosh Plus, had “Macintosh Plus” written on the front panel.
On the rear panel, the computer had a compartment for you to install a 4.5 V battery (known as TR133R, NEDA 1306A, 523, etc.) in charge of keeping the computer’s real time clock hardware working when the computer was turned off. Notice that this battery has the same physical size of a AA battery, but it is different (4.5 V vs. 1.5 V).
At the bottom part of the rear panel, the Macintosh 128K had a proprietary mouse port, a port for the installation of an external floppy disk drive, a serial port for a printer, a serial port for an external modem, and a 3.5 mm jack for an external speaker. The Macintosh 128K had an internal speaker as well.
To avoid regular users from opening the Macintosh 128K (and subsequent models), Apple used Torx TT15 screws, a very unusual type of screw to be used on computers (especially at the time), which require a special TT15 screwdriver at least 9 inches (230 mm) long.
Inside the computer, you would find the most commented on (and hidden) feature of the computer – the signatures of all the members of the team that designed the Macintosh, including, of course, Steve Jobs. See Figure 9. These signatures were kept on the successors of the Macintosh 128K, including the Macintosh 512K, the Macintosh Plus, and the Macintosh SE. Interestingly, the Macintosh Plus and the Macintosh SE were released after Steve Jobs left Apple. For some reason, Steve Job’s signature was kept inside the computer even though he was not related to the development of these computers, particularly the Macintosh SE.
You will see several people selling old Macs on eBay saying “this Mac is so special that it has Steve Jobs signature” or “rare – signed by Steve Jobs.” Let’s make something clear. All early Macintoshes were signed by the whole team, so that is not a “special feature.” And since millions of these computers were sold, they are not rare.
In Figure 10, you can see how the Macintosh 128K looked inside. It was comprised of two printed circuit boards: one containing the power supply and the electronics for the monitor; the other was the motherboard.
In Figure 11, you can better see the power supply board, which was officially called “Sweep / Power Supply,” part number 630-0102.
As mentioned, the Macintosh 128K was 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 128 kB of RAM available was comprised of 16 chips with 64 kbits each. Other notable chips available were 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.
The Macintosh used 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).
The motherboard had a reset and an interrupt button (seen at the top right corner in Figure 12) 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 these buttons.
More about the motherboard of the Macintosh 128K can be found here.