Inside the Macintosh 512K
By Gabriel Torres on December 19, 2012
Following the release of the original Macintosh in 1984, Apple released the Macintosh 512K in 1985. The main difference between the two was the increase in RAM to 512 kB. The original Macintosh had 128 kB and started to be called the 128K after plans to release the 512K were revealed.
Just like the original Macintosh, the Macintosh 512K came originally with a 3.5” 400 kB floppy disk drive (at the time, most computers used 5.25” 360 kB floppy disk drives) and a 9-inch black-and-white video monitor with a resolution of 512 x 342 integrated on the computer’s body. The 512K was based on the Motorola 68000 microprocessor, which was one of the most powerful CPUs available at the time.
An upgrade version of the 512K with an 800 kB floppy disk drive was released in April 1986 and was called the 512Ke (for “Enhanced”). This model had the part number “M0001E”.
At the end of the Macintosh 512K’s life, Apple cut its price and renamed it to Macintosh ED, and targeted it to the educational market. This model had the part number “M0001D” or “M0001ED.”
The computer didn’t come with a hard drive, so the operating system and programs must have been loaded through floppy disks. All of 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 512K kept the same yellow color as the Macintosh 128K and the Apple IIe.
Nowadays, the first thing you will notice looking at the Macintosh 512K 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. This keyboard was identical to the one used with the Macintosh 128K. Later, Apple released a version of the 512Ke with the same keyboard that came with the Macintosh Plus, dubbed the 512Ke/800. This model had the part number “M0001D.”
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 at the Macintosh 512K. Similarly to the Macintosh 128K, the Macintosh 512K had a brightness adjustment button on its front panel, below the Apple logo.
The easiest way to detect that this is the Macintosh 512K is that it has “Macintosh 512K” written on its back. The original Macintosh had only “Macintosh” or “Macintosh 128K” (models manufactured after September 1984) written on its back, and the successor to the 512K, the Macintosh Plus, had “Macintosh Plus” written on the front panel.
Another way to differentiate the original Macintosh from the Macintosh 512K is by the model number. The original Macintosh’s model number is “M0001,” while the 512K’s model number can be “M0001W” (for the original model with a 400 kB floppy disk drive), “M0001E” (for the 512Ke model, with an 800 kB floppy disk drive) or “M0001D” (for the 512Ke/800 model, with an 800 kB floppy disk drive and the same keyboard as the Macintosh Plus).
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 512K 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 512K had an internal speaker as well. This configuration is exactly the same as the Macintosh 128K.
It is interesting to note that while the serial port used on the PC used a male connector, the serial port used on the Macintosh used a female connector.
To avoid regular users from opening the Macintosh 512K, 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. The same applies to the original Macintosh and to the successors to the 512K.
Inside of the computer, you would find the most commented on (and hidden) feature of the computer: the signatures of all members of the team that designed the Macintosh, including, of course, Steve Jobs. See Figure 9. These signatures were also present on the original Macintosh and kept on the successors of the Macintosh 512K, such as the Macintosh Plus and the Macintosh SE. Interestingly enough, the Macintosh Plus and the Macintosh SE were released after Steve Jobs left Apple, but for some reason Steve Job’s signature was kept inside the computer, even though he was not related to the development of these computers, in particular the Macintosh SE.
You will see several people selling old Macs on eBay saying “this Mac is so special that it has Steve Jobs’s signature” or “rare – signed by Steve Jobs.” Let’s make something clear. All early Macintoshes were signed by the whole team, and that is not a “special feature.” Since millions of these computers were sold, they are not rare.
In Figure 10, you can see how the Macintosh 512K 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. This is exactly the same board used on the original Macintosh.
As mentioned, the Macintosh 128K and the Macintosh 512K 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 512K was identical to the 128K’s, except for the RAM chips. The 512K used 16 chips with 256 kbits each, while the 128K used 16 chips with 64 kbits each.
As you can see in Figure 12, the motherboard had two part numbers, “630-0101” for the Macintosh 128K or “630-0118” for the Macintosh 512K.
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 motherboard 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 512K can be found here.