Inside the Macintosh Plus
By Gabriel Torres on January 2, 2013
Following the release of the original Macintosh in 1984 and the Macintosh 512K in 1985, Apple released the Macintosh Plus in 1986, now with 1 MB of RAM, an 800 kB 3.5” floppy disk drive, and a SCSI port. Let’s check it out.
The original Macintosh (Macintosh 128K), the Macintosh 512K, and the Macintosh Plus had the same basic design. Thus, we can consider these three models as the first generation of Macintosh computers. They were all based on the Motorola 68000 microprocessor, which was one of the most powerful CPUs available at the time, and had a 9-inch black-and-white video monitor with a resolution of 512 x 342 integrated on the computer’s body. The main difference between the three models is the amount of RAM (128 KB, 512 KB or 1 MB). However, while the first two models didn’t allow the user to add more memory, the Macintosh Plus uses SIMM-30 memory modules, allowing users to expand the memory up to 4 MB, as we will explain. In fact, in January 1988, Apple released models of the Macintosh Plus with 2 MB or 4 MB of RAM.
Another difference between the three models was with the floppy disk drive. The Macintosh 128K used a 400 kB 3.5” floppy disk drive. The first models of the Macintosh 512K (part number “M0001W”) also used a 400 kB 3.5” floppy disk drive, but later models (part numbers “M0001E” and “M0001D”) used an 800 kB 3.5” floppy disk drive. The Macintosh Plus came with an 800 kB 3.5” floppy disk drive.
None of these computers 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 first-generation Macs on eBay, saying that the computer 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 Plus, however, was the first Macintosh computer to come with a SCSI port. This allowed you to install an external hard disk drive to this computer.
Originally, the Macintosh Plus was released in yellow, just like the Macintosh 128K and the Macintosh 512K. However, in January 1987, its color was changed to the light gray color (called “platinum”) that Apple started using on its computers from then on.
Nowadays, the first thing you will notice looking at the Macintosh Plus is how small it was. In Figure 2, we compare the Macintosh Plus 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 keyboard of the Macintosh Plus was different from the one used with the Macintosh 128K and the Macintosh 512K, as it now had a numeric keypad. The color was changed from yellow to platinum when the color of the Macintosh Plus changed (January 1987).
The mouse was rectangular with a single button, mechanically identical to the one used with the Macintosh 128K and the Macintosh 512K, which had its color changed from yellow to platinum when Apple changed the color of the Macintosh Plus. 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 Plus. Similarly to the Macintosh 128K and the Macintosh 512K, the Macintosh Plus had a brightness adjustment button on its front panel, below the Apple logo.
The Macintosh Plus is easy to identify as it has “Macintosh Plus” written on its front panel and on its rear panel as well. The original Macintosh had only “Macintosh” or “Macintosh 128K” (models manufactured after September 1984) written on its back, while the Macintosh 512K had “Macintosh 512K” written on its back.
Another way to differentiate the Macintosh Plus from the other first-generation models is through the computer’s part number, which was “M0001A.” The part number of the Macintosh 128K was “M0001” and the part number of the Macintosh 512K could 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). In Figure 7, you can also see the optional anti-theft device installed. This device, which was also available for the Macintosh 128K and the Macintosh 512K, allowed the use of a steel cable to prevent people from stealing the computer, which was highly desirable in public spaces such as schools.
In Figure 8, you can see the connectors available at the rear panel of the Macintosh Plus. The first connector was a 3.5 mm jack for an external speaker (the computer had an internal speaker as well), followed by a proprietary mouse port, and a port for the installation of an external floppy disk drive. These ports were identical to the ones used on the Macintosh 128K and the Macintosh 512K.
The other ports available on the Macintosh Plus, however, were completely different.
There was an external 25-pin SCSI port, which allowed you to install an external SCSI hard disk drive to the Macintosh Plus. Therefore, the Macintosh Plus was the first Macintosh to support a hard disk drive, even though the computer didn’t come with one.
Then there were two serial ports, one for a printer and one for an external modem. The Macintosh 128K and the Macintosh 512K also had two serial ports, but with the Macintosh Plus, Apple changed the connector type used for them from DE-9 to DIN-8, which became the standard for future Apple computers.
To avoid regular users from opening the Macintosh Plus, 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, the Macintosh 512K, and to the successor to the Macintosh Plus, namely the Macintosh SE.
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, the Macintosh 512K, 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 Jobs’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 Plus 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 Macintosh 128K and on the Macintosh 512K.
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.