Mac Musings

25th Anniversary of the Macintosh Plus

Dan Knight - 2011.01.17 - Tip Jar

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The Mac Plus changed the Mac from a closed box to an expandable computer system.


Macintosh 128K

On January 16, 1986, Apple introduced the Macintosh Plus. At first glance, it didn't look much different from the original Macintosh announced in January 1984, with its 128 KB of system memory, or the Mac 512K "Fat Mac", with four times as much system memory, released later that year.


Macintosh Plus

From the front, the biggest difference was that the new model said "Macintosh Plus" whereas the original Mac was unique and didn't need a nameplate.

Looks can be deceiving. Although from the front the Mac Plus looked a lot like the 128K and 512K with its 9" display, 3.5" floppy drive, and keyboard port, there were plenty of differences on the inside, on the back, and even on the keyboard.

A New Paradigm: Expandability

Until 1986, the Macintosh was designed as a closed box. There was no easy way to open up the computer, and there were no slots for upgrading memory. What you bought was what you got. The only expansion options were through external peripherals - a second floppy drive, an ImageWriter printer, and later on a relatively slow hard drive that connected to the Mac's floppy drive port.

With the Mac Plus, you got more to begin with. The Plus included an 800K double-sided floppy drive, giving it twice the capacity of the 400K single-sided drives used on the 1984 Macs. It shipped with 1 MB of system memory, twice as much as the 512K and 50% more than IBM compatible MS-DOS PCs with their 640 KB ceiling could access without special hardware.

Memory Expansion


30-pin 8-chip SIMM as used in Mac Plus.

But Apple went beyond simply giving it more memory and a higher capacity floppy drive; Apple built the Mac Plus for expansion. That megabyte of memory was installed as four 256 KB SIMMs, a new type of memory module that made it much easier to install banks of memory. No more fiddling with individual memory chips with their pesky, too easily bent pins. One SIMM replaced 8 conventional RAM chips, and SIMMs used a circuit board, making them easy to install.

AppleDesigned the Mac Plus so it could accept 1 MB SIMMs, allowing memory expansion to a mind-boggling 4 MB. By replacing two 256 MB modules, you had a 2.5 GB Mac. Replace all four for your 4 MB maximum.

SCSI Expansion

Although the floppy drive port was fast enough for floppy drives, it was excruciatingly slow for Apple's first Mac hard drive, the Apple Hard Disk 20 (also see Wikipedia), which had a 500 Kbps/62.5 KBps transfer rate. Several third-party vendors came to the aid of early Mac users, but they were all hacks that required special drivers, and many of them used one of the Mac's serial ports.


Mac Plus with SCSI drive.

With the Mac Plus, Apple adopted SCSI as its peripheral expansion bus. SCSI is an intelligent, buffered, peer-to-peer interface, meaning that every SCSI device has some smarts, which places less demands on the computer's processor. The SCSI bus on the Mac Plus allowed connecting up to 7 devices - hard drives, tape drives, scanners, and printers among the options.

So not only was SCSI a lot faster than the floppy drive port, it also gave Mac Plus users a lot more expansion options. Although the SCSI standard allowed for 5 MBps throughput and AppleDesigned its SCSI interface to support 1.25 MBps, the Mac Plus has a 2.104 Mbps/263 KBps ceiling, and you needed to use a system patch such as SCSI Accelerator 2.1 or SCSI Accelerator 7.0 to achieve maximum throughput.

Despite limitations in the way SCSI was implemented on the Mac Plus, it was over four times as fast as the old Hard Disk 20 connected to the floppy port.

A New Disk Format


Mac Plus with floppy drive.

The Mac's original file system allowed for folders, but not for folders within folders, which became a real nuisance with hard drives and their enormous capacity (relative to floppies). What System 3, introduced with the Mac Plus, Apple gave us HFS, a hierarchical file system. The Mac got subfolders, and today's Macs use an updated version of the filing system called HFS+.

Other Improvements

Steve Jobs, who had brought the Macintosh into the world, disdained the "extra" keys found on most personal computers, so the original Macintosh had used a minimalist keyboard that had just the keys you needed. There were no arrow keys, no function keys, and numbers were on the top row just like on a typewriter. In Jobs' mind, the mouse was the proper way to navigate the screen.

However, Steve Jobs left Apple in 1985, opening the door for a more expandable Mac and a new keyboard design. With the Mac Plus, Apple introduced that keyboard, adding useful things like a numeric keypad and arrow keys.

Sound, mouse, floppy, SCSI, printer, and modem ports on the Mac Plus
Sound, mouse, floppy, SCSI, printer, and modem ports on the Mac Plus.

The Mac Plus introduced a new connector for serial ports. It used the small, round DIN-8 connector instead of the larger DB-9 port found on earlier Macs. The smaller ports freed up the space necessary for adding a SCSI port.

The Mac Plus Legacy

The Macintosh Plus remained current until it was officially discontinued on October 15, 1990 - the longest model run in Mac history at 4 years, 10 months. For a lot of longtime Mac users, this is the model we cut our teeth on, as it was one of the first with sufficient power to handle Aldus PageMaker, the app that gave rise to the desktop publishing revolution.

Two other factor came into play there: The Apple LaserWriter wasn't just a high quality laser printer with Adobe Postscript, it was also very expensive at nearly $7,000. Fortunately, it was also a networkable laser printer, which meant that several Macs could share it. The second factor is the built-in LocalTalk networking support that became standard with the Mac Plus.

Although Apple moved to high-density floppy drives in 1989, Mac hard drives remained compatible with 800K disks until Apple began to phase out floppy drives in 1998. Likewise, SCSI and the DIN-8 serial ports remained on Macs until 1998, when Apple began its switch to new architecture with the first iMac.

The Mac Plus is the oldest Mac to support System 7.0 and was supported right through System 7.5.5. Like the Mac SE and Classic, both built around the same 8 MHz 68000 CPU, it's no speed demon under System 7, but it plods along reliably and provides real incentive for moving to a more powerful model to properly support that more powerful operating system.

For me, the Mac Plus was the first stepping stone along the Mac path, and I used it until mid 1993, when I sold it to buy a far faster Centris 610.

Further Reading on Low End Mac

Most Important Macs

Mac Plus 20th Anniversary

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Dan Knight has been using Macs since 1986, sold Macs for several years, supported them for many more years, and has been publishing Low End Mac since April 1997. If you find Dan's articles helpful, please consider making a donation to his tip jar.

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