Mac Plus – Apple Macintosh Museum

The Macintosh Plus computer is the third model in the Macintosh line, introduced on January 16, 1986, two years after the original Macintosh and a little more than a year after the Macintosh 512K, with a price tag of US$2599.^[1] As an evolutionary improvement over the 512K, it shipped with 1 MB of RAM standard, expandable to 4 MB, and an external SCSI peripheral bus, among smaller improvements. Originally, the computer’s case was the same beige color as the original Macintosh, Pantone 453^[2], however in 1987, the case color was changed to the long-lived, warm gray “Platinum” color.^[3] It is the earliest Macintosh model able to run Mac OS System 7.

The Macintosh Plus was the last classic Mac to have a phone cord-like port on the front of the unit for the keyboard, as well as the DE-9 connector for the mouse; models released after the Macintosh Plus would use ADB ports.

The Mac Plus was the first Apple computer to utilize user-upgradable SIMM memory modules instead of single DIP DRAM chips. Four SIMM slots were provided and the computer shipped with four 256K SIMMs, for 1MB total RAM. By replacing them with 1MB SIMMs, it was possible to have 4MB of RAM. (Although 30-pin SIMMs could support up to 16MB total RAM, the Mac Plus motherboard had only 22 address lines connected, for a 4MB maximum.)

It has what was then a new 3 ¹⁄₂-inch double-sided 800 KB floppy drive, offering double the capacity of floppy disks from previous Macs, along with backward compatibility. The then-new drive is controlled by the same IWM chip as in previous models, implementing variable speed GCR. The drive was still completely incompatible with PC drives. The 800 KB drive has two read/write heads, enabling it to simultaneously use both sides of the floppy disk and thereby double storage capacity. Like the 400 KB drive before it, a companion Macintosh 800K External Drive was an available option. However, with the increased disk storage capacity combined with 2-4x the available RAM, the external drive was less of a necessity than it had been with the 128K and 512K.

The Mac Plus has 128 KB of ROM on the motherboard, which is double the amount of ROM in previous Macs; the ROMs included software to support SCSI, the then-new 800 KB floppy drive, and the Hierarchical File System (HFS), which uses a true directory structure on disks (as opposed to the earlier MFS, Macintosh File System in which all files were stored in a single directory, with one level of pseudo-folders overlaid on them). For programmers, the fourth Inside Macintosh volume details how to use HFS and the rest of the Mac Plus’s new system software. The Plus still did not include provision for an internal hard drive and it would be over nine months before Apple would offer a SCSI drive replacement for the slow Hard Disk 20. It would be well over a year before Apple would offer the first internal hard disk drive in any Macintosh.

A compact Mac, the Plus has a 9-inch (23 cm) 512 × 342 pixel monochrome display with a resolution of 72 PPI, identical to that of previous Macintosh models. Unlike earlier Macs, the Mac Plus’s keyboard includes a numeric keypad and directional arrow keys and, as with previous Macs, it has a one-button mouse and no fan, making it extremely quiet in operation. The lack of a cooling fan in the Mac Plus led to frequent problems with overheating and hardware malfunctions.

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Here’s the actual item in my museum
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