Macintosh Plus

The Macintosh Plus is the third model in the Macintosh computer 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$2,599. The Apple Macintosh Plus features an 8 MHz 68000 processor, 1 MB of RAM, and an 800k disk drive in a beige or platinum all-in-one case with a 9" monochrome display. The Macintosh Plus was the first Macintosh to have a double-density 800k disk drive, a SCSI port to allow external expansion, and RAM slots to allow the RAM to be expanded beyond the pre-installed limit.

Macintosh Plus

Macintosh Plus
Specifications
Manufacturer	Apple Computer, Inc.
Type	Personal Computer
Released	January 16, 1986
Discontinued	October 15, 1990
Intro price	US$2,599
CPU	Motorola 68000 @ 8 MHz
Memory	1 MB RAM (expandable to 4 MB)
Storage	800 KB 3.5" floppy drive, External SCSI
Display	9" monochrome CRT (512×342 pixels)
Sound	Monaural 8-bit, 22 kHz (4-voice)
Dimensions	13.6" H × 9.6" W × 10.9" D
Weight	16.5 lbs (7.5 kg)
OS / Firmware	System 3.0 – System 7.5.5
Predecessor	Macintosh 512K
Successor	Macintosh SE
Codename	Mr. T
Model no.	M0001A

Although the Macintosh Plus would become overshadowed by two new Macintoshes, the Macintosh SE and the Macintosh II in March 1987, it remained in production as a cheaper alternative until the introduction of the Macintosh Classic on October 15, 1990. This made the Macintosh Plus the longest-produced Macintosh model, having been on sale unchanged for 1,734 days, a record not surpassed until the second-generation Mac Pro in 2018. Originally, the computer's case was the same beige color as the original Macintosh, Pantone 453; however, in 1987, the case color was changed to the long-lived, warm gray "Platinum" color.

Architecture and Processor

The Macintosh Plus is powered by the Motorola MC68000 microprocessor, operating at 8MHz. This 16/32-bit processor features:

• 32-bit internal registers with 16-bit data bus
• 16-bit address bus supporting up to 16 MB of addressable memory
• 8 MHz clock speed providing improved performance over earlier Macintosh models

The processor features 32-bit register width, 16-bit data bus width, and 32-bit address bus width, though the Mac Plus motherboard had only 22 address lines connected, for a 4 MB maximum memory configuration.

Memory and Storage

The Macintosh Plus shipped with significant memory improvements over its predecessors:

RAM Configuration

• Standard RAM: 1 MB of RAM standard, expandable to 4 MB
• Memory Architecture: The Mac Plus was the first Apple computer to utilize user-upgradable SIMM memory modules instead of single DIP DRAM chips
• SIMM Slots: Four SIMM slots were provided and the computer shipped with four 256 KB SIMMs, for 1 MB total RAM
• Maximum Capacity: By replacing them with 1 MB SIMMs, it was possible to have 4 MB of RAM

ROM and System Software

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). The Plus went through two ROM revisions during its general market relevance. The initial ROM was replaced after the first two months as it had a serious bug which prevented the Mac from booting if an external SCSI device was powered off. The second revision fixed a problem with some SCSI devices that could send the Mac into an endless reset at POST.

Storage Systems

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 drive is controlled by the same IWM chip as in previous models, implementing variable speed GCR.

Display and Graphics

The Macintosh Plus shipped with a 9-inch 512x342 display at 72 PPI (pixels per inch). The display specifications include:

• Screen Size: 9 inches (23 cm) diagonal
• Resolution: 512 × 342 pixels monochrome
• Display Type: Built-in CRT monitor
• Pixel Density: 72 pixels per inch

Audio Capabilities

The Macintosh Plus utilized one 8-bit, 22 kHz built in speaker. No audio in support was provided on this model.

Input/Output and Expansion

The Macintosh Plus introduced several important connectivity features:

SCSI Interface

The Macintosh Plus was the first Macintosh to have a SCSI port to allow external expansion. The SCSI bus on the Mac Plus is officially rated at 1.25 MBps/10 Mbps by Apple, although real world testing shows it to be barely over 0.26 MBps/2.1 Mbps – which is still 4x the speed of Apple's earlier floppy port hard drive.

Port Configuration

The back of the machine features ports, from left to right: Sound Out, Mouse, Floppy, SCSI, Printer, and Modem. The Macintosh Plus was the last classic Mac to have an RJ11 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.

General Maintenance

Over time, Macintosh Plus computers may experience issues common to vintage electronics, such as:

• Capacitor aging leading to power supply instability or complete failure
• Overheating problems due to lack of internal cooling fan
• CRT display issues including loss of deflection or brightness degradation
• SCSI termination problems affecting external device connectivity
• Analog board component failure particularly in power regulation circuits

The lack of fan could cause the life of a Macintosh Plus to end early for some users. As the power supply would heat up, solder joints inside it would fracture, causing many problems, such as loss of deflection in the monitor or a complete loss of power. Regular maintenance, including capacitor replacement and ensuring proper heat dissipation, can help preserve the functionality of these systems. For detailed procedures, refer to the Macintosh Plus General Maintenance.

PCB Schematics & Service Manual

The Macintosh Plus features two main PCBs: the Logic Board and Analog Board. Below are detailed schematics and the official Apple service manual for the Macintosh Plus.

Apple Service Manual

You can download the Service Manual here: File:Macintosh Plus.pdf

Logic Board & Analog Board Schematics

Macintosh Plus PCB Schematics

Logic Board	Analog Board
Macintosh Plus Logic Board Macintosh Plus Logic Board Schematic	Analog Board Analog Board Schematic

Capacitor Replacement Guide

Detailed capacitor replacement guidelines for both analog and logic boards can be found on the Macintosh Plus Capacitor Replacement Guide page.

Retrobrite

To address plastic yellowing, detailed instructions can be found on the Retrobrite page.

Troubleshooting

A comprehensive troubleshooting guide addressing no power, chime but no video, intermittent operation, Sad Mac errors, and floppy drive problems is available on the Macintosh Plus Troubleshooting page.

Technical Details

System Architecture at a Glance

Sub-system	Specification (Macintosh Plus, Jan 1986)
CPU	Motorola 68000 @ 7.8336 MHz (15.667 MHz ÷ 2)
Bus width	16-bit data • 24-bit address (16 MB logical)
ROM	128 KB "v4.0" Toolbox / HFS / SCSI Manager
RAM	1 MB on four 256-K × 1 DRAM SIMMs — expands to 4 MB via 30-pin SIMM slots
Video	512 × 342 monochrome frame-buffer (22.1 KB) DMA-shared with system RAM
Sound	8-bit PWM DAC via VIA timers • 22 kHz RC low-pass → LM380 amplifier
Disks	Sony OA-D34V-22 800 KB auto-inject 3.5″ floppy
I/O ports	DE-9 mouse/KB • DB-19 ext. floppy • DB-25 SCSI-1 • RS-422 serial × 2 • 3.5 mm audio-out

Memory Map (Physical)

Macintosh Plus Address Space

Range	Size	Purpose
$000000 – $3FFFFF	4 MB	DRAM (configured 1 MB – 4 MB)
$400000 – $41FFFF	128 KB	ROM (v4.0 Toolbox)
$480000 – $4BFFFF	256 KB	I/O A – VIA 1, VIA 2, SCC, IWM, VBL PAL
$580000 – $5BFFFF	256 KB	I/O B – WD 5380 SCSI DMA registers
$600000 – $FFFFFF	Mirrors / reserved

Frame-buffer note: still occupies the last 22 016 bytes of physical RAM (e.g. $3FE000 – $3FFFFF in a 4 MB system).

Technical Details

Processor Specifications

Component	Specification
CPU	Motorola 68000 @ 8 MHz
Data Bus	16-bit
Address Bus	24-bit
Internal Registers	32-bit
Performance	0.7 MIPS

Memory Configuration

Type	Specification
Standard RAM	1 MB
Maximum RAM	4 MB
SIMM Slots	4 × 30-pin
SIMM Speed	150ns
ROM	128 KB

Physical Specifications

Dimension	Measurement
Height	13.6" (34.5 cm)
Width	9.6" (24.4 cm)
Depth	10.9" (27.7 cm)
Weight	16.5 lb (7.5 kg)
Power Consumption	60 watts maximum

ROM Layout (128 KB, v4.0 "Plus-class")

Offset	Size	Module
$400000	8 KB	68000 vectors + Mini-Shell
$402000	20 KB	QuickDraw 1.2 (monochrome, relocatable)
$407000	52 KB	Toolbox, SANE, Font & Menu mgrs.
$414000	16 KB	Sony 800 KB floppy driver & IWM tables
$418000	24 KB	SCSI Manager, Disk Cache, HFS
$41E000	8 KB	I/O traps, Declarations, Copyright

SCSI Sub-system

• Western Digital WD 5380 NCR-compatible controller
• 5 MB s⁻¹ 8-bit synchronous (practically 1.2 MB s⁻¹ async)
• DMA-like handshake handled in software by 68000 — no true DMA

Floppy-Disk Format (800 KB GCR)

• 80 tracks • double-sided variable-speed
• 12 → 8 sectors/track (outer→inner) • 512-byte sectors
• 800 KB formatted • still uses IWM (not yet SWIM)

Key I/O ICs

• 6522 VIA × 2 — timers, sound PWM, keyboard/mouse handshake
• 8530 SCC — dual RS-422 serial (230 kbps)
• IWM — Integrated Woz Machine floppy controller
• WD 5380 — 8-bit SCSI controller
• MC6883 PAL — VBL interrupt & DRAM cycle arbiter

Sound Path

68000 → VIA Port A (8-bit sample) • VIA Timer 1 toggles Port B at 22.254 kHz → PWM → RC filter → LM380 → speaker / headphone jack.

RAM Upgrade Notes

• Uses four 30-pin SIMM sockets (Bank 0 & Bank 1).
• Accepts 256 KB or 1 MB SIMMs, 150 ns or faster.
• Mixed sizes allowed per bank (must match within a bank); 4 MB max.

Hardware Easter Egg

Boot while holding ⌘ + Option + ⇧ + Clear to hear the hidden *bass-only* startup chime — final compact-Mac model to include it.