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 |
The Macintosh Plus was the third model in Apple’s Macintosh line, introduced on January 16, 1986. It shipped with 1 MB of RAM (expandable to 4 MB) and introduced an external SCSI port, significantly improving over the Macintosh 512K. Initially beige-colored (Pantone 453), the case became platinum gray in 1987. The Plus could run up to System 7.5.5, introduced SIMM memory modules, and featured an 800 KB floppy drive.
General Maintenance
For detailed general maintenance practices including cleaning, PRAM battery handling, connector corrosion prevention, and voltage adjustments, please see the dedicated Macintosh Plus General Maintenance page.
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
| Logic Board | Analog Board |
|---|---|
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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)
| 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).
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.