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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. | 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.<ref>{{cite web |url=https://www.apple-history.com/plus |title=Macintosh Plus |publisher=Apple-History |access-date=2025-01-25}}</ref><ref>{{cite web |url=https://support.apple.com/kb/TA31885?locale=en_US |title=Macintosh Plus: Technical Specifications |publisher=Apple Support |access-date=2025-01-25}}</ref> 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.<ref>{{cite book |last=Hertzfeld |first=Andy |title=Revolution in the Valley: The Insanely Great Story of How the Mac Was Made |publisher=O'Reilly Media |year=2005 |isbn=0596007191}}</ref> | ||
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. | 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.<ref>{{cite web |url=https://www.computerhistory.org/timeline/1990/ |title=1990 Timeline of Computer History |publisher=Computer History Museum |access-date=2025-01-25}}</ref> 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.<ref>{{cite web |url=https://lowendmac.com/1986/macintosh-plus/ |title=Macintosh Plus |publisher=Low End Mac |access-date=2025-01-25}}</ref> | ||
== Architecture and Processor == | == Architecture and Processor == | ||
The Macintosh Plus is powered by the '''Motorola MC68000''' microprocessor, operating at 8MHz. This 16/32-bit processor features: | The Macintosh Plus is powered by the '''Motorola MC68000''' microprocessor, operating at 8MHz.<ref>{{cite web |url=http://www.easy68k.com/paulrsm/doc/dpbm68k1.htm |title=Design Philosophy Behind Motorola's MC68000 |author=Thomas W. Starnes |publisher=BYTE Magazine |date=April 1983 |access-date=2025-01-25}}</ref> This 16/32-bit processor features: | ||
* '''32-bit internal registers''' with 16-bit data bus | * '''32-bit internal registers''' with 16-bit data bus | ||
* '''16-bit address bus''' supporting up to 16 MB of addressable memory | * '''16-bit address bus''' supporting up to 16 MB of addressable memory | ||
* '''8 MHz clock speed''' providing improved performance over earlier Macintosh models | * '''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. | 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.<ref>{{cite book |last=Williams |first=Gregg |title=The Apple Macintosh Computer |publisher=BYTE |year=1984 |isbn=0070705151}}</ref> | ||
== Memory and Storage == | == Memory and Storage == | ||
| Line 39: | Line 39: | ||
=== RAM Configuration === | === RAM Configuration === | ||
* '''Standard RAM''': 1 MB of RAM standard, expandable to 4 MB | * '''Standard RAM''': 1 MB of RAM standard, expandable to 4 MB<ref>{{cite web |url=https://support.apple.com/kb/TA31885?locale=en_US |title=Macintosh Plus: Technical Specifications |publisher=Apple Support |access-date=2025-01-25}}</ref> | ||
* '''Memory Architecture''': The Mac Plus was the first Apple computer to utilize user-upgradable SIMM memory modules instead of single DIP DRAM chips | * '''Memory Architecture''': The Mac Plus was the first Apple computer to utilize user-upgradable SIMM memory modules instead of single DIP DRAM chips<ref>{{cite web |url=https://archive.org/details/AppleMacintoshPlusBrochure1986 |title=Macintosh Plus Product Brochure |publisher=Apple Computer, Inc. |year=1986 |access-date=2025-01-25}}</ref> | ||
* '''SIMM Slots''': Four SIMM slots were provided and the computer shipped with four 256 KB SIMMs, for 1 MB total RAM | * '''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 | * '''Maximum Capacity''': By replacing them with 1 MB SIMMs, it was possible to have 4 MB of RAM | ||
=== ROM and System Software === | === 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. | 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).<ref>{{cite web |url=https://support.apple.com/kb/TA32938?locale=en_US |title=Macintosh Plus: ROM Versions |publisher=Apple Support |access-date=2025-01-25}}</ref> 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 === | === 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. | 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.<ref>{{cite web |url=https://support.apple.com/kb/TA48218?locale=en_US |title=Macintosh Plus: Using the 800K Disk Drive |publisher=Apple Support |access-date=2025-01-25}}</ref> | ||
== Display and Graphics == | == Display and Graphics == | ||
The Macintosh Plus shipped with a 9-inch 512x342 display at 72 PPI (pixels per inch). The display specifications include: | The Macintosh Plus shipped with a 9-inch 512x342 display at 72 PPI (pixels per inch).<ref>{{cite web |url=https://support.apple.com/kb/TA31885?locale=en_US |title=Macintosh Plus: Technical Specifications |publisher=Apple Support |access-date=2025-01-25}}</ref> The display specifications include: | ||
* '''Screen Size''': 9 inches (23 cm) diagonal | * '''Screen Size''': 9 inches (23 cm) diagonal | ||
* '''Resolution''': 512 × 342 pixels monochrome | * '''Resolution''': 512 × 342 pixels monochrome | ||
* '''Display Type''': Built-in CRT monitor | |||
* '''Pixel Density''': 72 pixels per inch | * '''Pixel Density''': 72 pixels per inch | ||
== Audio Capabilities == | == Audio Capabilities == | ||
The Macintosh Plus utilized one 8-bit, 22 kHz built in speaker. No audio in support was provided on this model. | The Macintosh Plus utilized one 8-bit, 22 kHz built in speaker. No audio in support was provided on this model.<ref>{{cite web |url=https://support.apple.com/kb/TA31885?locale=en_US |title=Macintosh Plus: Technical Specifications |publisher=Apple Support |access-date=2025-01-25}}</ref> | ||
== Input/Output and Expansion == | == Input/Output and Expansion == | ||
| Line 67: | Line 67: | ||
=== SCSI Interface === | === 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. | The Macintosh Plus was the first Macintosh to have a SCSI port to allow external expansion.<ref>{{cite web |url=https://support.apple.com/kb/TA48273?locale=en_US |title=Macintosh Plus: SCSI Basics |publisher=Apple Support |access-date=2025-01-25}}</ref> 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.<ref>{{cite web |url=https://www.macworld.com/article/156994/scsi.html |title=SCSI Secrets |publisher=Macworld |date=1993-04-01 |access-date=2025-01-25}}</ref> | ||
=== Port Configuration === | === 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. | 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.<ref>{{cite web |url=https://support.apple.com/kb/TA31885?locale=en_US |title=Macintosh Plus: Technical Specifications |publisher=Apple Support |access-date=2025-01-25}}</ref> | ||
== General Maintenance == | == General Maintenance == | ||
Over time, Macintosh Plus computers may experience issues common to vintage electronics, such as: | Over time, Macintosh Plus computers may experience issues common to vintage electronics, such as: | ||
* '''Capacitor aging''' leading to power supply instability or complete failure | * '''Capacitor aging''' leading to power supply instability or complete failure<ref>{{cite web |url=https://68kmla.org/forums/topic/62234-mac-plus-analog-board-repair/ |title=Mac Plus Analog Board Repair |publisher=68kMLA Forums |access-date=2025-01-25}}</ref> | ||
* '''Overheating problems''' due to lack of internal cooling fan | * '''Overheating problems''' due to lack of internal cooling fan | ||
* '''CRT display issues''' including loss of deflection or brightness degradation | * '''CRT display issues''' including loss of deflection or brightness degradation | ||
| Line 81: | Line 81: | ||
* '''Analog board component failure''' particularly in power regulation circuits | * '''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 Maintenance Guide]]. | 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.<ref>{{cite book |last=Pogue |first=David |title=Macworld Mac Secrets |publisher=IDG Books |year=1993 |isbn=1568840594}}</ref> 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 Maintenance Guide]]. | ||
== PCB Schematics & Service Manual == | == PCB Schematics & Service Manual == | ||
| Line 124: | Line 124: | ||
! Sub-system !! Specification (Macintosh Plus, Jan 1986) | ! Sub-system !! Specification (Macintosh Plus, Jan 1986) | ||
|- | |- | ||
| '''CPU''' || Motorola 68000 @ 7.8336 MHz (15.667 MHz ÷ 2) | | '''CPU''' || Motorola 68000 @ 7.8336 MHz (15.667 MHz ÷ 2)<ref>{{cite web |url=http://www.easy68k.com/paulrsm/doc/dpbm68k1.htm |title=Design Philosophy Behind Motorola's MC68000 |author=Thomas W. Starnes |publisher=BYTE Magazine |date=April 1983 |access-date=2025-01-25}}</ref> | ||
|- | |- | ||
| '''Bus width''' || 16-bit data • 24-bit address (16 MB logical) | | '''Bus width''' || 16-bit data • 24-bit address (16 MB logical) | ||
|- | |- | ||
| '''ROM''' || '''128 KB | | '''ROM''' || '''128 KB "v4.0"''' Toolbox / HFS / SCSI Manager<ref>{{cite web |url=https://support.apple.com/kb/TA32938?locale=en_US |title=Macintosh Plus: ROM Versions |publisher=Apple Support |access-date=2025-01-25}}</ref> | ||
|- | |- | ||
| '''RAM''' || 1 MB on four 256-K × 1 DRAM SIMMs — expands to '''4 MB''' via 30-pin SIMM slots | | '''RAM''' || 1 MB on four 256-K × 1 DRAM SIMMs — expands to '''4 MB''' via 30-pin SIMM slots | ||
| Line 157: | Line 157: | ||
|} | |} | ||
'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 | === ROM Layout (128 KB, v4.0 "Plus-class") === | ||
{| class="wikitable styled-table" style="width:80%;" | {| class="wikitable styled-table" style="width:80%;" | ||
! Offset !! Size !! Module | ! Offset !! Size !! Module | ||
| Line 177: | Line 177: | ||
=== SCSI Sub-system === | === SCSI Sub-system === | ||
* Western Digital '''WD 5380''' NCR-compatible controller | * Western Digital '''WD 5380''' NCR-compatible controller<ref>{{cite web |url=https://support.apple.com/kb/TA48273?locale=en_US |title=Macintosh Plus: SCSI Basics |publisher=Apple Support |access-date=2025-01-25}}</ref> | ||
* 5 MB s⁻¹ 8-bit synchronous (practically 1.2 MB s⁻¹ async) | * 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 | * DMA-like handshake handled in software by 68000 — no true DMA | ||
| Line 184: | Line 184: | ||
* 80 tracks • double-sided variable-speed | * 80 tracks • double-sided variable-speed | ||
* 12 → 8 sectors/track (outer→inner) • 512-byte sectors | * 12 → 8 sectors/track (outer→inner) • 512-byte sectors | ||
* 800 KB formatted • still uses IWM (not yet SWIM) | * 800 KB formatted • still uses IWM (not yet SWIM)<ref>{{cite web |url=https://support.apple.com/kb/TA48218?locale=en_US |title=Macintosh Plus: Using the 800K Disk Drive |publisher=Apple Support |access-date=2025-01-25}}</ref> | ||
=== Key I/O ICs === | === Key I/O ICs === | ||
* '''6522 VIA × 2''' — timers, sound PWM, keyboard/mouse handshake | * '''6522 VIA × 2''' — timers, sound PWM, keyboard/mouse handshake<ref>{{cite web |url=http://www.easy68k.com/paulrsm/doc/dpbm68k2.htm |title=Design Philosophy Behind Motorola's MC68000 (Part 2) |author=Thomas W. Starnes |publisher=BYTE Magazine |date=May 1983 |access-date=2025-01-25}}</ref> | ||
* '''8530 SCC''' — dual RS-422 serial (230 kbps) | * '''8530 SCC''' — dual RS-422 serial (230 kbps) | ||
* '''IWM''' — Integrated Woz Machine floppy controller | * '''IWM''' — Integrated Woz Machine floppy controller | ||
| Line 199: | Line 199: | ||
* Uses four 30-pin SIMM sockets (Bank 0 & Bank 1). | * Uses four 30-pin SIMM sockets (Bank 0 & Bank 1). | ||
* Accepts 256 KB or 1 MB SIMMs, 150 ns or faster. | * Accepts 256 KB or 1 MB SIMMs, 150 ns or faster. | ||
* Mixed sizes allowed per bank (must match within a bank); 4 MB max. | * Mixed sizes allowed per bank (must match within a bank); 4 MB max.<ref>{{cite web |url=https://support.apple.com/kb/TA48210?locale=en_US |title=Macintosh Plus: Memory Upgrade |publisher=Apple Support |access-date=2025-01-25}}</ref> | ||
=== Hardware Easter Egg === | === Hardware Easter Egg === | ||
Boot while holding '''⌘ + Option + ⇧ + Clear''' to hear the hidden *bass-only* startup chime — final compact-Mac model to include it. | Boot while holding '''⌘ + Option + ⇧ + Clear''' to hear the hidden *bass-only* startup chime — final compact-Mac model to include it.<ref>{{cite book |last=Hertzfeld |first=Andy |title=Revolution in the Valley: The Insanely Great Story of How the Mac Was Made |publisher=O'Reilly Media |year=2005 |isbn=0596007191}}</ref> | ||
== References == | == References == | ||
Revision as of 13:26, 30 May 2025
 | |
| 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 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.[1][2] 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.[3]
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.[4] 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.[5]
Architecture and Processor
The Macintosh Plus is powered by the Motorola MC68000 microprocessor, operating at 8MHz.[6] 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.[7]
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[8]
- Memory Architecture: The Mac Plus was the first Apple computer to utilize user-upgradable SIMM memory modules instead of single DIP DRAM chips[9]
- 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).[10] 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.[11]
Display and Graphics
The Macintosh Plus shipped with a 9-inch 512x342 display at 72 PPI (pixels per inch).[12] 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.[13]
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.[14] 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.[15]
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.[16]
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[17]
- 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.[18] 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 Maintenance Guide.
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 |
|---|---|
  |
  |
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)[19] |
| Bus width | 16-bit data • 24-bit address (16 MB logical) |
| ROM | 128 KB "v4.0" Toolbox / HFS / SCSI Manager[20] |
| 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[21]
- 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)[22]
Key I/O ICs
- 6522 VIA × 2 — timers, sound PWM, keyboard/mouse handshake[23]
- 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.[24]
Hardware Easter Egg
Boot while holding ⌘ + Option + ⇧ + Clear to hear the hidden *bass-only* startup chime — final compact-Mac model to include it.[25]
References
- ↑ Macintosh Plus, Apple-History—link(accessed 2025-01-25)
- ↑ Macintosh Plus: Technical Specifications, Apple Support—link(accessed 2025-01-25)
- ↑
Revolution in the Valley: The Insanely Great Story of How the Mac Was Made, O'Reilly Media
- ISBN 0596007191}
- ↑ 1990 Timeline of Computer History, Computer History Museum—link(accessed 2025-01-25)
- ↑ Macintosh Plus, Low End Mac—link(accessed 2025-01-25)
- ↑ Design Philosophy Behind Motorola's MC68000– Thomas W. Starnes, BYTE Magazine, April 1983—link(accessed 2025-01-25)
- ↑
The Apple Macintosh Computer, BYTE
- ISBN 0070705151}
- ↑ Macintosh Plus: Technical Specifications, Apple Support—link(accessed 2025-01-25)
- ↑ Macintosh Plus Product Brochure, Apple Computer, Inc.—link(accessed 2025-01-25)
- ↑ Macintosh Plus: ROM Versions, Apple Support—link(accessed 2025-01-25)
- ↑ Macintosh Plus: Using the 800K Disk Drive, Apple Support—link(accessed 2025-01-25)
- ↑ Macintosh Plus: Technical Specifications, Apple Support—link(accessed 2025-01-25)
- ↑ Macintosh Plus: Technical Specifications, Apple Support—link(accessed 2025-01-25)
- ↑ Macintosh Plus: SCSI Basics, Apple Support—link(accessed 2025-01-25)
- ↑ SCSI Secrets, Macworld, 1993-04-01—link(accessed 2025-01-25)
- ↑ Macintosh Plus: Technical Specifications, Apple Support—link(accessed 2025-01-25)
- ↑ Mac Plus Analog Board Repair, 68kMLA Forums—link(accessed 2025-01-25)
- ↑
Macworld Mac Secrets, IDG Books
- ISBN 1568840594}
- ↑ Design Philosophy Behind Motorola's MC68000– Thomas W. Starnes, BYTE Magazine, April 1983—link(accessed 2025-01-25)
- ↑ Macintosh Plus: ROM Versions, Apple Support—link(accessed 2025-01-25)
- ↑ Macintosh Plus: SCSI Basics, Apple Support—link(accessed 2025-01-25)
- ↑ Macintosh Plus: Using the 800K Disk Drive, Apple Support—link(accessed 2025-01-25)
- ↑ Design Philosophy Behind Motorola's MC68000 (Part 2)– Thomas W. Starnes, BYTE Magazine, May 1983—link(accessed 2025-01-25)
- ↑ Macintosh Plus: Memory Upgrade, Apple Support—link(accessed 2025-01-25)
- ↑
Revolution in the Valley: The Insanely Great Story of How the Mac Was Made, O'Reilly Media
- ISBN 0596007191}