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Sourced to Apple Macintosh Plus Service Source + Guide to the Macintosh Family Hardware: SIMM speed corrected 120ns -> 150ns or faster (Apple); 2.5MB config corrected (1MB pair in Bank A, 256K pair in Bank B - page had 1MB in A with 256K described as Bank B incorrectly); R8 identified as the 256K BIT resistor and R9 ONE ROW added; eight-chip and complete-row rules added; refs and reflist
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The Macintosh Plus originally shipped with '''1 MB of RAM''', which was a significant improvement over earlier compact Macintosh models. However, many applications and system software updates required additional memory to function efficiently. Fortunately, the Plus supports memory upgrades up to '''4 MB''', vastly improving performance and usability.
The '''[[Macintosh Plus]]''' ships with 1 MB of RAM in four 30-pin SIMMs and can be taken to '''4 MB''', which is the maximum the machine supports.<ref name="ssplus">Apple Computer, ''Macintosh Plus Service Source'' — Specifications (Memory) and Upgrades (Memory Upgrade) chapters. Hosted on this wiki: [[:File:Macintosh plus.pdf]].</ref>


=== Memory Specifications ===
== Memory specifications ==


The Macintosh Plus uses '''30-pin SIMM (Single Inline Memory Module)''' chips. Below are the supported memory configurations:
* '''SIMM type:''' 30-pin, '''150 ns or faster'''. Apple notes that the speed is given by the −xx suffix after the manufacturer's part number, so −15 is a 150 ns part.<ref name="ssplus" />
* '''Eight RAM ICs per SIMM.''' A nine-chip SIMM will work, but the ninth device is not connected to the logic board.<ref name="gtmfh">Apple Computer, ''Guide to the Macintosh Family Hardware'', 2nd edition, Addison-Wesley, 1990, Chapter 5 "Memory", pp. 196–198 — SIMM installation rules and the RAM SIZE resistors for the Macintosh Plus and Macintosh SE.</ref>
* '''SIMMs go in rows of two.''' Row 1 is SIMMs 1 and 2, row 2 is SIMMs 3 and 4. A row must be either empty or full; one SIMM on its own is not a valid configuration.<ref name="gtmfh" />
* All the RAM ICs in a row must be the same size and the same speed.<ref name="gtmfh" />
* If the two rows differ, '''the larger SIMMs go in row 1'''.<ref name="gtmfh" />
* '''Maximum: 4 MB.'''<ref name="ssplus" />


<templatestyles src="Template:StyledTable/styles.css" />
<templatestyles src="Template:StyledTable/styles.css" />
{| class="wikitable styled-table" style="width:60%; text-align:center;"
{| class="wikitable styled-table" style="width:75%; text-align:center;"
|+ '''Macintosh Plus RAM Configurations'''
|+ '''Macintosh Plus RAM configurations (Apple)'''
! Total RAM !! Bank A (row 1) !! Bank B (row 2) !! R8 "256K BIT" resistor
|-
|-
! Total RAM !! SIMM Configuration !! Individual SIMM Capacity
| 1 MB (stock) || 2 × 256 KB || 2 × 256 KB || fitted
|-
|-
| 1 MB (stock) || 4 × 256 KB || 256 KB
| 2.5 MB || 2 × 1 MB || 2 × 256 KB || removed
|-
|-
| 2 MB || 4 × 512 KB || 512 KB
| 4 MB || 2 × 1 MB || 2 × 1 MB || removed
|-
| 2.5 MB || 2 × 512 KB + 2 × 1 MB || Mixed (512 KB & 1 MB)
|-
| 4 MB (maximum) || 4 × 1 MB || 1 MB
|}
|}


* The Macintosh Plus requires SIMMs with '''120 ns or faster''' access time.
== The RAM size resistors ==
* All four SIMM slots must be populated with memory of the '''same capacity''' unless using a mixed 2.5 MB configuration.
 
** If you are using the 2.5 MB configuration, install the 1 MB SIMMs in Bank A and 256 KB SIMMs in Bank B.
The Plus tells its general logic circuits what memory is fitted through two resistors in the area silkscreened '''RAM SIZE''':<ref name="gtmfh" />
* If you are using '''any''' 1 MB SIMMs, you will need to clip the '''R8''' resistor.
 
[[File:Mac-plus-memory.png|none|thumb|520x520px|Macintosh Plus Memory Guide]]
* '''R8''', labelled '''256K BIT''' — fitted only when '''every''' SIMM contains 256 Kbit DRAMs. If two or four SIMMs contain 1 Mbit DRAMs, R8 must be removed.
* '''R9''', labelled '''ONE ROW''' — fitted only when just two SIMMs are installed. With four SIMMs fitted it must be removed.


=== Required Tools ===
Apple's method is to '''clip one end of the resistor and bend it away from the clipped lead''' rather than desoldering it, so that it can be tacked back later if the machine is returned to its original configuration.<ref name="ssplus" />


To upgrade your Macintosh Plus memory, you will need:
Both of the upgrades in the table above use four SIMMs and at least two 1 Mbit modules, so both require R8 to be clipped. Do not clip R9 as well unless you are actually running only two SIMMs.


* '''Torx T15 screwdriver''' (to open the case)
== Tools ==
* '''Plastic spudger''' or '''pry tool''' (to carefully separate the case)
* Long T15 Torx screwdriver
* '''Anti-static wrist strap''' (to prevent electrostatic discharge)
* Case separator or plastic spudger
* '''Compatible 30-pin SIMMs''' (matching capacity and speed)
* Anti-static wriststrap
* Matching 30-pin SIMMs (see [[Recommended Tools]])


=== Memory Upgrade Procedure ===
== Procedure ==


'''Step 1: Prepare the Macintosh Plus'''
'''1. Prepare.''' Power off, unplug, work on a grounded surface.
Ensure the Macintosh Plus is completely powered off and unplugged. To protect components from electrostatic discharge, work on a grounded surface and wear an anti-static wrist strap.


'''Step 2: Remove the Case'''
'''2. Open the case.''' Remove the Torx screws — two at the bottom and two deep in the handle recess — and separate the halves. The machine contains a CRT; read [[CRT Discharge Procedure]] first, discharge the CRT before disconnecting anything, and do not fit a wriststrap until after it has been discharged.<ref name="ssplus" />
Using a Torx T15 screwdriver, remove the four screws securing the case. Carefully separate the two halves using a plastic spudger. Be mindful of the CRT, as it retains high voltage even when powered off.


'''Step 3: Locate the SIMM Slots'''
'''3. Reach the logic board.''' The SIMM sockets are on the logic board towards the rear ports.
The memory slots are located on the '''logic board'''. Identify the four 30-pin SIMM slots near the rear ports.


'''Step 4: Remove Existing SIMMs'''
'''4. Remove the existing SIMMs.''' Ease the retaining clips outward — they are brittle — and tilt each module out.
If replacing existing SIMMs, carefully push apart the retention clips on each side of the SIMM slot and remove the modules.


'''Step 5: Install New SIMMs'''
'''5. Fit the new SIMMs''', matching the notch to the socket keying, in complete rows.
Align the new SIMMs with the slots, ensuring the notch on the SIMM matches the slot’s keying. Gently push the module into place until the retention clips snap securely around it. If you are upgrading using 1 MB SIMMs, '''snip the leg on the R8 resistor'''.


'''Step 6: Reassemble the Macintosh Plus'''
'''6. Set the resistors''' as above before closing up.
Once the memory is installed, reassemble the case and secure it with the Torx screws.


'''Step 7: Verify the Upgrade'''
'''7. Reassemble and verify.''' Boot and check the memory figure. If it is wrong, the usual causes are a partly seated module, a mismatched row, or a resistor still fitted.
Power on the Macintosh Plus and check the memory count on startup. If the full capacity is not recognized, ensure all SIMMs are properly seated.


=== Common Issues and Troubleshooting ===
== Common problems ==


<templatestyles src="Template:StyledTable/styles.css" />
{| class="wikitable styled-table" style="width:100%;"
{| class="wikitable styled-table" style="width:100%;"
! Symptom !! Likely cause !! Action
|-
|-
! Issue !! Possible Cause !! Solution
| Sad Mac or beeps on startup || Wrong, faulty or partly seated SIMMs || Confirm 30-pin, 150 ns or faster, eight-chip, fully seated
|-
|-
| Macintosh Plus beeps on startup || Incompatible or faulty SIMMs || Ensure SIMMs are 30-pin, 120 ns or faster, and properly seated.
| Memory count unchanged after upgrade || RAM size resistor still fitted || Clip R8 when any 1 Mbit SIMMs are installed
|-
|-
| Memory not recognized correctly || Mismatched SIMM capacities || Use four identical SIMMs or a supported 2.5 MB configuration.
| Only half the memory seen || A row is half-populated, or the larger pair is in row 2 || Fill rows completely; larger SIMMs in row 1
|-
|-
| System crashes or instability || Bad or damaged SIMMs || Replace with known-good, compatible memory modules.
| Random crashes || Mixed speeds within a row, or a failing module || One speed per row; swap in known-good modules
|}
|}


== Additional Resources ==
Sad Mac codes are decoded on [[Sad Mac Error Codes]] and with the [[Sad Mac Decoder]].


For further details, refer to:
== Related pages ==
* [[Macintosh Plus]]
* [[Macintosh Plus General Maintenance]]
* [[Macintosh Plus Troubleshooting]]
* [[Macintosh Plus Troubleshooting]]
* [[Macintosh Plus General Maintenance]]
* [[Sad Mac Error Codes]]
* [[:Category:Apple Service Source|Apple Service Source]]
* [[:Category:Apple Service Source|Apple Service Source]]
== References ==
<references />


[[Category:Apple Vintage Computers]]
[[Category:Apple Vintage Computers]]
[[Category:Macintosh Memory Upgrades]]
[[Category:Macintosh Memory Upgrades]]
[[Category:Macintosh Hardware Upgrades]]
[[Category:Macintosh Hardware Upgrades]]

Revision as of 18:14, 23 September 2026

The Macintosh Plus ships with 1 MB of RAM in four 30-pin SIMMs and can be taken to 4 MB, which is the maximum the machine supports.[1]

Memory specifications

  • SIMM type: 30-pin, 150 ns or faster. Apple notes that the speed is given by the −xx suffix after the manufacturer's part number, so −15 is a 150 ns part.[1]
  • Eight RAM ICs per SIMM. A nine-chip SIMM will work, but the ninth device is not connected to the logic board.[2]
  • SIMMs go in rows of two. Row 1 is SIMMs 1 and 2, row 2 is SIMMs 3 and 4. A row must be either empty or full; one SIMM on its own is not a valid configuration.[2]
  • All the RAM ICs in a row must be the same size and the same speed.[2]
  • If the two rows differ, the larger SIMMs go in row 1.[2]
  • Maximum: 4 MB.[1]
Macintosh Plus RAM configurations (Apple)
Total RAM Bank A (row 1) Bank B (row 2) R8 "256K BIT" resistor
1 MB (stock) 2 × 256 KB 2 × 256 KB fitted
2.5 MB 2 × 1 MB 2 × 256 KB removed
4 MB 2 × 1 MB 2 × 1 MB removed

The RAM size resistors

The Plus tells its general logic circuits what memory is fitted through two resistors in the area silkscreened RAM SIZE:[2]

  • R8, labelled 256K BIT — fitted only when every SIMM contains 256 Kbit DRAMs. If two or four SIMMs contain 1 Mbit DRAMs, R8 must be removed.
  • R9, labelled ONE ROW — fitted only when just two SIMMs are installed. With four SIMMs fitted it must be removed.

Apple's method is to clip one end of the resistor and bend it away from the clipped lead rather than desoldering it, so that it can be tacked back later if the machine is returned to its original configuration.[1]

Both of the upgrades in the table above use four SIMMs and at least two 1 Mbit modules, so both require R8 to be clipped. Do not clip R9 as well unless you are actually running only two SIMMs.

Tools

  • Long T15 Torx screwdriver
  • Case separator or plastic spudger
  • Anti-static wriststrap
  • Matching 30-pin SIMMs (see Recommended Tools)

Procedure

1. Prepare. Power off, unplug, work on a grounded surface.

2. Open the case. Remove the Torx screws — two at the bottom and two deep in the handle recess — and separate the halves. The machine contains a CRT; read CRT Discharge Procedure first, discharge the CRT before disconnecting anything, and do not fit a wriststrap until after it has been discharged.[1]

3. Reach the logic board. The SIMM sockets are on the logic board towards the rear ports.

4. Remove the existing SIMMs. Ease the retaining clips outward — they are brittle — and tilt each module out.

5. Fit the new SIMMs, matching the notch to the socket keying, in complete rows.

6. Set the resistors as above before closing up.

7. Reassemble and verify. Boot and check the memory figure. If it is wrong, the usual causes are a partly seated module, a mismatched row, or a resistor still fitted.

Common problems

Symptom Likely cause Action
Sad Mac or beeps on startup Wrong, faulty or partly seated SIMMs Confirm 30-pin, 150 ns or faster, eight-chip, fully seated
Memory count unchanged after upgrade RAM size resistor still fitted Clip R8 when any 1 Mbit SIMMs are installed
Only half the memory seen A row is half-populated, or the larger pair is in row 2 Fill rows completely; larger SIMMs in row 1
Random crashes Mixed speeds within a row, or a failing module One speed per row; swap in known-good modules

Sad Mac codes are decoded on Sad Mac Error Codes and with the Sad Mac Decoder.

References

  1. ↑ 1.0 1.1 1.2 1.3 1.4 Apple Computer, Macintosh Plus Service Source — Specifications (Memory) and Upgrades (Memory Upgrade) chapters. Hosted on this wiki: File:Macintosh plus.pdf.
  2. ↑ 2.0 2.1 2.2 2.3 2.4 Apple Computer, Guide to the Macintosh Family Hardware, 2nd edition, Addison-Wesley, 1990, Chapter 5 "Memory", pp. 196–198 — SIMM installation rules and the RAM SIZE resistors for the Macintosh Plus and Macintosh SE.
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