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Macintosh Plus Memory Upgrade Guide

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The Macintosh Plus has no soldered RAM. It ships with 1 MB in four 256 KB 30-pin SIMMs and takes up to 4 MB.[1][2]

Memory rules

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Apple's Guide to the Macintosh Family Hardware gives these rules for the Plus:[3]

  • The RAM chips on each SIMM must have a RAS access time of 150 ns or faster. Apple's service manual adds that the speed is the -xx suffix after the chip maker's part number, so -15 means 150 ns.[1]
  • Each SIMM carries eight DRAM chips. A nine-chip SIMM works, but the ninth chip is not connected to the logic board.
  • The SIMMs work in pairs called rows. Row 1 is SIMMs 1 and 2 (Bank A), row 2 is SIMMs 3 and 4 (Bank B). A row must be empty or hold two SIMMs.
  • All the RAM chips in a row must be the same size and the same access time.
  • If the rows differ, the SIMMs with the larger chips go in row 1.

Configurations and the RAM size resistors

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Two resistors in the area of the logic board marked RAM SIZE tell the logic how many SIMMs are fitted and what size of DRAM they carry. The resistor labelled 256K BIT (R8) is fitted only when every SIMM uses 256 Kbit DRAMs. The resistor labelled ONE ROW (R9) is fitted only when just two SIMMs are installed. Both are 150 Ω.[3]

Macintosh Plus RAM configurations
Total RAM Row 1 / Bank A (SIMMs 1 and 2) Row 2 / Bank B (SIMMs 3 and 4) R8 (256K BIT) R9 (ONE ROW)
1 MB (as shipped) 2 × 256 KB 2 × 256 KB fitted not fitted
2 MB 2 × 1 MB empty not fitted fitted
2.5 MB 2 × 1 MB 2 × 256 KB not fitted not fitted
4 MB 2 × 1 MB 2 × 1 MB not fitted not fitted

Source: Guide to the Macintosh Family Hardware Figure 5-2.[3] Apple's service manual lists only the 1, 2.5 and 4 MB configurations.[1]

Apple's service method for R8 is to clip one lead and bend the resistor away from it, so it can be tacked back if the machine is returned to 1 MB.[1] Apple's Memory Guide calls the 256K resistor "R8 or R9", while the Guide to the Macintosh Family Hardware gives R8 as 256K BIT and R9 as ONE ROW.[2][3] Go by the silkscreen label next to the resistor.

Mac-plus-memory
Apple Memory Guide drawing of the Macintosh Plus logic board: SIMM Bank A, Bank B and resistors R8 and R9

Tools

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  • Long-shaft T15 Torx screwdriver; the screws under the handle are deeply recessed[4]
  • Case-cracking (pull-apart) tool
  • Anti-static wrist strap, to be put on only after the CRT has been discharged
  • Flush cutters for R8 and R9
  • Matching 30-pin SIMMs (see Recommended Tools)

Procedure

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The Plus contains a CRT. Read CRT Discharge Procedure first.

  1. Switch off and unplug the machine.
  2. Remove the battery compartment cover and any programmer's (reset/interrupt) switch. Remove the case screws, including the one inside the battery compartment, and prise the rear housing off with the pull-apart tool.[1]
  3. Discharge the CRT. Do not wear a grounding wrist strap until this is done.[1]
  4. Disconnect the floppy drive cable and the logic board cable from the logic board. Pull back on the top of the logic board and slide it out, taking care not to damage the SIMMs, especially the taller DIP-type SIMMs some boards carry.[1]
  5. Ease the retaining clips outward and tilt each old SIMM out of its socket.
  6. Fit the new SIMMs in complete rows, larger modules in Bank A, matching each SIMM's notch to its socket.
  7. Set R8 and R9 for the new configuration as in the table.
  8. Slide the logic board back in, reconnect both cables and refit the housing.
  9. Switch on and check the memory size in "About the Finder".

Problems after an upgrade

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Symptom Check
Sad Mac with a code from 030001 to 030080 SIMMs 1 and 3 (one per row). The Dead Mac Scrolls gives these codes for a fault in that pair.[5]
Sad Mac with a code from 030100 to 038000 SIMMs 2 and 4 (one per row).[5]
Wrong amount of memory reported R8 and R9 against the table; a row holding a single SIMM; the larger pair in row 2.[3]
Apple's MacTest reports a 128K/512K when testing a 1 MB Plus Apple's cure is a new logic board.[6]

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

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References

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  1. ↑ 1.0 1.1 1.2 1.3 1.4 1.5 1.6 Apple Computer, Macintosh Plus Service Source, hosted as File:Macintosh Plus.pdf (see Macintosh Plus Service Source). Specifications, Memory p. 3; Take Apart, Cover pp. 1-2 and Logic Board pp. 7-12; Upgrades, Memory Upgrade pp. 1-3.
  2. ↑ 2.0 2.1 Apple Computer, Memory Guide, "Early Macintosh" p. 1 (Macintosh Plus), reproduced in File:Mac-plus-memory.png.
  3. ↑ 3.0 3.1 3.2 3.3 3.4 Apple Computer, Guide to the Macintosh Family Hardware, 2nd edition, Addison-Wesley, 1990, chapter 5 "Memory", pp. 197-199 (SIMM rules and RAM size resistors) and Figure 5-2, p. 202 (Macintosh Plus SIMM configurations). Hosted as File:Apple Guide to the Macintosh Family Hardware 2nd Edition 1990.pdf.
  4. ↑ Torx T15 Macintosh Case Tool for 128k, Plus, SE, SE/30, Console5—link(accessed 2026-09-30)
  5. ↑ 5.0 5.1 Larry Pina, The Dead Mac Scrolls, Peachpit Press, 1992, pp. 115-116. Hosted as File:The Dead Mac Scrolls 1992.pdf.
  6. ↑ Apple, Macintosh Plus Service Source, Troubleshooting, Miscellaneous p. 10.


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