72-pin SIMM

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72-pin SIMM or as most people call it PS/2 SIMM, as it was first introduced with the IBM PS/2 line of personal computers, is the most common memory for late socket 4, socket 5, early socket 7 and socket 8 motherboards. It is used as add on memory in caching controllers or on soundcards like the Terratec EWS64 as well. The modules have 72 Pins and a data with of the modules is 32 bit without and 36 bit with parity. While most Pentium class machines need the modules installed in pairs, 386er and 486er boards can use single modules as well. 72-pin memory was superseded by 168-pin SDRAM.

72-pin SIMMs are availiable in the following capacities:

  • 1 MB, 256 Kbit x 32/36 (rare)
  • 2 MB, 512 KBIT x 32/36 (common in IBM PS/2)
  • 4 MB, 1 Mbit x 32/36
  • 8 MB, 2 MBIT x 32/36
  • 16 MB 4 MBIT x 32/36
  • 32 MB 8 MBIT x 32/36
  • 64 MB 16 MBIT x 32/36 (rare, not supported on many motherboards)
  • 128 MB 32 MBIT x 32/36 (rare, used on some hp 9000 workstations)

It first entered the market as Fast Page Mode (FPM). From 1995 on EDO memory modules were released which were 10 - 30 % faster but refuse to work in some older hardware. The difference can often be noticd fom the appendix on the memory chips or fom the chips partnunber itself. -70 ns chips and partnumbers ending on 0 like 5117400 are usually FPM.

Parity modules

A pair of 16 MB PS/2 SIMM modules with parity. The resistors required for presence detect are in the upper right corner.

Like 30-pin SIMMS before 72-pin SIMMS were availiable with and without parity. Vice versa from the 30-pin modules which usually came with parity, 72-pin SIMMs with parity are not very common. Quite a number of older Compaq and IBM or higher priced equipment like servers and workstations require parity modules installed. Parity modules can be easily detected as they usually cary extra chips. While non parity modules use 2, 4, 8 or 16 chips, parity modules have 3, 6, 9, 10, 18 or 20 chips.


A pair of 32 MB EOS modules

For some high priced servers from IBM and hp EOS-modules were availiable. These were sometimes advertised as ECC managleble SIMM, while EOS means ECC on chip. These modules can be easily recognized by the square chips on the modules.

Presence detect

72-pin modules have four pins used for presence detection. Tis ment the board can detect size and speed of the memory module installed by evaluating the 4 presence detect pins. While these are not commonly used by most motherboard manufacturers namely IBM made heavy use of them. Presence detect can easily modified with minor soldering skills.

Non PC modules

A pair modules from a hp 9000 PA-RISC workstation

Some manufacturers of non x86 workstation released 72pin SIMMs not compatile with PCs. Notably IBM made some 40 Bit SIMMs for their RS/6000 line and hp modules from hp 9000 series are not compatible with PC-hardware.