Ardent Tool of Capitalism

Identify SIMMs

PS/1, PS/2, and PC Memory Reference

Introduction
Visual Identification of SIMM Module
SIMM Module Presence Detection Pins
   PD Bits SIMM Identification - IBM Parity Modules
   PD Bits SIMM Identification - Non-Parity and ECC Modules
   PD Bits SIMM Identification (IBM / Industry)
Resistor to PD Bit
SIMMulator (Socketed PD Tester, cheap!)

Introduction

Most newer IBM PS/2 machines use the 72-pin Single In-line Memory Modules (SIMM) on either planar or memory expansion cards. Very often there are more than one of these 72-pin connectors available and one might wish to upgrade the memory by adding another memory module.
But the machine comes up with a error 215 and according to the error code list it means "Check memory, it might be wrong type or wrong speed".

How can the system know that? It's pretty simple. There are 4 pins reserved on the module, which are called "Presence Detect Pins" (PD-pins) and which tell the planar:

• if there is a module installed
• total capacity of the module
• access-time of used memory chips

Visual Identification of SIMM Module

Some early memory module kits are not labeled with a FRU number. The following information provides methods of identification.

There are four key identifiers:

1. Chip size: narrow or wide
   
2. Chip placement (horizontal, vertical, both, 1 side or 2 sides)
   
3. Speed suffix shown on the individual chips (7=70 ns, 8=80 ns, 85=85 ns)
   
4. Module ID resistors (R1=85 ns, R2=80 ns)

Memory-Module Kit 1A and 1B

Kit 1A and 1B, show 10 or more chips mounted horizontally, vertically or a mixed pattern. If there are chips on one side of the board, it is 1MB. If there are chips on both sides, it is 2MB. (A small number of 8MB kits were also manufactured in this configuration.) The suffix on the chip, for example, -8, -85 is important.

Memory-Module Kit 2L and 2R

SIMM Module Presence Detection Pins

For years, I have been living easy with the Models 90 and 95, both use 70 ns FPM SIMMs, which are fairly common. But when faced with a 8573-P70, I was brought to my knees by it's demands for ONLY a 2MB 85 ns SIMM. Many of the older SIMMs are marked only with a P/N, the size and speed are not shown. Even worse, many individual IBM chips are marked only with IBM P/Ns... These chips lack even a speed rating.

From the PC and PS/2 Pocket Reference Manual, Feb 1996 (edited):
  The presence detect pins 67-70 tell the system the size and speed of the IBM standard 72 pin parity SIMMs. The pins are either not connected or connected to ground. Using good ESD procedures and a digital Volt/ohm meter, measure the resistance between pins 67-70 and 72 which is ground.

When you look at a SIMM, the notch is to the right. Pin 72 will be the leftmost pin (usually silk screened or part of PCB).
Note that the PD bits are binary, with LSB to the right (pin 67).
Measure resistance of pins 70, 69, 68 and 67 to ground (GND) - pin 72.

Warning! If you use a Multimeter to check the connections on the Presence Detection Pins, do not touch other pins than the 5 shown above! The memory module might get damaged from the testing voltage of your multimeter. Also obey the usual rules for handling ESD-sensitive components!

PD Bits SIMM Identification - IBM Parity Modules

Pin 48 is Not connected on Parity modules

Table sorted by presence detect (PD) information.

Notes:

1. This is an accepted coding for the 8MB module used in the N51SX - see Jim Shorney's Modification and THIS for details.
2. This is an accepted coding for the 4MB/80 ns module P/N 79F3988 / 79F1000 (FRU 79F1003) used on the L40SX (8543), see THIS as well.
3. This is an accepted coding for the 8MB/80 ns module P/N 79F3989 (FRU 79F1004) used on the L40SX (8543), see THIS as well.
   However: It must be a single-sided module with chips that have the typecode -LJP8 after the sizecode!
4. This is an accepted coding for the 2MB/80 ns module P/N 79F3987 (FRU 79F1002) used on the L40SX (8543), see THIS as well.

PD Bits SIMM Identification - Non-Parity and ECC Modules

Pin 48 is to ground (GND, pin 72) on ECC modules.

Table sorted by presence detect (PD) information.

PD Bits SIMM Identification (IBM / Industry)

Table sorted by module capacity.

Notes:
   Some Industry standard PD codes are the same as IBM's, yet differ in speed.
   Some systems "mask" PD bits, ex. the 8573-P70 masks PD0 and PD1.

Pin - PD Bit - Resistor #

Note that the Resistor numbers (R1, R2...) are in reverse to the PD bit number.

SIMMulator

Jim Shorney says:
   Whacked the end SIMM socket off of an old Dell motherboard with a hacksaw and soldered up four LED's with 1K current limiting resistors to pins 67-70.  Feed it 9 to 12 volts DC against pin 72, and it makes a nice PD code reader for aSIMMilating those unidentified memory modules into the proper baggie.  No more fumbling with an ohmmeter.

Tony Ingenoso has a safety announcement:
   Sacrifice a clone mobo for the socket.  A propane torch from the rear will usually drop 'em off pretty clean.  Gotta burn through the little attach pegs, but once that's done they drop right off...
   A pail of water is nice to have standing by.  Sometimes the boards light up pretty good and really start stinking and billowing clouds of black smoke. Don't want the neighbors to call the FD ;->