Identify SIMMs
(Presence Detect and Chip Configuration)

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!)

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


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
In addition there is one other special pin which is used to detect modules with Error-Correcting Code (ECC) support.

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=70ns, 8=80ns, 85=85ns)
   4. Module ID resistors (R1=85ns, R2=80ns)

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


Kit 2L shows 10 wide chips with resistors across the top, but not on the end. If there are chips on one side, it is 4MB; two sides it is 8MB.

Kit 2R shows 9 or 10 chips per side, all mounted vertically. If there are two resistor locations on the right end labeled R1 and R2, it is 2MB. (R1 = 85ns, R2 = 80ns, R1 + R2 =
100ns.) There will either be 9 or 10 chips.

Memory-Module Kit 3


Kit 3 has 6 chips per side spaced evenly. If all the chips are the same size, (3R) it is 1MB. If the two center chips are smaller than the 4 outer chips, (3L) it is 4MB. These could be any speed.

Memory-Module Kit 4A and 4B


Kit 4A and 4B are both 4MB. 4A has 6 closely spaced chips on each side. There are no resistors, and it is 85ns (there might be a tiny FRU number on the end chip.) 4B shows the front and rear of another 4MB chip configuration. There are 8 chips on the front, and 4 on the rear.

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 Not connected

Pin
70
Pin
69
Pin
68
Pin
67
Size Speed Notes
oooo Free or Invalid
oooX1 MB120 ns
ooXo2 MB120 ns
ooXX2 MB70 ns
oXoo8 MB70 ns
oXoX16 MB70 ns
oXXo2 MB
32 MB
80 ns
70 ns
On older machines
IBM P/N 92F7205 or 92G7206
oXXX8 MB80 ns
Xooo Reserved (*1)
XooX1 MB85 ns
XoXo2 MB85 ns
XoXX4 MB70 ns
XXoo Reserved (*2)
XXoX1 MB100 nsNote (*3)
XXXo2 MB100 ns
XXXX4 MB80 nsNote (*4)
o - open circuit, X - connected to GND (via bridge or 0 ohm resistor)

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
ECC modules have Pin 48 tied to ground (GND) - pin 72

Pin
70
Pin
69
Pin
68
Pin
67
Size Speed Notes
oooo Free or Invalid
oooX16 MB 60 nsNon-Parity
ooXo32 MB 60 nsNon-Parity
ooXX4 MB 60 nsNon-Parity
oXoo8 MB
8MB
70 ns
70 ns
Non-Parity
IBM ECC (FRU 92F0098)
oXoX1 MB
16 MB
16MB
70 ns
70 ns
70 ns
Non-Parity
Non-Parity
IBM ECC (FRU 96F9100)
oXXo2 MB
32 MB
70 ns
70 ns
Non-Parity
oXXX4 MB70 nsNon-Parity
Xooo8 MB80 nsNon-Parity
XooX1 MB
16 MB
80 ns
80 ns
Non-Parity
XoXo2 MB
32 MB
80 ns
80 ns
Non-Parity
XoXX4 MB
4MB
80 ns
70 ns
Non-Parity
IBM ECC (FRU 92F0097)
XXoo16 MB70 nsIBM ECC
XXoX
XXXo
XXXX8 MB60 nsNon-Parity
o - open circuit, X - connected to GND (via bridge or 0 ohm resistor)


PD Bits SIMM Identification (IBM / Industry)

Pin
70
Pin
69
Pin
68
Pin
67
PD Bits
3 2 1 0
SIMM Type
o o X o 1101 32MB 60 ns Industry
o X X o 1001 32MB 70 ns Industry
X o X o 0101 32MB 80 ns Industry
o o o X 1110 16MB 60 ns Industry
o X o X 1010 16MB 70 ns IBM / Industry
X o o X 0110 16MB 80 ns Industry
o X o o 1011 8MB 70 ns IBM / Industry
o X X X 1000 8MB 80 ns IBM / Industry
o X X X 1000 4MB 70 ns Industry
X o X X 0100 4MB 70 ns IBM
X o X X 0100 4MB 80 ns Industry
X X X X 0000 4MB 80 ns IBM
o X X o 1001 2MB 70 ns Industry
o o X X 1100 2MB 70 ns IBM
X o X o 0101 2MB 80 ns Industry
o X X o 1001 2MB 80 ns IBM
X o X o 0101 2MB 85 ns IBM
o o o X 1110 2MB 100 ns IBM
o X o X 1010 1MB 70 ns Industry
X o o X 0110 1MB 80 ns Industry
X o o X 0110 1MB 85 ns IBM
X X o X 0010 1MB 100 ns IBM
o o o o 1111 Connector is empty
o - open circuit, X - connected to GND (via bridge or 0 ohm resistor)

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.

Pin PD Bit R#
70 PD3 R1
69 PD2 R2
68 PD1 R3
67 PD0 R4


SIMMulator
Jim Shorney sez:
   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 ;->


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