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Thread: PS/2 Model 25-XT Optional Memory Modules.

  1. #1

    Default PS/2 Model 25-XT Optional Memory Modules.

    Howdy folks. I have a PS/2 Model 25 with 512k installed, via two 256k 30 pin simms.

    There are six DIP sockets on the motherboard for expanding the RAM to 640k. Two are smaller than the others. The IBM documentation says....

    Code:
    Memory Module (64KB) (ZM1,ZM2) 	 	   00F2120
    Memory Module 64KB (U16,U22,U34,U35) 	   00F2121
    However, my google-fu is failing to tell me any manufacturer part numbers to search for on digikey or ebay, based on IBM's FRUs. Can anyone clue me in?

    Thanks!
    -- Lee

    May Be Interested In Acquiring: Mac IIsi, Quadra 840AV, Commodore PC(286+)/64C/128D, PC-era Tandy stuff, Peculiar Old Serial Terminals.
    Old Computer Fun! (Muh Projects Blahg) | Muh YouTube Channel that goes with it

  2. #2
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    Do you have a picture of the motherboard? Those chips sound like TAG SRAM and SRAM chips, which would be for CPU cache and not to add to the main system RAM.

  3. #3
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    According to the Model 25 Technical Reference,

    Code:
    The system board supports 640K bytes of read/write memory. The
    first 128K is optional and consists of four 64K by 4-bit and two 64K by
    1-bit chips. Sockets are provided on the system board for this
    optional memory.
    I think the 64k by 4-bit would be 41464 while the 64k by 1-bit would be 4164. There will be manufacturer specific suffixes and prefixes.

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    The reason it has the 1-bit chips is to store the parity bits for each 8-bit word. Most non-PC computers would only need the four 4464s to create a 128k memory.

  5. #5

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    Thank you, sirs!
    -- Lee

    May Be Interested In Acquiring: Mac IIsi, Quadra 840AV, Commodore PC(286+)/64C/128D, PC-era Tandy stuff, Peculiar Old Serial Terminals.
    Old Computer Fun! (Muh Projects Blahg) | Muh YouTube Channel that goes with it

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    Quote Originally Posted by Eudimorphodon View Post
    The reason it has the 1-bit chips is to store the parity bits for each 8-bit word. Most non-PC computers would only need the four 4464s to create a 128k memory.
    I wonder if the 4164s for parity are optional, or if the 30 pin SIMMs are the weird IBM proprietary modules that have parity on them.

  7. #7

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    Quote Originally Posted by GiGaBiTe View Post
    I wonder if the 4164s for parity are optional, or if the 30 pin SIMMs are the weird IBM proprietary modules that have parity on them.
    This may be a stupid question... but isn't parity support a common feature on 30pin SIMMs? All modules I have here contain a parity chip, and I remember that some of the ones back in my stash have, too. Or did IBM use a different pinout for parity?

  8. #8
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    Parity was an optional feature on 30 pin SIMM/SIPP modules. It was more expensive than normal non-parity modules due to the additional memory chip and required BIOS support to work. Having parity memory really didn't benefit the average user because it could only detect single bit errors and not correct them, it would just halt the system during a memory test and display an error.

    I have a pile of both parity and non-parity modules, both were equally common depending on how much you wanted to pay. Google Books doesn't immediately turn up anything, most memory was sold as system specific memory to make finding the right memory easier for the novice computer user, not necessarily the least expensive.

  9. #9

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    Quote Originally Posted by Svenska View Post
    T... isn't parity support a common feature on 30pin SIMMs? All modules I have here contain a parity chip, and I remember that some of the ones back in my stash have, too...
    I have a bag full of 30 pin SIMMs. Some are parity and some are non-parity, e.g., there are 9 chip (parity) and 8 chip (non-parity) modules as well as 3 chip (parity) and 2 chip (non-parity) modules. There are plenty of all flavors.
    PM me if you're looking for 3" or 5" floppy disks. EMail For everything else, Take Another Step

  10. #10

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    AFAIK, the Models 25 and 30 used standard 30-pin parity simms, while the rest of the PS/2 line used the awful proprietary ones.
    -- Lee

    May Be Interested In Acquiring: Mac IIsi, Quadra 840AV, Commodore PC(286+)/64C/128D, PC-era Tandy stuff, Peculiar Old Serial Terminals.
    Old Computer Fun! (Muh Projects Blahg) | Muh YouTube Channel that goes with it

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