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Revisting the IBM PC AT Extended Memory Card Problem...


Sep 24, 2013
Okay, mystery solving time. Everyone put on your thinking caps, please...

As those of you'se who have been around for a while know, I have an IBM PC AT which I've been trying to get 3MB of extended memory into, because it's the max amount of RAM the IBM extended memory card can address. The card is a 16-bit full-length ISA comprised of 12 30-pin SIMM slots, a 9 and 25 pin serial/parallel connector, and an 8-pin DIP switch.

I originally got the card with 2 sticks of 9-chip-parity 1MB RAM for a grand total of 2MB of extended memory. It is originally stated through IBM literature that this card was only meant to take 256K SIMM's (256K x 12 slots gives total of 3MB, max RAM), yet it clearly appears that larger SIMM's can work in here as well, obviously. With the card in this state it works in the system. Also stated in IBM literature is that the PC AT only works with parity RAM.

As most of you'se are aware, parity SIMM RAM comes in either 3-chip or 9-chip form. The 1MB SIMM's currently installed are 9-chip parity. I have tried various 3-chip SIMM's of both a parity and non-parity nature, and it doesn't show up in the system at all if installed alone; if installed with the existing 2 sticks of 1MB SIMM's (which are clearly known to be working) no extended RAM is registered in the system.

Also curious about this card is that it's serial and parallel ports do not work. They are strangely enough registered as addressable ports in the system (using Snooper.exe), yet cannot be used or show up in the installed system options (using the CMOS/setup disk).

Now, here come my questions:

  • Is there a difference between 3-chip parity and 9-chip parity SIMM's? It seems that 3-chip SIMM's do not work in the memory card, yet 9-chip SIMM's will, unless mixed with 3-chip parity SIMM's.
  • Do the settings on the 8-pin DIP switch on the card have to be altered everytime a different amount or combination of RAM, or are they just left alone no matter what? A 128KB base memory card has no DIP switch; is more or less plug-n-play. Why is this card so different? perhaps this is also related to the serial/parallel ports non-functionability as well?
  • For those who have the same card (IBM part no. 55x3543), can you tell me what type of RAM you have (3-chip, 9-chip, 256K, 1MB, etc?), how many sticks, if your ports work, and what DIP switch settings you have?
While most of you'se will just agree that 2MB of extended memory in an 80286 system is more then enough, I like the idea of having certain items maxed out in systems. In the case of my PC AT, I want it to have the maximum amount of historically correct RAM (12, 256K sticks) available in it. Plus I find mysteries fun to solve! Heehee.

Plus the information used to solve this problem will be readily available on the Internet, to help out anyone else using an IBM extended memory card and having the same problems as I...