On  9 Dec 98 at 21:57, Walt Ezyk wrote:

> Hi everybody, I've got a 486DX-75 with an IBM 486 Blue Lightning
> System Board. I've got 8 megs of RAM and I'd like to increase my
> RAM to 16 megs. According to the manual I need two 4 meg SIMMS
> specified as 1MX36 single density SIMM modules. I bought two 4 meg,
> parity 30 pin SIMMS (4MX9) and the pin arrangement doesn't seem to
> match my mother board and I sent them back. The new SIMMS had one
> row of 30 pins, and the mother board looks like it needs a SIMM
> with two in line pin segments with 36 pins in each segment with a
> small space(about 1/4") between the two pin segments. At this point
> I'm thoroughly confused and my questions are.

  SIMMs come in two sizes -- 30-pin, which are 8 or 9 bits wide, and
72-pin, which are 32 or 36 bits wide (the larger widths include a
parity bit for every 8 data bits).
  The 486 has a 32-bit-wide data bus, so if your system could use
30-pin SIMMs, it would need them in sets of four.

>        1. Did I do the right thing in sending the SIMMS back?

  They might be what you ordered, but they're not what your machine
will use.

>        2. Are there different configs of 30 pin SIMMS?

  Yes -- with and without parity.  Neither of which will fit your
board.

>        3. If the answers to 1 & 2 are affirmative, where can I get SIMMS for my
>           board?

  You need to get 72-pin SIMMs.  The "1x36" spec tells us that they
must be WITH parity.  I don't think your board is likely to support
EDO, so they should be FPM (Fast Page Mode).
  You might have to hunt around some.  Last year I installed a RAID
controller that was designed to take a 72-pin SIMM for cache memory,
in either 4MB or 16MB size.  I seem to recall that it was a little
challenging to find a new 16MB SIMM anywhere, and I don't think I saw
any 4MB 72-pin SIMMs for sale.

  [There are dughterboards available that will take a set of 4 30-pin
SIMMs and fit in a 72-pin slot.  I cannot recommend them, however.]

David G

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