11 Apr
2007
11 Apr
'07
6:17 a.m.
I am looking
for information about this part; I have something of a
mystery on my hands.
> At issue is a 512K IBM memory card that is part of the PC/XT 370
> option (from 1984).
> The card I have has 18 stacked chips comprising 512K of RAM; the top
> chip of one of the pairs was broken off. That's the first thing that puzzles me. It was my
understanding that
these were 128Kbit chips, so 18 of them would give you 256K bytes (+
parity) of memory. Can you confirm, please, that 18 of them really are 512K
> The top chips are stamped MCM66128L20; the bottom
chips have no part number.
My mistake; I should have said there are two rows of 18 chips, 36 in all, comprising
512K. You can see pictures at:- www.xt370.net
Thanks for the suggestion -- removing both the old lower chip and the new upper chip and
trying one of the new chips, on the assumption that the each of the new single-DIP parts
replaces one of the old piggyback pairs, is the next logical step.
But the board is so rare that I don't have the courage to rework it experimentally;
it has run 96 hours of memory / system board tests without failure, and so for now I guess
I will leave well enough alone.
I suspect, as you do, that the bottom, unmarked part has a different select line; the
chips are certainly soldered pin-to-pin. The lack of a part number on the bottom chip
suggests the stacked pair was sold as a single part number (and the part number suggests a
128K DRAM); the fact that the new chips have the same part number and no piggybacked
partner suggests that entire 128K eventually moved inside one DIP.
The Motorola books I have access to from back in the day don't list this part. I
guess, based on the munitions docs I had to sign, that it was mil-spec.
Thanks,
Mike