Looking for schematics of QBUS 32KW memory module.

Noel Chiappa jnc at mercury.lcs.mit.edu
Sun Sep 8 15:20:02 CDT 2019

    > From: Mister PDP

    > listed back there were numerous bad addresses all over memory.
    > ...
    > I cannot find schematics for any of the boards

You can repair MOS memory boards where the board is basically working, but
just has some failing memory chips, without schematics.

First you need to create a map which translates memory chip # to bits. You
have 32 chips in the array, so there are probably 2 32KB banks, each 16 bits
wide. Pull a chip, and then try and figure out which bit it is; then repeat
with other chips to try and figure out which bits are stored in which
chips. (Unless the designers were insane, each chip will hold the same bit in
all the words in that bank.)

On yours, the memory chips are in sockets, which makes this less painful.
(On boards where the chips are soldered in, a program which loops, storing a
word with a single 1 bit, can be used to the same effect; the chip data sheet
will tell you which pin is the data pin.)

Usually a missing chip results in bits stored in that chip reading as '0', but
it's possible they will read back as 1. Anyway, to test the first possibility,
start by finding a location in the each bank that can be written to all 0's
and all 1's (read back after writing to verify).

Next, pull a chip, and then try writing all 1's to that word in the low bank,
and read it back.  If it now has a 0 bit, congratulations i) you've verified
that missing chips read as 0, ii) that chip is part of the low bank, and iii)
the 0 bit tells you which bit that chip is - fill in that entry in your
chip<->bit chart.

If not, try the high bank word, and see if it now has a 0 bit. If not, try
writing 0's to the high and low words, and check for a '1' bit; if so, i)
missing chips read as 1, etc. If neither is true, check back here!

Otherwise, try pulling another chip, and work out which bit that one is, and
add it to the chart. Repeat for all 32 chips - although if you're lucky,
after a couple you might find a pattern, and be able to predict which chips
hold which bits. (But not always; many are random; see e.g.:


for some.) If so, do a few spot tests of your predictions to make sure your
pattern is correct.

With the completed chart in hand, given a failing word (address and bad
data), you can work out which chip is at fault, and replace it. Repeat
for all memory errors.


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