Upgrading NatSemi NS23C QBUS memory

Noel Chiappa jnc at mercury.lcs.mit.edu
Mon Jan 12 08:36:02 CST 2015

So it turns out the NatSemi NS23C QBUS memory card can really, really easily
be upgraded from 256KB to 1MB, as it has all the necessary traces, jumpers,
etc for this capability built into the card - even though the NS23C
documentation says nothing about this capability!

I found out about this capability when I bought a group of QBUS cards which
included two NS23C cards. Looking at the chips, trying to figure out how big
the cards were (I didn't at that point know the card model), I saw one had
64Kx1 chips, the other had 256Kx1 - one was 256KB, the other 1MB!

Later, looking at the prints, I noticed that the memory chips had all 9
address lines wired (unlike the very similar NS23M card), and there were a
couple of jumpers that appeared to adapt the card to 1MB operation.

So I tried upgrading the second card, and it worked!

The chips are all in sockets, so pulling the 64Kx1's and replacing them with
256Kx1's was easy. There are three jumpers one has to remove/move; alas, they
are in the PCB on the top surface, although there are jumper pins there -
there's a trace running between the two pins - so you have to cut the traces.

The first two jumpers one has to remove are W23 and W24 (right next to the
other memory size jumpers), which allow one to increase the maximum memory
size to 1MB.

The other jumper one has to move, is to move the 'jumper' from W40 to W41;
this moves the pickup point for the 'RS0' signal, which indicates which bank
of chips (there are 2x18 banks, i.e. 16 data, and separate byte parity) to
activate, from address line 17 to 19.

(Parenthetically, the way the address logic works on the card is slightly
odd; if the card is not on a 'natural' boundary [e.g. a 256KB boundary, if
it's a 256KB card], the memory contents are scrambled; the low memory, in bus
address terms, is at the top of the card, in chip terms, and the high memory,
in bus terms, is at the bottom of the card. I understand why they did it that
way, it's the most economical in logic/traces, etc but it's something one
would have to remember when looking for a bad memory chip, if the card is set
to an address which is not a multiple of its size.)

Not sure if anyone else out there has any of these cards, but if so, I'd be
interested to hear if anyone does this.


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