2 mysteries – 1 solved
MYSTERY PROGRAM FOR NAS-SYS
Type in the mystery program (use the ‘L’ command with the
checksums for error free loading), don’t contract the NOPs at the
beginning, they are important. Execute at 0D00H, and when you get fed
up with it, just RESET.
T D00 DB1
NASCOM 1 and 8K BASIC
-- a bit more
In the last two issues we have published details on how to fit
the Nascom Basic ROM to a series 1 memory card. We have since learned
the a good few hundred are working successfully, but that a couple of
dozen did some inexplicable things. We tended to ignore this as ‘finger
trouble’ until last week when one of the committee’s Nascom 1 suffered
an unfortunate accident. (It was discovered that the PIO input
protection is not proof against 240V AC !!!).
As only about three chips were still working, it was decided to
re-chip the Nascom from square one. As the Nascom was pretty well fully
expanded this was expensive, but all worked well except the Basic which
normally wouldn’t initialize, or when it did, only worked a few minutes
without crashing. We poked around and found the only difference made
when re-chipping was that IC9 on the buffer was originally a 74S04 (see
Memory plague notes in INMC 3), and as we didn’t have any spare S04s, a
74LS04 was used instead. Well we never believed that the extra 5nS the
S04 gave to the leading edge of MREQ was of any signicance, so how
about the extra capactiance an S04 imposed on MREQ ?
We tried zapping 150pF across pin 1 of IC9 to ground, and
lo !!!, working Basic. Just to prove this wasn’t a flook, we got in
touch with a dealer we knew had three boards supposedly not working,
borrowed them, and without the capacitor the Basic didn’t work, with
the capacitor it worked.
So if you have a Basic ROM fitted to a Nascom Series 1 memory,
and its giving you trouble try 150pF between pin 1 and earth of IC9 on
the buffer board. Don’t ask us what it’s doing, we haven’t dared look
(something horrible to MREQ we bet), but its worth a try.