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

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.