INMC 80 News

Instant diagnosis

In my pile of letters I’ve found a number of small queries which I haven’t replied to. I decided that it probably wasn’t worth it as most of my mail is at least 3 months old. So instead I’ll precis them here and maybe answer some of those unasked questions you feel some one might have the answer to.

M. R Hughes asks about the fault on the video where the video dot generator (IC65) on a Nascom 2 loads the front half of the video data twice. If you use a scope you’ll notice that the LD waveform on pin 1 of IC65 has a nasty spike in it. It comes from IC71. The cure, to change IC71 to a 74LS13.

Another is the sideways ‘weave’ of the N2 display. This is caused by the fact that the frame sync pulses are not exactly 50 Hz (about 50.15Hz actually). If you’re using a TV or monitor with poor smoothing, or there is ripple on your Nascom power supply then this will be apparent to a greater or lesser extent as the 50 Hz beats with the 50.15 Hz, and moves the line sync pulses in sympathy. There is no simple cure except to ensure that both Nascom and video monitor PSUs are ripple free. Bits and PCs have a mod up their sleeves which alters the frame rate to 50Hz, but this involves cutting tracks and wire links.

There have been some comments about the INMC book of Basic programs. One thing we totally missed when we printed it was that the old faithfull IBM didn’t have an up-arrow, instead it printed a fullstop. This had the unfortunate affect that some thing like X=30^2 got printed as X=30.2, which if course is something else entirely. We haven’t been through the book to find them, but watch out!!!

Another has been about saving a Basic program after a system ‘crash’. Now this should never ever happen in Basic, and if it does, then suspect a touch of memory plague (read the index in INMC 7 for all references to memory speed and plague). Anyway, given that something nasty has happened, a Basic program is likely to be complete up to a point, followed by rubbish. This is because Basic has lost the pointer to the next line. It is possible to reconstruct the program by going into memory using the machine code ‘M’ and ‘T’ commands, and once you understand the way Basic puts pointers into each line, terminate the last valid line with three NOPs. I won’t go into detail, ’cos if you’re clever enough to figure out how it works, you’re clever enough to sort the problem yourself. Otherwise ‘LIST’ the program to tape (refer to INMC 7 again) and then reload it just short of the point where the data becomes corrupt. Moral, always dump any program to tape before running it.

Watch out, pin connections to the serial and parallel outputs of Nascom 1 and 2 differ, make sure which version is meant before connecting anything to these pins.

Turning a parallel printer on and off from Basic is easy, assuming the U command is used and has already been activated. To turn on simply DOKE 0C78H (3192 decimal … I think) with the address of your print handler. To turn off, take a DEEK at 3192 when the ‘U’ command is unactivated (’cos it varies between NAS-SYS 1 and 3) and use that number when DOKEing location 3192.

One thats in the manual but yet remains a hardy perennial. You can enter lines up to 72 characters in length into the 8K Basic by the following sequence. Note that 72 is the maximum number of characters that can be entered.

MONITOR
X0
Z
Ok
1233 REM This is the line you wanted to enter and may be up to 72 chars
MONTTOR
N
Z
Ok