80-Bus News

   

January–February 1984 · Volume 3 · Issue 1
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Utters from the gutters

By Mick Waters

Before getting down to the meaty bits, I should say that I am one of Dr. Dark’s dodos in that I have never before submitted anything for publication. In the past I haven’t subscribed regularly to either the INMC or 80-BUS News and used to pick up my copies about once a year on my rare excursions to one of the London dealers. This year I made a resolution to persuade (con) a relative into buying me a subscription as a Christmas present and so now I should be able to read the latest scandal almost as it happens. I don’t pretend to be an authority on Nascom hardware but have managed over the past five years or so to become reasonably acquainted with the way they work. Mind you, being an RAF technician, I am an expert on modern electronic techniques (circa 1960) and can write volumes on pentodes, triodes and other glass encapsulated transistors with heaters. Since I have been about 12 months behind everyone else, this article may be too late for solving the problems of a couple of readers who asked for advice as far back as the March/April 83 issue but here goes regardless.

Nascom/SIMON

The first problem(s) of interest came from S. Willmott and from the Sept/Oct 83 issue of Dr. Darks diary. First Dr. Dark, your problem with SIMON getting in the way when verifying a page-mode style RAM-DISK appears to me to be due to the fact that the Nascom computers do not support page mode and SIMON will be present (and generating a RAMDIS signal) no matter what page of RAM is selected. The answer? Read on.

Mr Willmott asked how SIMON may be removed from the system once its job is done. I did (yet) another mod to my N2 as I objected to losing 4k of RAM and also to the prospect of buying a page mode EPROM board for one chip. My solution is not very elegant but it works. Below is a table of advantages and disadvantages which should be weighed up before going any further:

For it

  1. Its CHEAP. (Free in fact)
  2. It doesn’t involve hacking your faithful Nascom around.
  3. It works.

Against it

  1. The tape drive may no longer be used for anything else.
  2. SIMON doesn’t work apart from booting the system up as it effectively switches itself out.
  3. You must power up with a disk in drive A.
  4. Pressing RESET doesn’t work any more. The only way to reset is to switch off then switch back on. This creates problems when a program goes into a loop as you can’t then RESET and examine the memory.
  5. This mod. may only be used if an IVC is fitted as chips are borrowed from the redundant Nascom video circuitry.

In my opinion, the advantages outweigh the disadvantages, you may not think so. If you decide to try this mod, proceed as follows:

  1. Pull out the chips used for the video circuitry except IC8.
  2. On LKS1 remove all links except for the link between pins 3 and 14.
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