The “How to fit 64K of EPROM to your Nascom Mods
& “How to Page It”

by Paul Anderson

By special request (and much pestering after seeing my system – sorry chaps) I will proceed to enlighten those of you that asked for details of how it’s done – it is in fact quite easy to do and forms the basis of a very flexible system especially when using 64k of RAM , however I modified my system so that I always had a full 48k of RAM available – this of course meant that all firmware must reside between D000 and FFFF which by the by means that all software needs to be relocated ...... After many hours I have produced source listing for most of the software and ended up as follows....

Page switching is very easy and consists of outputting to Port FF the page number which incidently are NOT 1, 2, 3, 4 – but 0 to disable the card and 1, 3, 5, 7 for each of the pages. This provides the simplest modifications with maximum flexibility.

Now to the “Nitty Gritty” – the mods are in two parts, firstly to allow the fitting of 4k EPROMs, and. secondly to provide the switching under software control.

Modification to fit 2732 EPROMS

Ready – REMOVE all capacitors which are situated along side the EPROM sockets and with the following numbers 28, 30, 32, 34, 36, 38, 40, 42, 44, 46, 48, 50, 52, 54, 56 and 58. This frees the highest address line (which normally goes to the +5v supply).

With reference to fig 1 wire each of the link blocks as shown, each one being wired the same way. Identify the track plate through hole near IC8 and link it to the right-hand centre pin of all the link blocks. This provides the new highest order address line.

Selection of address blocks is just as before except that each block is now 16k long (not 4/8k). If for any reason you decide to start a block on an odd numbered address (ie B000) then the address locations of each EPROM in the block will not