NEW EPROMS FOR OLD

by R.A. Gibson

If you look back at the first issue of Personal Computer World, which featured the Nascom 1 on it’s front cover you will find that 2708 EPROMs were offered for sale at a price of £31.15 each. In the latest issue of the magazine 2708s are advertised at £2 each in one off quantities, and 2716s are £2.50. Now a 2708 can hold 1024 bytes of data, while a 2716 has twice the capacity, so on economic grounds it would seem to be sensible to use 2716s in your system rather than the 1K device. This will double the amount of memory that you can cram into a given number of sockets, and there are other advantages too. The 2716 is most readily available in the single-rail form, which requires only a +5 volt supply. The details given in this article are specifically for the single rail 2716. The 2K device consumes less power than the 2708, and therefore runs much cooler. However, the most significant advantage is that while many 2708s do strange things at 4 mhz, I have never come across a 2716 which did not operate perfectly at this clock rate without wait states.

Fortunately the two chips are very similar in pin assignment so the changes necessary to run 2716s on the RAM A card or in the monitor sockets of a Nascom 1 are fairly simple. In both the 1K and 2K EPROMS pins 8 – 1,23 and 22 (in that order) are address lines A0 – A9, pins 9 –11 and 13 – 17 are data lines D0 to D7, pin 12 is Ground, pin 18 is used for programming the EPROMs (also for power-down mode in the 2716), pin 20 is the chip select line, CS, and pin 24 is the +5 volt supply line. This leaves, only two pins which are used differently in the devices for normal operation; pin 19 is the +12 volts supply for the 2708, but is the high address line, A10, in the 2716; pin 21 is taken to −5 volts in the 2708, and +5 volts in the 2716.

Here then are the full details of the modifications necessary to fit 4 x 2716 to a RAM A card, which means that this card can carry 8K of EPROM data. The description assumes that you are holding the RAM card in front of you with the plated edge connector at the bottom and the component side away from you, the soldered side towards you.

1. Disconnect pin 19 of the EPROM sockets from the +12 volt line. A short wide copper track runs from pin 19 of the topmost socket to a plated through hole. Either cut this track with a sharp knife, or cut away the copper around the hole by hand, using a small sharp twist drill. This disconnects all the EPROM sockets from + 12 volts.
2. Similarly cut the longer wide track running from pin 21 of the topmost socket. This disconects the – 5 volt line.
3. Connect pin 21 of the top socket to + 5 volts, A convenient point to conect to is the wide track coming from the right hand edge of the board to within 1/2 inch of pin 24 of the top EPROM. The +5 volt lines can be identified by a short downward branch approximately 1/4 inch from the lefthand end.
4. A thin track approximately 1 inch long connects two plated through holes