4Ks Via 2 Megs.

by A. Want

How to convert the popular Gemini/​Bits and PCs EPROM Programmer to program 2732s and 2716s instead of 2708s and 2716s, at a cost of two 1M resistors.

That excellent article by D. A. Boyd (Micropower, Volume 2, Number 2) showing how to put 32K of EPROM on a Nascom 2 board was exactly what I needed for our school Nascom 2s. Trouble is that our Gemini/​Bits and PCs EPROM programmer only blows 2708s and 2716s, or does it? This article explains how to convert this very popular ‘blower’ into a 2716/2732 model. The same warning applies as in D. A. Boyd’s article, it will not work with 2532s, although making it do so would be easier than it was for the 2732. I chose the more difficult route because the 2732 is now available at just over £4 for the 350 nsec version. Having had timing problems with 2708s, I thought the extra 50-100 nsecs needed by the 2532 might be critical. (It shouldn’t be I know).

The 2732 is programmed by:

1. Placing 25 volts, steady, on pin 18, known as /0E Vpp.
2. Putting the right address on the 12 address pine A0 to a11.
   Putting the eight bits of the required data on the eight data pins D0 to D7.
3. When all the above are stable, drop pin 20, know as /CE, to 0 volts for 50 milli-secs from its standby voltage of 5 volts.

Thanks to the superb documentation with the Gemini​/Bits and PCs blower, I realised that this was very easily arranged. (Yes, I did say ‘superb documentation’.) Mind you, it took a week of evenings to sort out, but then I am not an electrical engineer or a programmer.

Hardware Modifications.

Please do not write in saying how hamfisted these are, it is easy and it works. First study the switch diagram in Figure 1, the IC pin out diagram in Figure 2 and the voltage/​pin out in Figure 3. Do this in conjunction with the circuit diagrams supplied with the board. Note that the pins of switch 2 are labelled as if it were an IC, orientated like the other ICs on the board. Now, work as follows…

1. On the top of the board, cut the two thick tracks between the two switches close to switch 2. On the underside of the board, close to switch 2, cut two thin tracks, one going to pin 6 of the switch (the furthest from the edge of the board) and the other to pin 3 (the track that crosses the switch). All four 2708 switch pins are now ‘floating’ and ready to become 2732 pins.