80-Bus News

This article describes the construction of a very cheap EPROM programmer for the Nascom owner who, like me, has to run his Nascom on a budget. It can be built for less than £20 including the cost of a zero insertion force socket.

Most inexpensive programmers that use only one PIO generate the address lines from a CD4040 12-stage counter giving a range of 4096 (2^12) addresses, i.e. the largest EPROM that they can handle being the 2732. However, the 2732 is no longer cost-effective, the best value as far as bits per pound at the moment is given by the 2764, and the 27128 will no doubt fall in price.

I wanted to get the programmer built and working as quickly as possible and the following points were the main constraints on the design:

1. Low cost
2. Minimum hardware
3. Simple software
4. Capable of programming up to 27128

With low cost being the main priority, points 2 and 3 naturally follow. Minimum hardware means that there is no automatic switching of power supplies or selection of EPROM type. Power supplies are switched manually and the EPROM type is selected by inserting a suitably wired header plug into the selector socket. Since all switching is manual the software can be very simple. However certain ‘awkward’ EPROMS do not follow the usual pattern, particularly the 2732 which does not have a ‘VERIFY’ mode and uses a common pin connection for /OE and Vpp. This would require additional circuitry to do the switching so I just left the 2732 out. I don’t find this a problem since I doubt if I will ever use a 2732, unless somebody gives me some! Its brother, the 2532 is also a bit awkward since it does not have a ‘VERIFY’ mode either. In this mode the contents of the EPROM may be read with Vpp at +21V (or +25V) so that each location may be verified immediately after programming without having to switch off Vpp. The original control program I wrote uses this feature so that any faulty locations are reported immediately. However this meant that the 2532 was excluded also. So, in order to include at least one 4K EPROM, I modified the program to do all verifying when programming is complete so that Vpp can be switched off before verification if a 2532 is being programmed. Thus the following EPROMS can be programmed :– 2716, 2516, 2532, 2764, 27128 (and perhaps the 27256, but I don’t have any data on this one and I think it uses a different Vpp).

Leaving everything to the human operator is, of course, asking for trouble, but with prompts on the Nascom screen together with warning lights to show when power is applied to the EPROM I have found it easy to get into a routine when programming an EPROM. So as long as you take some care you shouldn’t have any problems – although it’s probably best to keep off the home-brew while you are using the programmer!

Circuit Description

Referring to fig 1, the 14 address lines are supplied from a PIO. The 8 data lines and 3 control lines are supplied from another PIO. The data lines and most of the address lines are taken directly to the pins of the 28-pin ZIF socket, since these connections are common to all EPROMs. Address lines A11