80-Bus News

be read in and when this has been done the NAS-SYS Tabulate command is used to display the contents of the EPROM (note: the addresses shown are RAM addresses).

3. Program

After entering the EPROM type you enter the start and end address (inclusive) of the RAM block which holds the data to be blown into the EPROM. Next, you are asked for the base address of the EPROM. The default value is 0000. If you wish you can change the base address to the actual address at which the EPROM will reside in memory. When this has been entered the start and end addresses of the EPROM will be displayed, assuming programming is to start at the first location in the EPROM. If you are programming from a different address then change the values accordingly. The next prompt tells you to switch the programmer to Program and then to press key ‘P’.

When programming is complete the locations just programmed will be verified against RAM by reading in each Location and comparing it with the corresponding byte in RAM. If a discrepancy is detected a message is given which displays the EPROM location concerned. If there are a lot of errors press ESCAPE to abort the verification. Then you can try the same EPROM again or a fresh one.

The signals used to control the EPROM are /OE which is low to read and high to program, and the programming pulses PGM (active high) and /PGM (active low). The duration of the pulses is 50ms +or- 5ms and to implement this I used a simple delay loop. However a program crash during the middle of the delay loop would leave the PGM signals permanently active and the EPROM would not like this! So a safer alternative would be to use a 74LS121 monostable to provide the 50ms pulse and this would simply need to be triggered from the PIO.

As I already had a Nascom I/O board which holds up to three PIO’s I took advantage of this by using two PIO’s to provide all the necessary signals and therefore simplify the hardware. If you only have the on-board PIO but still would like to build the programmer have a look at fig3. This is something off the top of my head (yes, I know it’s a funny place to keep circuit diagrams!) and has not actually been tried but something along these lines should work, although the control program will need modifying.

Construction

The circuitry can be built on a piece of Veroboard and mounted in a plastic box with the ZIF and 16-pin dil sockets poking through holes cut in the top of the box, together with the switch and LED’s. Connection to the PIOs on the I/O board is made using two 26-way ribbon cables with IDC headers. See fig 4 for pin connections (but note that I use different addreses for the PIOs from those recommended in the manual). The power supply could be included in the programmer box or use a separate one. It should be able to deliver 30V at about 300ma. Wire up the header sockets as per fig 2 and I strongly suggest you clearly mark each one with the EPROM type!