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
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
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.
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!