Each 4K block used must also be gated into the IC44a/RDB circuitry and this can be
conveniently effected by wiring small signal diodes from the block pulse (pins 10, 11
or 12) to pin 7 on LKSI (XROM).
Sometimes it may be required to switch different EPROMS into a given memory
location and fig. 1 shows a method of doing this for, in my case, alternative monitors.
The appropriate block pulse is switched to the appropriate pin 20 but it is necessary
to disable the non used EPROM by allowing its Pin 20 to go high via a 1K resistor to
+5v. The 1K for the original monitor is already on board (R53) but must be provided
for any non-used EPROM. The gating diode for the switched monitor is also taken to
All the EPROMS used are single rail as these are readily available and hence the +12v
and –5v connections are not used.
As I am still waiting for delivery of a 4K EPROM programmer, I have not tried this
yet, but it looks to me as if a 4K EPROM can simply be plugged into a socket with
A11 on Pin 18, in which case A11 will not be necessary.
Most of the wiring has been on the solder side of the board as I find wiring onto the
link pins is not easy – and in any case it does not show on the bottom! Do not try
using the spare IC7b and IC7h gates as A11 inverters as they are only buffer gates
– not buffer inverters. It took me quite a time to realise this.
The diagram shows how I have populated my sockets but of course any socket can be
used by suitable connections of A11 or A11 and
the relevant block pulse to pin 20.
MODS TO POPULATE
WITH 2K (or 4K?)