BASIC CHIPS ON NASCOM 1’s
So there you are with your brand new (and expensive) MK36271
BASIC ROM, and wondering where to fit it. There don’t seem to
be any holes for it. When the memory board was designed it
wasn’t envisaged that anyone would want to fit an 8K ROM to
it, so no holes were provided. Never mind, a little bit of
surgery, and all will soon be working.
Take a careful look at fig. 1, and carefully cut the tracks as
as shown. Then connect wire jumpers as
IC27 Solder side
1) IC26 pin 19 to IC27 pin 20.
2) IC24 pin 2 to IC27 pin 19.
3) IC24 pin 3 to IC27 pin 18.
4) IC25 pin 7 to IC27 pin 21.
5) Decode pads 11 and 12 together to pad P5.
6) Check it!
7) Plug the board back in the Nascom and try
it. Lot You should have BASIC.
Now this mod. means that you have lost the use of the other 3
sockets, so for those more adventurous souls, you can get them
back again. The tracks to pins 19 and 21 were the +12 and –5
volt supplies to the EPROM sockets
so these must be restored to make
ICs 28, 29 and 30 operable. Two
jumpers from the points where the
tracks were cut to pins 19 and 21
on IC28 will restore the juice.
But this will also put juice back
on IC27, as these pins are linked
on the top side of the pcb. Some
deft knife work is required to cut
these two tracks between ICs 27 and
28 sockets (and get it right ’cos
you’ll never repair a mistake!!!).
Now the way we have mod’ed the chip
select on the Basic ROM means that
it will come on regardless, so a
switch is called for to disable the
ROM when not required. So make it a
double pole switch, and use it for
selecting the decaodes as well. Note that alternative arrangements
are shown, for any other decode apart from ‘E’ and ‘F’, and
decode for ‘F’. Why? Well some idiot will want to run Big BASIC
and Super Tiny BASIC on one board. Apart from that our version of
Naspen is an early one, and runs from F800 to FFFF. Naspen usually
resides between B800 and BFFF.
Can we make one thing quite clear, these mods have been tried and
work, but the INMC can not take responsibility for any failures,
or chewed up pcbs as a consequence of trying them. If you don’t
think you are capable of attempting these mods, DON’T TRY. Seek
the help of someone competant, then you can blame him if he chews
up half the tracks on the pcb.
STOP PRESS; 100nF capacitor required across pins 24 and 12 of the