2.7 inches from the bottom edge and 3.3 inches from the left edge of the
board. Cut this track somewhere in the middle.
-
Connect the left hand plated through hole of the latter track to A12 at the
edge connector – this is line 42 of Nasbus.
-
Connect the right hand plated through hole of the track to pin 19 of the
bottom EPROM socket.
-
Select the addresses for the four sockets by connecting pad 5 to the
appropriate two decode pads (remember, you’ve got 8K there now). The two
4K blocks selected need not be adjacent, but they must have different
values for A12, because this line is used in the decoding. For example you
could use blocks C000 and F000, but not C000 and E000.
-
Because the modifications have inverted the order of the decoding lines the
addresses of the EPROM sockets are no longer in the correct order. Thus if
you have selected addresses C000 – DFFF you will find that the start
address are:–
IC27 C000 IC28 D000 IC29 C800 IC30 D800
That completes the mods for the
RAM A
card. For just a little work, and using no extra
chips, you get an extra 4K out of the board by replacing 2708s.
The Nascom 1 owner can also modify his main board to take 2716s in the monitor
position; if you put a 2716 in each socket you can have two monitors on your machine,
selected by a switch on the CS lines. Here are details of the modifications required.
-
On the soldered side of the board a wide track runs from pin 21 of IC38 to a
plated through hole near the edge of the board. Either cut this track with a
sharp knife, or break the plated through connection with a small sharp twist
drill. This disconnects both EPROM sockets from the −5 volt line.
-
Also on the soldered side of the board, a wide track connects pin 19 of IC38
to the +12 volts lines. This track should be cut with a sharp knife near to pin
19.
-
Connect pins 21 and 24 of IC38 with a short wire link
-
Connect an insulated wire link from the solder pad of pin 19, IC38 to the
solder pad of pin 2, IC36. Thus provides address line A10 for the 2716s.
-
Tie pins 4 and 5 of IC36 together. This gives a 2K decode instead of two
separate 1k decodes.
-
Remove IC44 from its socket, bend pins 8, 11 and 13 to a horizontal
position, and replace the IC.
-
Fit a single-pole double-throw switch at some convenient point in your
computer. Link the centre of this switch to pin 8 of IC44. The two outer
connectors of the switch are tied to +5 volts through 4k7 resistors; they are
then linked to pads 8 and 11 of IC44.
You can now fit a 2K monitor in each of sockets IC38 and IC39. The switch selects
which monitor is in use.