The RAM BLK 0 – 2 signals are taken to pins 2, 5 and 10 of IC36, where they
are ANDed with the RFSHB signal, taken to pins 1, 4 and 9. Another gate is
required to support the RAM BLK 3 signal. Two spare gates are available on IC41,
which is also a 74LS00. Pins 12 and 13 of IC41 are connected to +5 volts by a
wide track. After double checking, carefully cut this track to isolate these pins. Join
pin 13 of IC41 to the new RAM BLK 3 line using light hookup wire, or Verowire; the
best point to join the wire is at pin 10 of RP5. The RFSHB signal can be tapped from
the plated through hole beside pins 10 and 11 of IC36; it should be connected to pin
12 of IC41.
If you hold the board up to a strong light, and look at a point halfwasy
between pins 8 and 9 of IC35 and pins 6 and 7 of IC36 you will see that there
is quite a large area where there are no tracks on either side of the board. Mark the
centre of the clear area and, after rechecking, drill a hole through the board. The
hole only needs to be large enough to permit hookup wire to pass through. One
end of this wire should be soldered to pin 11 of IC41, and the other end of the wire
is passed through the hole to the component side of the board where it passes
between IC36 and C61 and is soldered to a point vertically above pin 1 of IC37.
We now have to find a new OR gate to generate a RAS signal for the exta
memory bank, and we do this by piggybacking a new 74LS32 onto IC37. The
only signals needed from IC37 for the new IC are +5 volts (pin 14), 0 volts (pin 7),
and RAS, which is at pins 2, 5 and 10. For purey mechanical reasons, pin 2 is
used, as it spaces the mounting pins evenly.
Now make up a DIL socket for the new IC. This requires some ‘nice’ soldering,
and I would strongly advise against soldering the new IC directly on top of IC37.
Take a 14 pin DIL socket and break off pins 6, 8 and 11. Solder pins 2, 7 and 14 to
the 74LS32; a touch of instant glue between the IC and socket is recommended.
Carefully bend out pins 1, 3, 4, 5, 9, 10, 12 and 13. Pins 4 and 5 should be taken
low by linking them to pin 7. Similarly, pins 9, 10, 12 and 13 should be taken to +5
volts by linking them to pin 14. Insert the joined up IC and DIL socket, and link the
wire from pin 11 of IC41 to pin 1 of the socket. Next soldedr a hookup wire to pin 3
of the socket, and connect this through a 33 Ohm resistor to the wire which links the
pin 4s on the new memory bank. Plug a 74LS32 into the socket, check the board
thoroughly, and when satisfied insert the board in the Nascom and check for normal
operation. If all is well, you can rewire the DIL header plug that fits in SK1; the
only difference from the addressing shown in the RAM-B instructions is that pin
20 is now used to decode the new memory bank. You should now have a RAM-B
board which will run 64K of dynamic RAM at 4 Mhz.