The link LK1 must be left in the internal position (to select internal
decoding IOEXT must now be connected to IORQ. Now with IOEXT lo A2
selects between the internal ports, while IOEXT hi disables the on
board ports as it should.
To make this modification disconect pin 2 of IC45 and pin 5 of
IC46 by cutting the tracks on the component side, then join these two
points together and to the IOEXT line at LK1, on the solder side. You
can then build your IO board with the following decoding:
Circuit Diagram for modifying RAM A
See ‘32K – 48K Expansion’ article.