Date   : Wed, 14 Feb 2007 11:33:28 -0000 (GMT)
From   : splodge@... (Richard Gellman)
Subject: BBC Master 128 problem

> Ahh, but what I want is the GoMMC inside the machine but occupying
> something outside banks 4-7 -.o

The address lines are common between the two, as well as the R/_W and _CLK
lines, so there shouldn't be much effort.

The socket which occupies two ROM slots however is a 32K socket and has an
additional line. This line appears to the ROM at the topmost address line,
but is a bank selector coming from one of the M128's controller ULAs.

The cartridge slot employs a similar mechanism, but I can't remember if it
uses an address line and chip select or just two chip select lines (one
for each "bank" of ROM space).

If there are two chip select lines you will need to AND them together (an
AND gate makes an OR gate for inverse logic) for the CS line on the GoMMC,
and use one of the CS lines as an address line.

If there is already an address line and a CS line you simply need to patch
in these two to the chip socket.

Obviously the original CS and top A lines need disconnecting.

I would recommend consultation of the advanced reference manual so you can
check the circuit diagram for details. I'll have a look later myself too.

-- Richard