[Coco] modpack hack was Re: Drivewire question

[Darren A]

The address decoding in the serial and modem paks works like this:

A3 through A15 are connected to a 13 input NAND gate (74LS133) with A4
and A7 being inverted first. The output of this gate goes to chip
select CS1* on the ACIA.  This decodes the address range FF68-FF6F.

A2 goes to chip select CS0 on the ACIA.  If inverted it narrows the
addres range to FF68-FF6B (serial pak), otherwise FF6C-FF6F (modem
pak).

Darren