🚀 go-pugleaf

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I put a math chip (NS32081) on the cartridge port of my ST using the
following hardware mod of the ST that I proposed as a standard:

Cut pins 1, 39 and 40 behind the cartridge connector (they are redundant
5V and gnd pins).

Connect pin 1 to the CPU R/W line.

Build a decoder inside the ST (I used one chip, a 74HCT138) that passes
the CPU ~AS signal but only when the address on the address bus is in the
fourth megabyte ($3xxxxx).  Connect that signal to pin 39.

Connect pin 40 to the CPU clock (8 MHz).

You can now write to the cart port using any address in the $3xxxxx range,
and read using the original cartridge port addresses (only!).

You can still use standard cartridges if you cut the traces to pins
1,39,40 inside them.

- Moshe Braner