[Coco] How does the cartridge port work, anyway?

[Phill Harvey-Smith]

On 15/02/2013 06:50, Darren A wrote:
> If the SAM/GIME is configured for RAM/ROM mode, then the device select
> code which is produced for the cartridge address range (C000-FFEF)
> results in the 74LS138 asserting the line that goes to the CTS* pin
> (Cartridge Select) on the cartridge port.  Inside the ROM pak, the
> signal from the CTS* pin is typically connected to the ROM's Output
> Enable pin, Chip Select pin or both.

The other way that this can happen of course is if the cartridge has 
it's own address decoding and holds the "device select disable" line low 
it can pretty much override the internal memory decoding.

This could for example be used to map an external ROM / RAM into the
area that the basic roms normally occupy.

This method is much more complex to implement, and the majority of 
cartridges just use the CTS* pin.

Likewise the SCS* pin is decoded to go active between $FF40 and $FF5F 
and is typically used to access the disk controller chip & registers.

Cheers.

Phill.