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

[Darren A]

On Thu, Feb 14, 2013 at 9:03 PM, Allen Huffman wrote:
> I have wondered this for ages, and thought this might be a good place to ask...
>
> Could someone explain how the CoCo cartridge port works? How are cartridges able to show up as memory locations in that final area of the 64K?
>
> And, perhaps, in terms a software guy with little hardware background could understand?
> -

You should probably read through the Color Computer Technical
Reference Manual. The "Theory of Operation" section does a reasonable
job explaining how the machine works. One of many places it can be
found is:

<http://archive.org/details/Trs-80ColorComputerTechnicalReferenceManual>

As far as the cartridge ROM appearing in the memory map, when the CPU
presents an address on the address bus, the SAM / GIME will produce a
3-bit device selection code (S0..S2) based on a set of "sections" of
the address space.  The device selection code is then decoded by a
74LS138 to produce individual "chip select" signals.

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.

Darren