[Coco] Where can I find information on using the Cartridge port for I/O ?

[Rick Ulland]

On 12/24/20 12:47 PM, Neil Cherry wrote:
> I'll start out simple as I'm sure there's a lot I don't know.
>
> I want to build a cartridge with an interface to a USB keyboard using 
> the CH376. What information
> is available for using the Cartridge port for I/O with ROM on the 
> board. I'll need the board to
> be read at startup so that it can add the interrupt chain.
>
>
> CH376
> https://www.mpja.com/download/31813MPSch.pdf
>


Most keyboard mods I am familiar with emulate the physical connections 
of the CoCo keyboard, and are plugged into the keyboard connector. 
Because...

The cartridge port doesn’t give you a lot to work with. It was designed 
for one software ROM select (CTS) and one hardware select (SCS), and 
includes one return (CART) that ECB/DECB interprets as ‘run the ROM’. 
Since there will only ever be one, a disk controller with onboard ROM 
can ask to be auto-executed and lean on these three signals, needs 
little decoding hardware onboard.

The MPI can switch between 4 different 1+1 devices, but you can’t 
effectively manage that from the MPI side, and worse, it switches the 
CART return to the one & only active device this scheme can handle. OS9 
folks physically ‘strap’ all four slot’s CART lines together so any slot 
can trigger an IRQ poll sequence, but that is both 'hardware hacking' 
and polling doesn’t exist in DECB anyway - no IRQ for you!

Sorry to be no help whatsoever.

-ricku