[Coco] Educate me about Y-Cables

[tim lindner]

On Fri, Feb 20, 2015 at 11:48 AM, Joe Grubbs <jsgrubbs at hotmail.com> wrote:
> I've seen a number of references to people using Y-cables to facilitate the use of more
> than one program pak. Is there a vendor for these? If not, is there a description of how to make one?
>
> What kind of paks can someone use simultaneously with one of these? I was always under the
> impression that paks could only be operated one at a time (hence the selector switch on the MPI),
> however a Y-cable seems to imply this isn't always the case.

It all comes down to how a cartridge decodes the addresses it needs.

The CoCo supplies a useful signal (called SCS*) for ROMs at $C000.
Pretty much all cartridges use this when they want their ROM to be
read.

So a multi-pak will direct this signal to only one of the ports at a time.

There is another signal from the CoCo (called CTS*) used to select the
addresses between $FF40 - $FF5F.

But cartridges are not required to uses these two signals. They could
be designed to fully decode the address bus on their own. In that way
two cartridges could be connected at the same time with no conflicts.



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tim lindner