[Coco] Consensus on using two floppy disk controller at the same time?

[Darren A]

On Sun, Mar 19, 2017 at 10:29 AM, tim lindner wrote:

> Of couse this requires a Multi-Pak, but it also depends on how the
> cartridges put the NMI* and RESET* lines when not active.
>
> Are these lines generally left floating or pulled high?
>


As far as the hardware goes, there is no problem having multiple FDCs in
the MPI.  The Tandy controllers all use open-collector outputs to assert
the NMI* and HALT* lines low.  Pull-ups in the MPI (or CoCo) keep them high
otherwise.

Software switching of the SCS* line is the key to making it work.  SDC-DOS
and the SDC driver for Nitros9 do this to allow both the SDC and a single
floppy controller to co-exist.

Floppy controllers made for the Dragon are a different story. They connect
the DRQ line from the Western Digital FDC directly to the CART* interrupt
line.  The CART* line is switched in the MPI, but the switching is
associated with the CTS* slot selection rather than SCS*.  This poses a
problem since changing the CTS* slot also changes the active cartridge ROM.


- Darren