[Coco] Utility of large MPI (was: Re: ROM Cart registers)

[RETRO Innovations]

My comment was more around the utility of such a device in the context 
of existing HW (no driver changes/rewriting).  The only value the MPI 
provides is to select among existing devices that reside in $ff4x, which 
is mainly:

FF40:FF4F

	

FD-502 (FDC)

	

Floppy Drive Controller (all?)

FF40:FF5F

	

Color Burner

	

Dennis Kitsz' Color Burner


Otherwise, a Y cable would be just as useful (a 4 tap cable, or 8 tap) 
and be significantly cheaper.  Maybe a Y cable with some buffer/driver 
ICs to beef up the address/data/control line integrity.

I'm just thinking what is needed is a next generation MPI, with possibly 
an MPI-emulator mode, where:

  * Each port is truly isolated.  If you have selected slot #3, then
    slot #3 (and only slot #3) is connected to the Coco.  Each cart
    would be completely isolated from any other cart.
  * In MPI-emulate mode, only CTS, CART, and SCS would be isolated.

Of course, in this case, any software that is not MPI aware would need 
to be modified (I would assume HDB-DOS is top of the list) so it would 
select the correct slot before executing any code.

It's not terribly hard to modify the MPI design to support full 
isolation (essentially, the quickest option would be to put 3 state 
buffers on some activity lines like E and Q, and put a buffer on DATA.  
BUt, with buffers on each individual data line on each individual slot, 
You'd need to mux all of the data ports on a read cycle (since you'd 
have no idea which slot is driving the DATA bus).  That adds a lot of 
complexity.

At that point, it's probably cheaper/better to simply make a super IO 
cart, like used to be on PCs.

  * Glenside IDE
  * CocoSDC
  * ROM
  * serial
  * RTC
  * centronics parallel?
  * not sure what else (sound generator?)

Of the above, IDE, serial, parallel, rtc, and flash are easy, looks like 
you have FDD figured out, and not sure what else folks would need.  With 
an MPI, you'd still have options for Speech, SC-90, special purpose 
existing carts, etc. paks.

Just thinking out loud.

Jim