[Coco] Utility of large MPI (was: Re: ROM Cart registers)
go4retro at go4retro.com
Tue Feb 2 13:21:48 EST 2016
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
Floppy Drive Controller (all?)
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
At that point, it's probably cheaper/better to simply make a super IO
cart, like used to be on PCs.
* Glenside IDE
* 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.
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