[Coco] MPI

[Gene Heskett]

On Tuesday 10 July 2007, Chester A Patterson wrote:
>Hi. Through the years I've read that in the MultiPac Interface, in order
>to work better with OS9, needed to have all four of the pin X jumpered
>for some reason. I never did it. Silly me. What pin/signal was it and
>what was the reason? Thanks. /Chester

That's pin 8 of all sockets Chester, the IRQ line.  And the reason is so that 
an IRQ can get through the mpi & back to the coco from any cartridge that's 
plugged in regardless of the switch position or the current state of a driver 
that may have it switched to some other than #4 slot.  

That means your rs232 pack will never again miss an incoming character or 20 
while the mpi logic is switched to some other slot, either by the switch or 
by the interrupt handling code in the clocks IRQ handler in the case of 
os9/nitros9.

It also means there is a caveat.  Each pin 8 has its own pullup resistor, 
usually r1-r4 along the front of the board.  If some of them aren't removed, 
the combined power supply drain during an interrupt assertion can be well 
over 20 milliamps and the video flickers like crazy when a download is in 
progress.

As each cart that is capable of asserting an IRQ also has its own pullup, I'd 
remove all 4 of those in the MPI, usually 2.2k's, and replace just one with a 
10k to 22k, just enough to hold the line high even if no carts are plugged 
in.  The video will still flicker enough to let you know the download is 
working, and will probably continue to flicker some even if you go through 
all the carts and replace whatever is in there with a 10k, up to maybe 22k 
but I've not tried it personally.

Folks often forget that ALL the logic in the MPI is coco powered, and the coco 
has precious little surplus power in its budget.  The MPI's power supply only 
powers the cartridges that are plugged in.

-- 
Cheers, Gene
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