[Coco] MPI
Gene Heskett
gene.heskett at verizon.net
Tue Jul 10 17:30:48 EDT 2007
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
"There are four boxes to be used in defense of liberty:
soap, ballot, jury, and ammo. Please use in that order."
-Ed Howdershelt (Author)
"You have been in Afghanistan, I perceive."
-- Sir Arthur Conan Doyle, "A Study in Scarlet"
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