Xref: utzoo comp.os.os9:278 comp.sys.m6809:964
Path: utzoo!attcan!uunet!husc6!think!ames!ncar!noao!asuvax!nud!mcdchg!clyde!ag@floyd.ATT.COM
From: ag@floyd.ATT.COM
Newsgroups: comp.os.os9,comp.sys.m6809
Subject: MIDI Interface Driver for CoCo 3
Keywords: OS-9, Driver, MIDI
Message-ID: <36830@clyde.ATT.COM>
Date: 23 Nov 88 14:54:51 GMT
Sender: feg@clyde.ATT.COM
Lines: 41


Hi, y'all,

I have built a MIDI interface for my CoCo 3.  The hardware seems to
work fine.  Now I would like to try to write a stripped-down version
of the ACIA driver, so that the driver will keep up with the
31,250 baud rate.  One question I can't seem to find the answer
to is how OS-9 handles interrupts.  In the OS-9 manual, it mentions that
with the multipak interface, there are no real IRQ interrupts available,
just this pseudo-interrupt, VIRQ.  The problem with VIRQ is that the fastest
I will be able to read MIDI bytes is once every 1/60 sec.  Since MIDI data
can be available as fast as every 1/3125 seconds (1 start, 8 data, 1 stop),
I will definitely lose charaters.

I can connect the 6551 IRQ line to CART, but does OS-9 use the CART signal
at all?  Can it be told to use the CART signal as an interrupt source by
setting some values in the GIME chip (around 0xFFA2, I believe)?  If these
are all impossible, then should I hardwire the 6551 IRQ line directly to 
the 6809 IRQ input?

Another similar question: Since the multipak interface switches only
CTS, SCS, and CART, how does a system with 2 RS-232 paks or 2 DC Modems
operate?  Neither of these paks use the SCS signal for their enable.
How does OS-9 tell them apart (using /T1 and /T2, or /M1 and /M2)?  And
how does the multipak keep from dual-sourcing the bus when trying to
read either one, since they are at the same address space?

So many questions, so little money....

Happy Turkey Day!

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