Re: MIDI for the //c???

[musjndx@gsusgi2.gsu.edu (Jonathan N. Deitch)]

irsman@iastate.edu (That Ian Guy) writes:

>In article <jmk3.721939227@crux1.cit.cornell.edu> jmk3@crux3.cit.cornell.edu (Jay M. Krell) writes:
>>The Apple MIDI interface connects to the serial port, so it should at least
>>work on the //c hardware wise. I'm not sure about software.

>Nope.  The Apple MIDI (and compatibles...I happily use an Altech 'Mac' 
>interface on my GS) all require the host serial port to run at the MIDI data
>rate, which is 31.something kbaud.  This is well out of range of the 6551 chip,
>so MIDI will not work on a //c.  Sorry.

Well ... my solution was to design an add-on board that fits the addresses
for the 6850 serial chip and 6840 timing chip that the Passport MIDI 
interfaces use around the four bytes the 6551 uses.  It looks like this :

Port 2 : C0A0-C0AF

C0A0-C0A7 : 6840 registers
C0A8-C0AB : 6551 registers
C0AC-C0AD : 6850 registers

The 6551, then, is untouched and still functions as a serial port.  The two
68xx chips are the same chips used in the Passport //e MIDI interfaces and
generate 31.25 kbaud output which I hook into a standard Mac interface. The
only problem with this setup is the following :

1) You have to hack your //c or design a memory board type piggy back arrage-
   ment (which means you have to remove your memory board).
2) The 6850 registers on the regular Passport card are C0A8 and C0A9, which
   means they have to be moved on the //c.  If you don't, the built in
   interupt handler coughs and dies on you.  Also, this means any software
   written for the Passport interface will have to be edited to change all
   references to the 6850 to the new addresses.

I'm working on a new version that includes a memory card, a clock, and a MIDI
interface all on a single piggyback card, but I haven't had the time to 
actually sit down and make it all work.

- Jonathan
-- 
         ** Atlanta Braves - Back to Back NL Champs '91 and '92 **

 Internet: jdeitch@gisatl.fidonet.org   Fidonet: Jonathan Deitch@1:133/411.7
           musjndx@gsusgi2.gsu.edu      Bellnet: 1 - (404) - 261 - 3665           
-----------------------------------------------------------------------------
Atlanta 1996 !! | Play Pinball !! | Don't Panic ! | "I hate it when I can't
--------------------------------------------------| trust my own technology!"
"Thrills!  Chills!  Magic!  Prizes!" -- Hurricane |    -- Geordi LaForge

Gene Roddenberry, Isaac Asimov, Jim Henson, Dr. Seuss, Mel Blanc ... Sigh ...