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From: jwt@atari.UUCP (Jim Tittsler)
Newsgroups: comp.sys.atari.st
Subject: Re: midi to rs232?
Message-ID: <886@atari.UUCP>
Date: Tue, 10-Nov-87 19:24:01 EST
Article-I.D.: atari.886
Posted: Tue Nov 10 19:24:01 1987
Date-Received: Fri, 13-Nov-87 07:30:20 EST
References: <8711100411.AA12628@ucbvax.Berkeley.EDU>
Organization: Atari Corp., Sunnyvale, CA
Lines: 29
Summary: Using the MIDI port for another terminal

In article <8711100411.AA12628@ucbvax.Berkeley.EDU>, AIPS@BRANDEIS.BITNET (Greg Lindahl [chimps@brandeis.bitnet]) writes:
> 
> if the MIDI port is run by some sort of ACIA and is actually some fancy
> sort of rs232 port, could it possibly be reprogrammed a bit to be a
> "normal" rs232?
> 

The MIDI port and the ikbd line are each provided by (Motorola) 6850 ACIAs.
They are being clocked with a 500 kHz signal.  The MIDI port uses the
divide-by-16 mode of the 6850 to produce 31.25 kbaud, and the ikbd port uses
divide-by-64 to produce 7812.5 baud.

The things that make your suggestion a tad messy:
1.  The baud rate divisors provided by the 6850 have no finer granularity
than /16 and /64.  This precludes getting "standard" baud rates.
(Unofficial hint:  If you were willing to hack up your hardware, you could
separate the MIDI 6850's TxClock and RxClock lines from the 500 kHz signal
and provide them with your own baud rate clock.  If you were willing to
live with the restriction that the "MIDI" port would always have the same
baud rate as the MFP serial port, you could probably get away with stealing
the clock from the MFP's timer D output which is used as the MFP baud rate
generator.)

2.  There are no modem control lines available on the MIDI output connectors.

3.  You will have to convert the MIDI current loop to RS-232C (or whatever
your terminal expects).

Jim Tittsler     {ames, imagen, pyramid}!atari!jwt