Subject: Re: Serial Clock
To: Grant Stockly <gussie@alaska.net>
From: Bill Studenmund <wrstuden@nas.nasa.gov>
List: port-mac68k
Date: 04/20/1999 14:08:29
Why did "tech@openbsd.org" show up as the "to" address in a message to
port-mac68k?

I'm going to assume you're running NetBSD, not OpenBSD, as that's what I'm
familiar with.

On Mon, 19 Apr 1999, Grant Stockly wrote:

> I'm interested in using the Mac IIci's serial port at 128,000bps.

You can try, but I doubt you'll be happy with the results. There's a
reason why MacOS doesn't support this baud rate on '030 class hardware.

> I hear this is possible by supplying an external clock (there is a little
> box where I can enter in the frequency [in the booter]).
> 
> Does anyone have any information on how I'd go about using the serial port
> at this speed?  What type of clock to give the serial port?

You need to give a clock signal at 2.048 MHz. While a clock at 128 kHz
would work, that clock would have to be synchronized with your datastream,
which would be a pain.

The clock needs to be a valid RS-422 signal - +/- 5V should be fine.

> If you could, please send me any info.
> 
> HSKi and GPi are valid clock inputs?

Yes.

Though to be honest, I think you'll be very unhappy with the results.
It'll work fine for transmitting data, but you stand a GREAT chance of
having overruns on receive. Or the machine will be unusable while
receiving.

What exactly are you wanting to do?

Take care,

Bill