🚀 go-pugleaf

In article <8254@clyde.ATT.COM>, saf@moss.ATT.COM writes:
> 
> Another way is to supply an external clock to the SCC (through the same wire
> used for DSR).  Note that you can never supply a clock of more than 4Mhz
> because of limitations in the SCC chip itself, so this in and of itself
> doesn't let you go faster, but it does allow you to use an external divider
> and get all those non-standard baud rates you may need.  This is what is
> done for MIDI interfaces; the programmable divider is turned off, and an
> external clock of 500kHz, 1MHz, or 2MHz is fed in.  By choosing the
> appropriate sampling divisor, the MIDI rate of 31.25kbaud is achieved.
> 
> The only problem is that again, you can't do this with the standard driver,
> you have to write your own.
> 	Steve Falco   AT&T Bell Laboratories

The rest of Steve's posting seems impeccably accurate, but I must take issue
with the comment that the standard driver won't support external clocking.
Indeed, you have to retrieve the SCC's control address from low memory and
poke the chip into "clock channel A from TRxC" and "divide-by-(16,32,64)",
but having done so, I've had no trouble running MIDI data capture, filtering,
rechannelizing and echoing utilities written entirely in Lightspeed Pascal
(even the poking for setup is done in Pascal).  The ROM serial driver of
the Mac+ works fine and is remarkably efficient.
I can't guarantee that the 64K ROM driver or the RAM driver work,
but I suspect suspect that they do.

	Lee Story @ Wang Labs