IIGS 115,200 Baud Serial Port - How?

[Hugh Hood <hughhood@earthlink.net>]

For many years, it's been generally accepted that the (2) IIGS built-in
serial ports (Zilog 8530 SCC chip) max out at 57,600 baud.

When pros like David Empson say so, I believe it.

And yet, along comes David Schmidt, with his ADTPro, and he's transferring
files at 115,200 baud from the IIGS modem port.

David Schmidt is nice enough to post his source for his program, and after
examining it, it appears he does 115,200 baud by changing the SCC clock
factor from 16x to 32x.

OK, pretty neat. Time to look at the Zilog manual again.

So, I get out my block editor and start patching away on ProTERM 3.1. First,
I change its settings from a 16x clock factor to a 32x clock factor, and
then recalculate the values for the Time Constants for the various bauds.
Finally, I patch everything into its correct place.

My results - failure, after failure, after failure. And it wasn't just at
115,200 baud. Whenever the 32x clock factor was selected, I failed even at
_much_ slower speeds.

So, I ask, how in the world does Schmidt do it? What am I missing? It seems
he may be throwing the SCC into transmit only mode, but I just don't know.

Any takers?

Hugh Hood