MATT: ignore this, it has nothing to do with your problem, as to that, this information is irrelevant!

Jon, I was in the "I do not want to start something new Friday afternoon" mode and decided to take a peek at your document. I found some small errors/oversights

from "the bible":
set TI. This occurs at the 10th
divide-by-16 rollover after “write to SBUF.

You make the common mistake of stating "the TI bit in SCON. When this bit is set we know that the last character has been transmitted".

I know this is totally irrelevant for RS232, but for turning the bus when using RS485 this, and the RI timing, is probably the most common source of trouble.

I did not check your "treatment" of RI, but here is the skinny: from "the bible":
RI Receive interrupt flag. Set by hardware halfway through the stop bit time (I abbreviated)

This together with TI timing shown above is extremely critical knowledge when doing RS485.

Of course, it you want to take the position that your document is a RS-232 document ONLY, you can do so, but most 485 developers got their feet wet on 232 and then they goof on the above because something they "know" was not known. I made this maistake 10+ years ago and heve seen countless others make it since.

Have a great weekend

Erik