There's no way a local protocol of any sort would help in this case because (a) dumb terminals don't provide/support them and (b) having to switch with in the current phase of a 33 MHz clock would probably not allow for much action by the MCU.

I need a handshaking trick that will, at least under some circumstances, be meaningful beyond just reporting a garbled ASCII character. If there's a dumb terminal there, it wouldn't know what to do anyway, and will probably just echo some error character, though if there's something intelligent at the remote end, perhaps some convenient manipulation of the handshake tags (CTS, RTS, DSR, DTR, RLSD, etc.) would illicit a useful response.

Since I'm not given to using low (<10Mbps) transmission rates, generally speaking, I'm not "up" on the latest alchemy with otherwise unprescribed handshaking on RS232. I could, conceivably accommodate some simple sort of thing, but I absolutely need to signal that the channel is momentarily closed, and, of course, when it is, once again, available. Most often, one can tolerate a single missing/damaged character. With a quick hardware signal, it should be possible to signal change of status such that no additional ASCII characters are sent.

My hope was that someone had a "clue" how one might effect such a feat. If it's too big a challenge, I'll just delete the async serial channel.

RE