I actually contacted Michael, and here is his answer :
There's a blast from the past.
I'm afraid I have absolutely no documentation on the SeriAll. I sold the company in 1989 and the buyer (Hayes Microcomputer Products – long gone), had all the documentation.
The only other comment would be to look at a PrinterFace – parallel version of this. Some of the commands may be the same.
Sorry I couldn't be of more help.
So I'll start disassembling, time permitting. Will also dump the eprom.
-Fred