Hmmm - the MT985 likely uses a polled protocol at RS-485 levels. It wants to hook up to the B1955 Multi-line controller fer instance. You would hang a bunch of these on a multi-drop line. The Multi-line would throw-out a poll sequence and get either an ACK or NACK from each terminal on the line. If the ACK happens, the terminal is then told to send in a block of data. If memory serves - it has 3 separate micros in it, and the software running on it was written in Pascal. These came out around 1981-2. Steve Wilson (worked on the B1955/B1965 design)