I think Black Box used to carry multiplexers that could carry multiple RS232 signals over a single twisted pair, or maybe two pairs.
But if the signal on the actual wire/PHY layer adheres to RS232, there is no standard/supported way to multiplex on it.
I have seen a primitive multidrop implemented on RS232, by a creative Mac user who didn't know better. He got an electronic balance and a diluter with a serial interface to coexist, using the Mac's one serial port at two different baud rates in alternation. There was some interference, where some characters at the low baud rate, intended for one device, would be received as different characters at the high baud rate receiver of the other device, but the minimal command sets of the two devices didn't overlap enough for it to be a problem.
Thinking about that led to an attempt at commercializing a multidrop system that used one PC as a master, and up to 32 control/sensor devices that used a PIC at each to organize themselves by device number, decode standardized packets, and act accordingly. It worked okay, but never really found a market, mostly because it needed an external host adapter sort of thing to drive the multidrop lines because not all PC serial ports provided the proper voltage and current for RS232, and some PC serial ports could be damaged by a return signal at what would otherwise be a standard level.
Mike Halloran
Pembroke Pines, FL, USA
