reply to Subaru
Re: Anyone know about Serial Y cables? It is possible to Y a serial connection, but serial is designed for point-to-point between two devices and if a 3rd connection attempts to do more than "listen", it will cause problems.
That's partially correct, although in one of my previous job I managed to get RS232 serial communication working with 3 senders and 3 receivers on one shared line. All that needed were some Schottky diodes and a SW protocol tailored to that particular situation.
I never said it wasn't possible. If all the software/firmware is aware that it may not have exclusive access and the wiring is properly done, it most certainly can be done.
Getting 3 to work over RS-232 must have been a headache, was there some reason why you couldn't use RS-485 (which is designed for multiple connections)?
Originally, the system contained only 2 devices (perfectly OK for RS-232) and because of legacy issues, the bus had to stay the same, and the 3rd device (a PC with 2 serial ports connected to aforementioned RS-232 bus) had to be, and was literally "plug and play".