Hello Tony,

Can you share some more information about your Multiprocessor communications system.

1. Use 2 I/O pins on each controller so that its identification can be set (of course use 3 I/O pins and select up to 8 devices, etc.).
Question.
Do you select each 89C4051 using a separate output pin from the IBM PC?
What do you mean 3 I/O pins select 8 devices (74LS138)?

2. Connect all RX pins of the microcontrollers together and all TX pins together with suitable pull-ups.
Question.
Do the TX outputs have open collector outputs (or high impedance outputs)?
How do you avoid the output of the RS232 fighting each other?

3. Now you can treat the RX and TX as if it were 1 device and put these 2 signals through appropriate buffers so that they can be connected to the PC. Simple transistor inverters will work quite well unless going over long distances.

I can see how each 89C4051 would act as one unit although three are actually hooked up in parallel if there was another signal telling each one to awaken (at the proper time) and the TX output were open collector.

Regards,

Charles Bannister