thank you Andy and Erik,

Andy,one UART will be enough.
Erik, my sniffer is a '51...no Hyperterminals!
My protocol analyser helps me a lot and in the meanwhile I could proceed in my little project .
My RX pin will alternatively receive TX of master and the corresponding TX of slave.(half duplex).
Do you think I can simply 'or' these signals using two diodes in MML (mikey mouse logic similar to DTL)?or is it better to use active components?
In the end these data will be converted into 4 to 20 mA.

Ed