There is no point in fighting of any means. It is always better to state facts and discuss. Erik at least states facts, even if his style might irritate others; but the last 2 posts contained no facts.

So the facts are:
- Rx and Tx on '51 are open collector, so they CAN be tied together, they won't fight (the HW won't suffer from overcurrent) even if transmitting simultaneously (but of course the message to be transmitted gets corrupted)

- the "line driver" (here: pullup) output impedance of such arrangement is relatively high (~kΩ)

- there is considerable hum (50-60Hz and overtones) and other noise coupling in common house installation, you can think of it as a noise voltage source with certain impedance (the impedance depending on the coupling factor, e.g. if the communication line runs on a long distance together with the power lines, considerable coupling can be expected; this coupling is typically common (hence common mode) to all comm lines (Rx, Tx, GND) running together but depending on the termination this can turn into differential interfering voltage)

- noise pickup depends on the ratio of the "driver" output impedance (and voltage) and "noise source impedance" (and voltage) (you can think of it as a sort of a resistor divider)

- long lines (tens of metres) also present considerable capacitance (well, exactly speaking some sort of transmission line, but let's assume we are at a low frequency domain) which together with the output impedance of the "driver" yields to signal distortion - long rise times (this alone can be overcome by lowering the baudrate; but not the noise pickup)

- with more nodes problems increase (faster than linearly)

Hence, what works on a workbench with 3 nodes over 10m won't work (or can have unpredictible reliability) in a typical house automation installation.

I'd recommend to go for RS485 for the hardware level - it's proven and working. The protocol etc. is an another issue.

Jan Waclawek