RS-422 is a full-duplex, point-to-point data communication standard.
It uses two twisted wire pairs; one to transmit and one to receive.
Data is transmitted by a differential voltage signal. The two wires
in a pair are not a loop -- a common misconception. Both are '+'
signals sourcing current to a third "virtual" ground conductor. This
differential voltage signal is quite robust and not susceptible to noise
or minor shifts in signal reference ground.
Warning! The "true" RS-422 standard absolutely does not allow multi-drop to 32 PLC. So if you have a PLC which claims to have RS-422 and supports multi-drop, then what you really have is 4-wire RS-485. Now, these misguided PLC makers can call their RS-485 "late-for-supper" for all I care - however, they cannot expect people who know what they are doing to blindly follow suit! Real EIA/RS-422 cannot be multi-dropped.
So if you expect to support multi-drop, you must not buy commercial
RS-422 data comm products (including our RobustDC products!). True
RS-422 does not allow the transmitter to be disabled, therefore 2 or 10
or 32 devices can never share a wire pair for transmitting on. If you support
multi-drop, then you must buy 4-wire RS-485 products instead. Don't
worry about converting such "multi-drop" RS-422 to 4-wire RS-485 - it already
is 4-wire RS-485 (period!) so no conversion required.
