TReX User Manual v2.14 Firmware
RS-422/RS-485
RS-422 and RS-485 use a differential electrical signal, as opposed to unbalanced signals
referenced to ground as used by RS-232. Differential transmission uses two lines each for
transmit and receive signals which results in greater noise immunity and longer distances
between devices. These advantages make RS-422/RS-485 a better fit for industrial
applications.
RS-422 is point to point interface that allows for up to 10 receivers but only a single
transmitter. This would be a common serial interface for NMEA devices all sharing GPS data
from a single GPS receiver for example.
RS-485 is a multipoint interface that allows each device to be a transmitter and also a receiver.
This interface also allows for more devices to be connected on a common serial bus. Because
each device on the same bus can transmit, each device must be capable of setting all transmit
lines to a high impedance state when not in use to ensure that all devices have the ability to
transmit. RS-485 interfaces can be presented as 4 or 2 wires. When there are 2 wires required,
the RS-485 device must also be able to isolate its own receiver when transmitting to prevent
looping data back to itself.
The TReX satisfies the requirements for both 4 and 2 wire RS-485 communications. The
TReX will also be able to operate on a RS-422 bus.
© WTE Limited, 2018 – Christchurch New Zealand
Page 130 of 158
