TReX User Manual v2.14 Firmware
2 Wire Connection
Note:
the TReX must have wire links fitted as shown in order to operate correctly in this
configuration.
Bus Terminating Resistor
RS422/RS485 may require the fitting of a terminating resistor. The purpose of termination is
to match the impedance of a transmission line to the hardware impedance of the interface it is
connected to. There is more than one way to add termination to an RS485/422 serial
connection. The most commonly used is DC Termination, accomplished by attaching a resistor
between the signal lines on the extreme ends of the transmission line.
The rule of thumb for termination is:
•
If the propagation delay of the data line is much less than one bit (pulse) width,
termination is not needed.
This assumes reflections will damp out in several trips back and forth on the data line.
Typically for slow speeds of 9600bps or below, no termination resistor is required.
If termination is required a resistor value of 120Ω or greater should be used, and no more than
2 termination resistors should be used, one at each end of the RS422 transmission line.
Do not use termination resistors with a value of less than 90Ω.
© WTE Limited, 2018 – Christchurch New Zealand
Page 132 of 158
Illustration 5: TReX RS-485 2 Wire Multi-Point
