Line Termination - Advantech ADAM 4100 User Manual

E. 2 Line Termination
Whenever the cable is long or the modules are different in the network,
signal reflections are very likely to occur. As a result, the quality of the
signals will be affected, and the signals will be distorted. In order to
eliminate this problem, a resistor should be implemented at the
beginning and the end of the cable.
The value of the resistor should be as close as possible to the
characteristic impedance of the line. Although the receiving devices
will add some resistance to the transmission line, having the resistor
impedance equals to the characteristic impedance of the line should be
sufficient enough.
Example:
Each input of the receivers has a nominal input impedance of 18 kW
feeding into a diode transistor- resistor biasing network that is
equivalent to an 18 kΩ input resistor tied to a common mode voltage of
2.4 V. It is this configuration which provides the large common range
of the receiver required for RS-485 systems! (See Figure E-5 below).
Figure E-4 Signal Distortion
Appendix E
Appendix E RS-485 Network E-5