Appendix E
Appendix E RS-485 Network E
-5
E. 2 Line Termination
Each discontinuity in impedance causes reflections and distortion.
When an impedance discontinuity occurs in the transmission line the
immediate effect is signal reflection. This will lead to signal distortion.
Specially at line ends this mismatch causes problems. To eliminate this
discontinuity terminate the line with a resistor.
Figure E - 4 Signal Distortion
The value of the resistor should be a close as possible to the
characteristic impedance of the line. Although receiver devices add
some resistance to the whole of the transmission line, normally it is
sufficient to the resistor impedance should equal the characteristic
impedance of the line.
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)