User Manual

Appendix E
Appendix E RS-485 Network E
-5
E. 2 Line Termination
Each discontinuity i n i mpedance causes reflections an d di stortion.
When a n i mpedance di scontinuity occ urs i n t he t ransmission l ine t he
immediate effect is sig nal reflection. This will lead to signal distortion.
Specially at line ends t his mismatch causes problems. To eliminate this
discontinuity terminate the line with a resistor.
Figure E-4 Signal Distortion
The val ue of t he resi stor sh ould be a cl ose as p ossible t o t he
characteristic im pedance of the lin e. Although recei ver devices a dd
some resistance to the whol e of the transmission lin e, normally it i s
sufficient to th e resisto r imp edance should eq ual t he ch aracteristic
impedance of the line.
Example:
Each input of the receivers has a nominal input im pedance of 18 kW
feeding i nto a di ode t ransistor- re sistor bi asing network t hat i s
equivalent to an 18 kΩ input resistor tied to a common mode voltage of
2.4 V. It is th is configuration wh ich provides the large common range
of the receiver required for RS-485 systems! (See Figure E-5 below)