Advantech Adam - 4021, User Manual, Page: 262 / 286

ADAM-4000 Series User Manual 250
E.3 Line Termination
Each discontinuity in impedance causes reflections and distortion. When an imped -
ance 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 imped -
ance 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)
Figure E.5 Termination resistor locations