Specifications

Chapter 2 - System Configuration RS-422 And RS-485 Applications Ebook
Page 28 of 137 Manual Documentation Number: <DocNumber>
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transmission line, if the line is not terminated with an impedance equal
to the line impedance, the transmitted signal is not completely
absorbed and a portion is reflected back into the transmission line.
Reflections interfere with real data and can cause errors. If the source,
transmission line and load impedances are equal, reflections are
eliminated.
A practical solution is to add a resistor in parallel with the receiver's A
and B lines in order to match the characteristic impedance of the
communications cable. This value is specified by the cable
manufacturer. (120 ohms is a common value.) This value describes the
intrinsic impedance of the transmission line and is not a function of the
line length. A terminating resistor of less than 90 ohms should not be
used.
Figure 19: Transmission line with termination
Deciding whether to use termination
Although termination may be necessary in some situations, there are
also disadvantages to implementing termination. Termination increases
load on the drivers, increases installation complexity, changes biasing
requirements and makes system modification more difficult.
Note: If termination is added, and its effects on biasing are not considered, it can compromise the
performance of the communications system.
The decision whether or not to use termination should be based on the
cable length and data rate used by the system. A good rule of thumb is
if the propagation delay of the data line is much less than one bit
width, termination is not needed. This rule makes the assumption that
reflections will damp out in several trips up and down the data line.