723PLUS Hardware Manual Manual 02877
14 Woodward
Termination
For RS-422, termination should be located at the receiver when one or more
transmitters are connected to a single receiver. When a single transmitter is
connected to one or more receivers, termination should be at the receiver
farthest from the transmitter. Figure 2-3 is an example.
Figure 2-3. RS-422 Terminator Locations
Communicating with the OpTrend® operator interface is a special case of Figure 2-
3. Because the OpTrend master cannot put its transmitter into a high-impedance
state, and our bit rate is less than 90k bits/sec, termination is not needed at the
slave. Termination is needed at the OpTrend receiver, but because there is no way
to locate it there, it has to be put at the alternate location.
For RS-485, termination should be at each end of the cable. If termination can’t
be located at the end of a cable, put it as close as possible to the ends. Figure
2-4 is an example.
Figure 2-4. RS-485 Terminator Locations
If you do not know if a master can put its transmitter into a high-impedance state,
terminate the line as shown in Figure 2-3. It does not hurt to terminate the line
even when it is not needed as in the case of the OpTrend operator interface.
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