MVI56-MCM ♦ ControlLogix Platform Reference
Modbus Communication Module
Page 110 of 159 ProSoft Technology, Inc.
July 24, 2008
8.3.4 RS-485
The RS-485 interface requires a single two or three wire cable. The Common
connection is optional and dependent on the RS-485 network. The cable required
for this interface is shown below:
Note: Terminating resistors are generally not required on the RS-485 network, unless you are
experiencing communication problems that can be attributed to signal echoes or reflections. In this
case, install a 120 ohm terminating resistor on the RS-485 line.
RS-485 and RS-422 Tip
If communication in the RS-422/RS-485 mode does not work at first, despite all
attempts, try switching termination polarities. Some manufacturers interpret +/-
and A/B polarities differently.
8.3.5 DB9 to RJ45 Adaptor (Cable 14)
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