Rockwell Automation 1771-DMC_DMC1_DMC4_DXPS Control Coprocessor User Manual - Manual (Page 125)

Chapter 7
Using the Serial Ports
7-17
Although the control coprocessor and expander are able to communicate
successfully with RS-422 devices, the RS-422 that the control coprocessor
and the expander support is not true RS-422 communication. True RS-422
communication can go to 1,200 meters (3,937 ft) at the coprocessor’s
maximum baud rate of 19.2 kbps; the coprocessor can only go a distance
of 200 ft at that rate.
True RS-422 can go long distances because it uses a balanced (differential)
driver and receiver with a balanced wiring pair—i.e., a pair of wires in
which neither the signal nor the signal-return line is connected to the
ground—which makes it highly immune to common-mode noise, as well
as because it uses a fast rise time. The coprocessor and expander use an
unbalanced—single-ended—configuration where the signal-return line is
connected to the ground, and the rise time is slower. A balanced
transmitter/receiver can be connected to and communicate with an
unbalanced transmitter/receiver as long as the maximum cable length is
reduced to that of the unbalanced specification—in this case, 200 ft.
Using a Serial Port for
RSĆ422 Communication