Rockwell-automation 1771-DMC_DMC1_DMC4_DXPS Control Coprocessor User M Uživatelský manuál stáhnout pdf (Strana 125)

Chapter 7

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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

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Rockwell-automation 1771-DMC_DMC1_DMC4_DXPS Control Coprocessor User M Uživatelský manuál Strana 125