Rockwell Automation 1771-DMC_DMC1_DMC4_DXPS Control Coprocessor User Manual - Manual (Page 118)

Chapter 7
Using the Serial Ports
7-10
The control coprocessor and the expander support RS-485
communications. The modules have the necessary hardware and
low-level drivers on COMM1, COMM2, and COMM3. To communicate
on this network:
1. Set up the hardware.
Set the switches as shown on the side label of the coprocessor or
serial expander. Connect the signal pair of wires to pins 11 and 25.
Within the control coprocessor, pin 7 is connected to the logic
common of the transceiver and pin 1 is connected to chassis ground.
An isolated power supply powers the module’s transceiver, so there
is no internal connection of the logic common to the chassis ground.
For more information, see appendix C. You can also refer to the
EIA-485 standard.
The coprocessor and expander use a bidirectional transceiver. The
sense of the voltages appearing across the RS-485 outputs:
for a binary 0 (SPACE or ON) state, pin 11 is positive with respect
to pin 25
for a binary 1 (MARK or OFF) state, pin 11 is negative with
respect to pin 25
You can enable and disable the transmitter (under software control)
to allow it to function in typical RS-485 networks where multiple
transmitters are present. You cannot disable the receiver.
The RS-485 transmitters default to transmit at power-up. If the
coprocessor is connected in a multiple transmitter network, you
should include a call (in the listing above) to a program that will
disable the transmitter to prevent the coprocessor from disrupting
the network.
2. Use the port.
Follow this order of events when writing and reading over an
RS-485 network:
a. Make sure that the input buffer is empty.
b. Turn the transmitter on.
c. Write the data.
Using a Serial Port for
RSĆ485 Communication