Rs485 And Rs422 Communications - GE Feeder Management Relay 750 Instruction Manual

3.2 ELECTRICAL 3 INSTALLATION

3.2.10 RS485 AND RS422 COMMUNICATIONS

The 750/760 relays provide the user with two rear communication ports which may be used simultaneously. Both support a
subset of the AEG Modicon Modbus protocol as well as the Harris Distributed Network Protocol (DNP) as discussed in the
communications chapter. Through the use of these ports, continuous monitoring and control from a remote computer,
SCADA system or PLC is possible.
The first port, COM1, can be used in a two wire RS485 mode or a four wire RS422 mode, but will not operate in both modes
at the same time. In the RS485 mode, data transmission and reception are accomplished over a single twisted pair with
transmit and receive data alternating over the same two wires. These wires should be connected to the terminals marked
RS485. The RS422 mode uses the COM1 terminals designated as RS485 for receive lines, and the COM1 terminals des-
ignated as RS422 for transmit lines. The second port, COM2, is intended for the two wire RS485 mode only.
To minimize errors from noise, the use of shielded twisted-pair wire is recommended. Correct polarity should also be
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observed. For instance, SR type relays must be connected with all B1 terminals (labeled COM1 RS485+) connected
together, and all B2 terminals (labeled COM1 RS485–) connected together. Terminal B3 (COM1 RS485 COM) should be
connected to the common wire inside the shield. To avoid loop currents, the shield should be grounded at one point only.
Each relay should also be daisy-chained to the next one in the link. A maximum of 32 devices can be connected in this
manner without exceeding driver capability. For larger systems, additional serial channels must be added. It is also possible
to use commercially available repeaters to add more than 32 relays on a single channel. Star or stub connections should be
avoided entirely.
Lightning strikes and ground surge currents can cause large momentary voltage differences between remote ends of the
communication link. For this reason, surge protection devices are internally provided at both communication ports. An iso-
lated power supply with an optocoupled data interface also acts to reduce noise coupling. To ensure maximum reliability, all
equipment should have similar transient protection devices installed.
Figure 3–19: RS485 WIRING DIAGRAM
3-16 750/760 Feeder Management Relay GE Multilin
Courtesy of NationalSwitchgear.com