Rs485 / Rs422 Communication Ports - GE 745 TRANSFORMER MANAGEMENT RELAY Instruction Manual

3.2 TYPICAL WIRING 3 INSTALLATION

3.2.16 RS485 / RS422 COMMUNICATION PORTS

The 745 provides the user with two rear communication ports which may be used simultaneously. Both imple-
ment a subset of the AEG Modicon Modbus protocol as outlined in Chapter 8: COMMUNICATIONS.
The first port, COM1, can be used in the two wire RS485 mode or the four wire RS422 mode, but will not oper-
ate 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 designated as RS422 for transmit lines. The second port,
COM2, is intended for the two wire RS485 mode only. Through the use of these ports, continuous monitoring
and control from a remote computer, SCADA system or PLC is possible.
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To minimize errors from noise, the use of shielded twisted-pair wire is recommended. Correct polarity should
also be observed. For instance, the relays must be connected with all B1 terminals (labeled COM1 RS485+)
connected together, and all B2 terminals (labeled COM1 RS485–) connected together. Terminal B3 (labeled
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 in the link. A
maximum of 32 relays 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 include
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 com-
munication ports. An isolated power supply with an optocoupled data interface also acts to reduce noise cou-
pling. To ensure maximum reliability, all equipment should have similar transient protection devices installed.
Figure 3–13: RS485 CONNECTION
3-12 745 Transformer Management Relay GE Power Management