Rs485 / Rs422 Communications - GE TRANSFORMER MANAGEMENT RELAY 745 Instruction Manual

3.2 TYPICAL WIRING 3 INSTALLATION

3.2.15 RS485 / RS422 COMMUNICATIONS

The 745 provides the user with two rear communication ports which may be used simultaneously. Both implement a subset
of the AEG Modicon Modbus protocol as outlined in Chapter 7: Communications.
The first port, COM1, can be used in the two wire RS485 mode or the 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 termi-
nals 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 pos-
sible.
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, 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 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 3–13: RS485 WIRING
3-12 745 Transformer Management Relay GE Multilin
Courtesy of NationalSwitchgear.com