GE Feeder Management Relay 750 Instruction Manual

3-16 750/760 Feeder Management Relay GE Multilin3.2 ELECTRICAL 3 INSTALLATION33.2.10 RS485 AND RS422 COMMUNICATIONSThe 750/760 relays provide the user with two rear communication ports which may be used simultaneously. Both support asubset of the AEG Modicon Modbus protocol as well as the Harris Distributed Network Protocol (DNP) as discussed in thecommunications 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 modesat the same time. In the RS485 mode, data transmission and reception are accomplished over a single twisted pair withtransmit and receive data alternating over the same two wires. These wires should be connected to the terminals markedRS485. 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 beobserved. For instance, SR type relays must be connected with all B1 terminals (labeled COM1 RS485+) connectedtogether, and all B2 terminals (labeled COM1 RS485–) connected together. Terminal B3 (COM1 RS485 COM) should beconnected 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 thismanner without exceeding driver capability. For larger systems, additional serial channels must be added. It is also possibleto use commercially available repeaters to add more than 32 relays on a single channel. Star or stub connections should beavoided entirely.Lightning strikes and ground surge currents can cause large momentary voltage differences between remote ends of thecommunication 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, allequipment should have similar transient protection devices installed.Figure 3–19: RS485 WIRING DIAGRAM