GE 750, Instruction Manual, Page: 58 / 504

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750/760 Feeder Management Relay GE Power Management
3.2 TYPICAL WIRING 3 INSTALLATION
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3.2.16 RS485 / RS422 COMMUNICATION PORTS
The 750 and 760 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 designated 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 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 sys-
tems, 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 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 3–19: RS485 CONNECTION