GE 750 Instruction Manual page 81

CHAPTER 3: INSTALLATION
750 FEEDER MANAGEMENT RELAY – INSTRUCTION MANUAL
The RS485 port is disabled when the Ethernet option is ordered.
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 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 isolated power
supply with an opto-coupled data interface also acts to reduce noise coupling. To ensure
maximum reliability, all equipment should have similar transient protection devices
installed.
FIGURE 3–18: RS485 Wiring Diagram
ELECTRICAL INSTALLATION
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