GE 745 Instruction Manual page 50

TYPICAL WIRING
3–18
The COM1 port 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 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 COM2 port 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.
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 communication ports. An isolated 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 3–17: RS485 wiring
745 TRANSFORMER PROTECTION SYSTEM – INSTRUCTION MANUAL
CHAPTER 3: INSTALLATION