GE D90 Plus Instruction Manual page 63

CHAPTER 3: INSTALLATION
RS485 port
PLUS
D90 LINE DISTANCE PROTECTION SYSTEM – INSTRUCTION MANUAL
RS485 data transmission and reception are accomplished over a single twisted-pair with
transmit and receive data alternating over the same two wires. Through the use of this
port, 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 must also be observed. For instance, the relays must be connected with all
RS485 "+" terminals connected together, and all RS485 "–" terminals connected together.
The COM terminal should be connected to the common wire inside the shield, when
provided. 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 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 increase the number of relays on a single channel to more than 32. 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.
Both ends of the RS485 circuit should also be terminated with an impedance as shown
below.
Figure 22: RS485 serial connection
ELECTRICAL INSTALLATION
53