GE B30 Instruction Manual page 84

WIRING CHAPTER 3: INSTALLATION
3.3.9.2 RS485 port
RS485 data transmission and reception are accomplished over a single twisted-pair wire with transmit and receive data
alternating over the same two wires. Through the use of the port, continuous monitoring and control from a remote
computer, SCADA system, or Power Line Carrier (PLC) is possible.
To minimize errors from noise, the use of shielded twisted-pair wire is recommended. Correct polarity must be observed.
For instance, the relays must be connected with all RS485 "+" terminals connected together, and all RS485 "–" terminals
connected together. Though data is transmitted over a two-wire twisted pair, all RS485 devices require a shared reference,
or common voltage. This common voltage is implied to be a power supply common. Some systems allow the shield (drain
wire) to be used as common wire and to connect directly to the B30 COM terminal (#3); others function correctly only if the
common wire is connected to the B30 COM terminal, but insulated from the shield.
To avoid loop currents, ground the shield at only one point. If other system considerations require the shield to be
grounded at more than one point, install resistors (typically 100 ohms) between the shield and ground at each grounding
point. Each relay needs to be daisy-chained to the next one in the link. A maximum of 32 relays can be connected in this
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manner without exceeding driver capability. For larger systems, additional serial channels must be added. It is also
possible to use commercially available repeaters to have more than 32 relays on a single channel. Avoid star or stub
connections 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 provided internally at both communication ports. An
isolated power supply with an optocoupled data interface also acts to reduce noise coupling. To ensure maximum
reliability, ensure that all equipment has similar transient protection devices installed.
Terminate both ends of the RS485 circuit with an impedance as shown in the figure.
Figure 3-32: RS485 serial connection
3.3.9.3 100Base-FX fiber optic ports
The fiber-optic communication ports allow for fast and efficient communications between relays at 100 Mbps. Optical
fiber can be connected to the relay supporting a wavelength of 1310 nm in multimode.
3-34 B30 BUS DIFFERENTIAL SYSTEM – INSTRUCTION MANUAL