GE G30 Instruction Manual page 81

CHAPTER 3: INSTALLATION WIRING
Figure 3-27: CPU module communications wiring
3
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 G30 COM terminal (#3); others function correctly only if the
common wire is connected to the G30 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
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.
G30 GENERATOR PROTECTION SYSTEM – INSTRUCTION MANUAL 3-29