GE L30 Instruction Manual page 88

3.2 WIRING 3 HARDWARE
3
Figure 3–23: CPU MODULE COMMUNICATIONS WIRING
b) RS485 PORTS
RS485 data transmission and reception are accomplished over a single twisted pair with transmit and receive data alternat-
ing over the same two wires. Through the use of the 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. 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 L30 COM terminal (#3); others function cor-
rectly only if the common wire is connected to the L30 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 with-
out exceeding driver capability. For larger systems, additional serial channels must be added. It is also possible to use com-
mercially available repeaters to have more than 32 relays on a single channel. Avoid star or stub connections entirely.
3-24 L30 Line Current Differential System GE Multilin