GE C30 Instruction Manual page 68

3.2 WIRING 3 HARDWARE
3
Figure 3–22: 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 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 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 C30 COM terminal (#3); others function cor-
rectly only if the common wire is connected to the C30 COM terminal, but insulated from the shield.
To avoid loop currents, the shield should be grounded 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 should also be daisy-chained to the next one in the link. A maximum of 32 relays can be connected in this
3-22 C30 Controller System GE Multilin