3-20 D30 Line Distance Relay GE Multilin3.2 WIRING 3 HARDWARE3Figure 3–21: CPU MODULE COMMUNICATIONS WIRINGb) RS485 PORTSRS485 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 port(s), 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 beobserved. 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 pro-vided. To avoid loop currents, the shield should be grounded at one point only. Each relay should also be daisy chained tothe next one in the link. A maximum of 32 relays can be connected in this manner without exceeding driver capability. Forlarger systems, additional serial channels must be added. It is also possible to use commercially available repeaters toincrease 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 thecommunication link. For this reason, surge protection devices are internally provided at both communication ports. An iso-lated power supply with an optocoupled data interface also acts to reduce noise coupling. To ensure maximum reliability, allequipment should have similar transient protection devices installed.Both ends of the RS485 circuit should also be terminated with an impedance as shown below.842765A2.CDRUCTUCT
