3-22 N60 Network Stability and Synchrophasor Measurement System GE Multilin3.2 WIRING 3 HARDWARE3b) 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 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 beobserved. 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 sharedreference, or common voltage. This common voltage is implied to be a power supply common. Some systems allow theshield (drain wire) to be used as common wire and to connect directly to the N60 COM terminal (#3); others function cor-rectly only if the common wire is connected to the N60 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 tobe grounded at more than one point, install resistors (typically 100 ohms) between the shield and ground at each groundingpoint. Each relay should also be daisy-chained to the next one in the link. A maximum of 32 relays can be connected in thismanner without exceeding driver capability. For larger systems, additional serial channels must be added. It is also possibleto use commercially available repeaters to have more than 32 relays on a single channel. Star or stub connections shouldbe 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.Figure 3–22: RS485 SERIAL CONNECTIONSCADA / PLC / computerOptocoupler DataUR-series deviceShield827757AA.CDRLast deviceZ (*)TZ (*)Teach end (typically 120 Ω and 1 nF)Terminating impedance atTwisted pair RS485 +RS485 –COMP 485COMRelayRelayGround shield at SCADA / PLC /computer only or atUR-series device onlyData OptocouplerUp to 32 devices,maximum 4000 feet(1200 m)Z (*)TRS485 +RS485 –COMP 485COMRS485 +RS485 –COMP 485COMCOM
