3-22 D30 Line Distance Protection System GE Multilin3.2 WIRING 3 HARDWARE3To 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 D30 COM terminal (#3); others function cor-rectly only if the common wire is connected to the D30 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 groundedat more than one point, install resistors (typically 100 ohms) between the shield and ground at each grounding point. Eachrelay 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.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.Terminate both ends of the RS485 circuit with an impedance as shown below.Figure 3–25: RS485 SERIAL CONNECTION
