3-34 F60 FEEDER PROTECTION SYSTEM – INSTRUCTION MANUALWIRING CHAPTER 3: INSTALLATION3Figure 3-30: CPU module communications wiring3.3.9.2 RS485 portRS485 data transmission and reception are accomplished over a single twisted-pair wire with transmit and receive dataalternating over the same two wires. Through the use of the port, continuous monitoring and control from a remotecomputer, SCADA system, or Power Line Carrier (PLC) is possible.To minimize errors from noise, the use of shielded twisted-pair wire is recommended. Correct polarity must be observed.For instance, the relays must be connected with all RS485 “+” terminals connected together, and all RS485 “–” terminalsconnected 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 (drainwire) to be used as common wire and to connect directly to the F60 COM terminal (#3); others function correctly only if thecommon wire is connected to the F60 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 begrounded at more than one point, install resistors (typically 100 ohms) between the shield and ground at each groundingpoint. Each relay needs to be daisy-chained to the next one in the link. A maximum of 32 relays can be connected in this
