CHAPTER 3: INSTALLATION WIRINGN60 NETWORK STABILITY AND SYNCHROPHASOR MEASUREMENT SYSTEM – INSTRUCTION MANUAL 3-193The CPU modules do not require a surge ground connection.Figure 3-18: CPU module communications wiring3.3.9.2 RS485 portRS485 data transmission and reception are accomplished over a single twisted pair 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 N60 COM terminal (#3); others function correctly only if thecommon wire is connected to the N60 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 thismanner without exceeding driver capability. For larger systems, additional serial channels must be added. It is alsopossible to use commercially available repeaters to have more than 32 relays on a single channel. Avoid star or stubconnections entirely.842722A4.CDR100Base-TXCOMMON++——D1aD2aD3aD4bD4aBNCIRIG-BinputCPU VCo-axial cableShieldedtwisted-pairsGround atremotedeviceRS485COM2100Base-TX100Base-TXCOMMON++——D1aD2aD3aD4bD4aBNCIRIG-BinputCPU UCo-axial cableShieldedtwisted-pairsGround atremotedeviceRS485COM2100Base-TX100Base-FXPort 1Port 2100Base-FXTx1 Rx1COMMON++——D1aD2aD3aD4bD4aBNCIRIG-BinputCPU TCo-axial cableShieldedtwisted-pairsMM fiber-optic cableGround atremotedeviceRS485COM2100Base-FX100Base-FXTx2 Rx2Tx3 Rx3Tx1 Rx1Port 3Port 1Port 2Port 3Port 1Port 2Port 3100Base-FXTx1 Rx1
