GE Multilin C70 Capacitor Bank Protection and Control System 3-233 HARDWARE 3.2 WIRING3Figure 3–23: 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 the port, 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 C70 COM terminal (#3); others function cor-rectly only if the common wire is connected to the C70 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.842722A3.CDR100Base-TCOMMON++——D1aD2aD3aD4bD4aBNCIRIG-BinputCPU VCo-axial cableShieldedtwisted-pairsGround atremotedeviceRS485COM2100Base-T100Base-TCOMMON++——D1aD2aD3aD4bD4aBNCIRIG-BinputCPU UCo-axial cableShieldedtwisted-pairsGround atremotedeviceRS485COM2100Base-T100Base-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
