RS485 Port

RS485 Communication Bus Connection

Multiprotocol I/O modules provide an RS485 serial port designed for BACnet MS/TP or Modbus RTU/ASCII communication and should be connected using a shielded twisted-pair cable in a daisy-chain bus topology with 120 Ω termination resistors at both ends of the bus (see RS485 Network Termination section below for details).

RS485 Network Termination

Transmission line effects often present problems for data communication networks. These problems include reflections and signal attenuation.

To eliminate the presence of reflections of signal from the end of the cable, the cable must be terminated at both ends with a resistor across the line adequate to its characteristic impedance. Both ends must be terminated since the propagation is bidirectional. In case of an RS485 twisted pair cable this termination is typically 120 Ω.