Connecting Modbus devices

Data loggers can communicate with Modbus on all available communications ports. Consideration should be given to proper surge protection of any cabled connection. Between systems of significantly different ground potential, optical isolation may be appropriate. For additional information on grounds, see Grounds.

The common serial interface used for Modbus RTU connections is RS-485 half-duplex, or two-wire RS-485. This connection uses one differential pair for data, and another wire for a signal ground. When twisted pair cable is used, the signal can travel long distances. Resistors are often used to reduce noise. Bias resistors are used to give a clean default state on the signal lines. For long cable lengths, termination resistors, which are usually 120 ohms, are needed to stop data corruption due to reflections. Signal grounds are terminated to earth ground with resistors to prevent ground loops, but allow a common mode signal. The resistors to ground are usually integral to the equipment. The resistive ground is labeled as RG on Campbell Scientific equipment.