Troubleshooting RS485 / Modbus RTU Wiring

Generally, a shielded & twisted-pair wire such as Belden 9841 (Single pair) or 9842 (Two pair) with a characteristic impedance of 120 ohms is recommended which should be also terminated at each end with a 120 ohm, quarter watt (or greater) resistor.

 

1. Ensure you are not using random electrical wires to avoid issues in the long term.
2. Shielded & twisted pair RS485 cable is always recommended- The shield helps protect your signals from picking up stray noise from nearby wiring and other electrical equipment. The shield will provide little benefit just on your bench or for short runs with no other equipment around, but its value goes up as exposure to sources of noise goes up.
3. The gauge of wire is typically 22 gauge or something close to that. The gauge number itself is not important. What is important is keeping both resistance and capacitance as low as possible. The longer the network will be, the more important it is that you use good quality wire. The wire manufacturers have wire designed specifically for data networks.

Several types of network topologies are possible, but the daisy-chain bus configuration is the most efficient. A “Star” or a network with “Stubs (Tees)” is not recommended as reflections within the cable may result in data corruption.

Many common network topologies exist,but the daisy chain is the most reliable for RS485 networks.

Avoid routing cables for communication with power cables. When connecting the cables to the devices, make sure not to reverse the wires. Connection inversion at a single point can prevent operation of the entire network.

RS485 network shall include cable termination as per Modbus protocol specifications. Check if Termination resistors are not set up properly.

Yes you need it, although most things will work without any termination for short distances but in the field, it is needed, and must be located at both ends of the line and only at the two far ends of the line. Typically this termination consists of a 120 ohm resistor between the two data lines at each end of the line.

The termination is required to avoid reflections on the line. Think of tossing a pebble into calm water near a stone wall. When the ripples hit the wall, they bounce back toward where the pebble was tossed in. Data on a long unterminated transmission line will do essentially the same thing. When the reflection comes back to a point where something is trying to listen to the line, the receiver will see both original data and reflected data, and the receiver will quickly get confused and end up with garbled data. The garbled data will get tossed out since it does not pass framing and/or checksum tests.

RS485 network shall have only 1 Master Device (Data Logger) until & unless you are using additional equipments to achieve multiple masters setup.

To make it more easy to understand in general situations – One Inverter should only be connected to 1 single Data Logger.

The Modbus-RTU protocol allows only one master and the addition of a second one can interfere the operation of the network or can completely prevent communications.