1. Modbus RTU Troubleshooting

1.1. Verify the baud rate, stop bits, data bits, and parity.

Factory default settings of Modbus parameters:

address = 1
baudrate = 38400
parity = odd

Troubleshooting serial settings

Figure 1. Troubleshooting serial settings.

1.2. Check for reversed polarity on RS485 lines; swap wires A and B.

Troubleshooting reverse polarity

Figure 2. Troubleshooting reverse polarity.

1.3. Slave address: With RS485 RTU, up to 32 slave devices can be daisy-chained (don't forget termination). For a Modbus RTU master, make sure all slave device addresses match what the master is trying to connect to. The slave address can be set in RCP on the "Communication" tab.

Troubleshooting slave address

Figure 3. Troubleshooting slave address.

1.4. Make sure the biasing resistors, termination resistors, and proper RS485 network setup are installed.

Troubleshooting network topology

Figure 4. Troubleshooting network topology.

1.5. Check that the slave device is enabled for Modbus communication; this must be done by a Rheonics agent. See Updating Sensor Configuration.

1.6. Use the speed that is necessary for the application. The figure below is a good reference for selecting the speed, which helps achieve more efficient communication.

Troubleshooting speed

Figure 5. Troubleshooting speed.

1.7. When routing cables, keep communication cables separate from power cables. Shielded, twisted-pair RS485 cable is always recommended for communication wiring.

2. CRC Errors

A CRC error usually means the device is at least recognizing bit activity on the network line, but the bits probably make no sense to the device. If getting these errors, check the following:

  • Check baud rate and character format.
  • Check wiring – if everything else is correct, CRC errors mean noise on the line.
  • Check for reversed polarity on RS485 lines. Reversed polarity often looks like noise.
  • Check that Pre-Delay is at least 50 ms.

3. Modbus Debugging Tools

Modbus Poll is a Modbus master simulator designed primarily to help developers of Modbus slave devices, or others who want to test and simulate the Modbus protocol.

Download link (official Witte Software download page): https://www.modbustools.com/download/ModbusPollSetup64Bit.exe