Troubleshooting in Modbus RTU

Troubleshooting in Modbus RTU

In this article, We will see steps to solve communication issues between Master and Slave Device using Modbus RTU protocol. You are attempting to establish communication between a Master Device (such as a PLC/HMI) and a Slave Device (like a Temperature Controller, Barcode Reader, etc.), but encountering difficulties in achieving successful communication. In such a scenario, consider the following steps:

1.Wiring of Modbus RTU

In any communication setup, your initial step should involve confirming the wiring connections. Begin by determining the physical layer available in the slave device, whether it’s RS232, RS485, or RS422. Subsequently, verify the pinout of the slave device and confirm the integrity of the connecting cable.


Verify parameters such as Baud rate, Parity, Data Bit Length, Start Bit, and Stop Bit with the settings on the slave side. Ensure that the parameters on both sides are configured to match accurately.

3.Address and Function Code

Next, validate the address and Function code with the device.

If you are still encountering communication issues, follow the steps below to determine whether the problem lies on the Master Side or the Slave Side.

Communication issues in Master Device?

To check communication issues in the master device, connect it to ModSim software, which can be utilized as a simulated slave device.

The ModSim software’s appearance is illustrated in the image below. By configuring simple settings, you can validate the master-side configuration. If the master device is able to successfully exchange data with ModSim, then the master-side settings are likely correct. Additionally, this test can help assess the performance of your cable. Here, Modsim software will acts like slave device.

Communication issue in slave Device?

To verify the settings on the slave device side, connect it to Modscan software. Through straightforward configuration, assess its communication with the Modscan software, which functions as a Modbus Master. If the communication is successful, then the settings on the slave device are correct. If not, please review and verify the settings on the slave device.

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