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ModBus / RS485 ​

1. Introduction ​

MODBUS RTU over RS485 is a widely-used industrial communication protocol that enables reliable data exchange between control and monitoring devices. This application note describes how to set up MODBUS RTU communication on Xatellite series devices using a USB-to-RS485 converter.

2. Overview of MODBUS RTU ​

MODBUS RTU is an open, serial communication protocol developed for industrial applications. It follows a master/slave architecture where one master device can communicate with up to 247 slave devices on the same network. The protocol's simplicity, reliability, and efficiency make it popular for:

  • Industrial automation
  • Building management systems
  • Energy monitoring
  • Process control
  • Data acquisition

3. RS485 Interface ​

RS485 (also known as TIA-485) is the physical layer used to carry MODBUS RTU communications. Key features include:

  • Differential signaling for noise immunity
  • Lightning Prtotection
  • Multi-drop network capability (up to 32 devices without repeaters)
  • Cable lengths up to 1200 meters
  • Half-duplex communication
  • Data rates up to 921,600 bps (depending on cable length)

4. USB to RS485 converter ​

The USB to RS485 converter should be inserted into the USB port. Here is the wire connection diagram:

5. RS485/ModBus Configuration ​

Login into the Xatellite's Web administration page and go to "Services/RS485/ModBus":

and click the "ADD" button to create a ModBus RTU service (RS485 over TCP):

NOTE:

  • Don't forget to check the "Enabled" checkbox to enable this service
  • The Serial Device should be always "/dev/ttyCH9344USB0"

Once the above parameters are set and click the "SAVE" button, you will see:

Finally, click the "SAVE & APPLY" button.

NOTE: To make sure the ModBus RTU service daemon is running properly, you might check the System Log. It will show ".....:mbusd-0.4.0 started..." that means the service is running properly.

Fri Oct 25 16:51:18 2024 daemon.err mbusd[20139]: 25 Oct 2024 16:51:18 mbusd-0.4.0 started...
Fri Oct 25 16:51:40 2024 daemon.err mbusd[20139]: 25 Oct 2024 16:51:40 Terminated by signal: SIGTERM
Fri Oct 25 16:53:26 2024 daemon.err mbusd[20345]: 25 Oct 2024 16:53:26 mbusd-0.4.0 started...

6. Suggested ModBus Testing Tools ​

For the sake of a quick test, there are many free ModBus Tools. Here is one of open-source ModBus Tools

https://github.com/ClassicDIY/ModbusTool/releases

You might download it and install it on your PC running Windows.

For example, here is the quick test topology:

Here is the setup for Modbus Master:

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