modbus.rst

ModBus

Modbus <https://modbus.org/> is a communication protocol from the 1970s designed to support industrial use cases. Its main modes of operation are Modbus RTU (Remote Terminal Unit) and Modbus TCP. The RTU variant is mostly used over RS-485 serial ports, while the newer Modbus TCP variant allows for a communication over Ethernet for more flexibility.

Besides its core industrial use cases in SCADA systems it also moved closer to the consumer when being integrated into homes e.g. in home heating systems and PV systems.

|main_project_name| maintains support for Modbus in its Linux and Zephyr flavours. On the Linux side the user-space Modbus implementation libmodbus <https://libmodbus.org/> is used while on Zephyr the native Modbus subsystem <https://docs.zephyrproject.org/latest/services/modbus/index.html> is used.

RTU and TCP operation modes are supported.

Enabling Modbus in Oniro:

Modbus functionality is not enabled by default in the |main_project_name| and there is currently no Blueprint demonstrating its use cases. The basic integration is working none the less and Modbus can be enabled in the Linux images by adding libmodbus to the image you are building. This can be done by adding the following line to your conf/local.conf file:

IMAGE_INSTALL:append = " libmodbus"

On Zephyr you would need to enable the CONFIG_MODBUS options for your modes of operations. The Zephyr upstream project does offer samples for RTU as well as TCP operations modes in different roles <https://github.com/zephyrproject-rtos/zephyr/tree/main/samples/subsys/modbus>

Modbus-testing:

The libmodbus library does not come with any examples on how to use it by default. To fill this gap and allow integrators and developers to have a head-start when developing an application using libmodbus we developed a small set of examples to demonstrate the usage. These examples can be found at our modbus-testing repo <https://gitlab.eclipse.org/stefanschmidt/modbus-testing> and included in the image by the following addition to your conf/local.conf file:

IMAGE_INSTALL:append = " libmodbus modbus-testing"

To test Modbus RTU communication over an existing RS-485 serial port (e.g. /dev/ttyUSB0) under Linux you can run a server as follows:

$ modbus-server /dev/ttyUSB0

To execute the client side instead run the following command:

$ modbus-client /dev/ttyUSB0

To run the same example for Modbus TCP instead of RTU adapt the parameters as shown below:

$ modbus-server -tcp

$ modbus-client -tcp SERVER_IP_ADDRESS_HERE

Keep in mind that these examples have hard-coded values for many parts (e.g. device id, register values, etc) as they are only suited for demonstration purpose and as a head-start for your own development.