How can a temperature sensor, an electricity meter, and a control system exchange information even though they come from different manufacturers? In industrial and building automation, the answer is often Modbus.
Modbus is a communication protocol used to read measurements, transfer status information, and send control commands between devices. Developed by Modicon in 1979, it has become a familiar feature in everything from manufacturing plants to ventilation systems.
Modbus follows a simple pattern: a client sends a request, and a server responds. The client might be a supervisory control system, while the server could be an electricity meter or a temperature controller. Older documentation often uses the terms master and slave for these roles.
A typical exchange might look like this:
The client can also write values, for example to change a setpoint or activate an output, provided the device supports it.
Modbus organizes information into four basic data types:
The device manufacturer determines what each address means. One register might contain a temperature, while another indicates an alarm or an operating mode.
This makes the device’s register map essential. It specifies which addresses are used, how values should be interpreted, and which functions the device supports.
For example, a temperature reading might be stored as the integer 235. If the register map specifies a scaling factor of 0.1, the temperature is 23.5 °C. Larger numbers and floating-point values may span multiple registers, so their order must also be interpreted correctly.
