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What is Modbus RTU communication?

Open communication protocol
Modbus is an open communication protocol, meaning that it's free for manufacturers to build into their equipment without having to pay royalties. Modbus is often used to connect a supervisory computer (or master device) with a Remote Terminal Unit (RTU) in supervisory control and data acquisition systems.
Master-slave communication 
The Modbus RTU communication protocol works by the master-slave principle. The Modbus master device sends a request, the slave device(s) send their answer back. It uses RS485 technology for data transfer. On a standard Modbus RTU network, there is one master and up to 247 slaves, each with a unique slave address from 1 to 247. The master can also write information to the slaves or adjust their settings. 
Line topology
Ideally, the Modbus network is built up in a line topology. The main line can have a maximum length up to 1.000 m, branches should be kept short. More details can be found in the Sentera Modbus RTU installation guidelines.
 Modbus topology
Industry standard
Thanks to its reliability and affordability, Modbus has become the standard communications protocol in the HVAC industry, and is now the most commonly available means of connecting (industrial) electronic devices. 
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