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In Modbus master-slave communication, the Modbus master initiates transactions or queries, whereas the Modbus slaves respond by providing the requested data or performing the requested action. Typical Modbus master devices include host processors and programming panels. Typical Modbus slave devices include programmable controllers. The messages that Modbus masters and slaves exchange are called frames. Different Modbus communications methods, such as Modbus Serial and Modbus Ethernet, lead to different types of Modbus master-slave communication.
In this communications method, a Modbus master and one or more Modbus slaves connect to the same bus simultaneously. The maximum number of Modbus slaves that a serial bus can connect to is 247. The Modbus master initiates communication with Modbus slaves using one of the following modes:
|Note Each Modbus slave must have a unique address, also known as unit ID, so that other Modbus slaves can communicate with this Modbus slave independently.|
In this communications method, one or more Modbus masters establish a TCP connection to communicate with a Modbus slave. The Modbus masters connect to the Modbus slave with a specific address and port. The Modbus slave determines the number of Modbus masters that can connect to itself.