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General The ISO-OSI model for the standardization of communication protocols,defines 7 hierarchical levels for the division of tasks. To easy understanding, and for the purposes dedicated to embedded field, I'm going to regroup the 7-layer ISO-OSI layers 2, the transmission level and the level information. Modbus In this subdivision the term Modbus is part of the transmission level. Fact, the Modbus defines how information is transferred, and thus defines: a) physical layer transmission: - Serial 232 -> point-to-point, defined Modbus RTU - Serial 485 -> multipoint connection with a master and several slave - Ethernet -> Define Modbus TCP> b) defining the packet transmission mainly composed of: - No. Node recipient -> (8 bit) - Command, each command is defined in the protocol-> 8 bit - Data, data, and number is defined in the protocol depending on the command - CRC -> 16 bits c) Protocol of Master-Slave. In reality, the Modbus has some aspect that also affects the level of information, it defines 4 different data areas, usable for interchange information. Specifically, the 4 areas are: * Input coils * Coils * Input Register * Holding Register The systems will exchange the values of the areas specified above, then the Master calls for information value of information of a specific area, then is given by the specific device to explain what information field is associated with that value. Modbus projects Differents projects were developed using Modbus technology: AVEthBoardMB , Chip Web Server with Modbus interrogation for HTML visualizzation. AVModbus , software for PC with Modbus interrogation. AVSDLog , board sor Modbus interrogation and SD Card storage. AVCanModBridge,board to connect automotive to industrial field using Modbus standard