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General

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The Modbus protocol is one of the most widely used fieldbus communication protocols in the industry. The main reason for this is that no licence is needed and it is free to any user. Many producers of automation equipment have it as a standard integration into their in devices.
Modbus is a master-slave protocol that transmits over a bus through serial lines, ethernet or a wireless network. It can mainly be classified in RTU and TCP mode, where RTU directly transmits binary data over serial communication, while TCP allows transmission over a network. The figure shows a combination of the different modes using gateways.

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A Modbus message being sent from the master includes what action it is to perform in a function code. It could be read registers and would have information of which register to start at. In the message there is an error check field that allows the slave to check the message's integrity.

The function code from the master should be echoed back in a normal response, with the data collected by the slave. This would be as register values or status. Additionally, the code can be changed to respond there is an error, if one has been detected. 

• https://help.codesys.com/webapp/_mod_general;product=core_modbus_configuration_editor;version=3.5.17.0
• https://help.codesys.com/webapp/_mod_f_configurator;product=core_modbus_configuration_editor;version=3.5.14.0
• https://modbus.org/docs/Modbus_Application_Protocol_V1_1b3.pdf
• (siste 2 min)
• (fra 15 min)

 og TM FLOW:

• https://www.tm-robot.com/wp-content/uploads/2021/05/robot_modbus-instruction_rev1.0_20171213_en.pdf
• https://uchimurarobotics.com/wp-content/uploads/2020/01/1.64_ModBus_V1.1_ENG.pdf