To define multiple actions, some functions will have sub-function codes added to them. When the Master sends a message to the Slave, it is the function code field which informs the server of what type of action to perform. Only codes within the range of 1 through 255 are considered valid, with 128-255 being reserved for exception responses.

The function code field is then coded into one byte. The format of a request initiated by a Master is established by the Modbus application protocol. It is the function which informs the server as to which type of action to perform. In order to build the Modbus application data unit, the client must initiate a Modbus transaction. The function codes of Modbus are elements of Modbus’ request/reply PDUs (Protocol Data Unit). Modbus is intended to be a request/reply protocol and delivers services specified by function codes. On the OSI model, Modbus is positioned at level 7. Modbus enables Master/Slave communication between devices connected through buses or networks. A Modbus Master is typically a host supervisory computer running software that will communicate with one or more Modbus Slave devices. Meaning, any application that utilizes the Modbus RTU protocol will have a Modbus Master and at least one Modbus Slave. The Modbus RTU protocol uses a Master/Slave technique to communicate between devices. Modbus is often used to connect a supervisory computer with a remote terminal unit (RTU) in supervisory control In building automation, for example, temperature and humidity are often communicated to a computer for long term storage.

Modbus is typically used to transmit data from control instrumentation to a logic controller or a system for archiving data. Modbus is used widely by many manufacturers throughout many industries. It is the most pervasive communications protocol in industrial automation and is now the most commonly available means of connecting industrial electronic devices. Modbus RTU is an open standard, meaning that manufacturers can build it into their equipment without having to pay royalties. Almost as old as the first Programmable Logic Controller, the Modicon 084, which in those days was called a PC for programmable Controller. In today’s age of Internet connectivity and Web Services, Modbus’ unconnected message and simple request-response communication structure are almost quaint. It truly is as old as the hills and has the whiskers to prove it. You might call the Modbus protocol the grandfather of industrial networking.

Ccs c modbus rtu