Application layer of OSI model of RS485 interface

OSI  A model is not a set of rules , It is more like a model to help engineers characterize the system .RS485 Interfaces are well contained in OSI  In the first three layers of the model , The actual implementation of the bus is characterized in the application layer . This layer covers the address or command set used by the device and the interpretation of data . It also includes how much data the designer expects , And the control of the bus itself .

for example ,CUI Devices  Encoder RS485 The application of the interface will be the host that requests the absolute location from the device . When the host sends the position command of the encoder （ Address ） when , The encoder responds with two complete bytes . Then the host decrypts these bytes to know what the absolute position is , At the same time, determine the frequency of sending commands and which devices it wants to send them to . Simply speaking , The application layer is RS-485 Implementation of bus .

chart 12： Fully defined  OSI  Model

because RS-485 The standard only defines the physical and data link layers with addressing requirements , Therefore, the application layer can adopt various proprietary or open communication protocols . Engineers can use existing protocols , for example Modbus agreement , Or they can define their own transport protocols for their applications . For example, use CUI Devices  Our encoder uses a very simplified addressing structure to assert devices , So as to achieve rapid turnover and the shortest processing time . The address of each encoder has only the upper six bits of one byte , The lower two are commands . This allows the encoder to start responding after a single byte from the host , Thus ensuring fast turnaround time , This is crucial in motion control applications .