ABSTRACT
The Modbus protocol, created in 1978, is the single, most supported protocol among automation devices today. Modbus is an application layer messaging, protocol placed at OSI layer 7, for client/server communication between devices connected on different types of buses or networks. This chapter describes the full protocol and its client/server behavior in the abstract, with no particular instantiation of the layers. The transfer of information between a Modbus client and a Modbus server is initiated when the client sends a request to the server to transfer information, to execute a command, or to perform one of many other possible functions. Server devices are addressed using a unit ID. This ID is assigned to servers, not to clients. The code field accommodates the encoding of the service asked to the server when in a request and signals the success or failure of the requested service in a response (excluding the broadcast case that does not have a response).
