Modbus is an open protocol for communication between various devices (PLC, touchscreens, I/O interfaces, etc.), which allows to transfer data across different networks and buses. Communication works by transmitting data words between a client and a server (master and slave).
Modbus protocol defines the structure of the message at the protocol level (PDU – Protocol Data Unit) regardless of the type of communication layer. Depending on the type of network where the protocol is used, the PDU can be extended to take part at the application level (ADU – Application Data Unit).