Modbus Protocol Performance Analysis in a Variable Configuration of the Physical Fieldbus Architecture

Few developments have changed the evolution of the automation process as substantially as the development of industrial local area networks and communication protocols. Especially fieldbus systems, networks created for the lowest levels of the automation hierarchy, have an enormous influence on the flexibility and performance of modern automation systems in all application areas. The technique of updating data on Modbus type devices involves a procedure of reading data at regular time intervals called polling. Some automation processes include interdependence between different control variables belonging to different hardware devices. This paper presents and analyzes various scenarios related to a variable architecture configuration of physical modules based on Modbus communication protocol. The proposed Modbus Extension (ModbusE) concept is presented by defining the new optimized message format, and the structure of the acquisition cycle to obtain a deterministic temporal behavior, solutions for describing devices at the Modbus protocol level being presented. The status update of each Modbus module is done according to the address of the device, but also the number of registers per device. The paper analyses the worst-case scenario of communication involving Modbus devices on the same network and exchanging data corresponding to one or more server registers.