PDU and its processing code constitute Modbus Application Protocol SpecificationCore. This achieves the separation of the core protocol which defines the protocol data unit (PDU) and the network layer which defines the application data unit (ADU). Over time, the protocol introduced different application data units to change the data packet format used in serial communication, Or allow the use of TCP/IP and User Datagram Protocol (UDP) networks. In the original approach, Modbus is a single protocol built on serial ports, so it cannot be divided into multiple Floor. Master-slave network relationship Modbus protocol layer The content of these requests and responses and the network layer that sends these messages are defined by the different layers of the protocol.įigure 1. Usually, the master device is a human-machine interface (HMI) or a monitoring and data acquisition (SCADA) system, and the slave device is a sensor, programmable Programmable logic controller (PLC) or programmable automation controller (PAC). In a master-slave relationship, communication always occurs in pairs-a device must initiate a request, then wait for a response-and the initiating device (master device) is responsible Initiate each interaction. Modbus is a request-response protocol implemented with a master-slave relationship. Access data in Modbus and Modbus data models.

Modbus read multiple registers