Fieldbus Communication • 117MODBUS FunctionsWAGO-I/O-SYSTEM 750ETHERNET TCP/IP4.2 MODBUS Functions4.2.1 GeneralMODBUS is a manufacturer-independent, open fieldbus standard for diverseapplications in manufacturing and process automation.The MODBUS protocol is implemented for the transmission of the processimage, the fieldbus variables, different settings and information on the coupleraccording to the current Internet Draft.The data transmission in the fieldside takes place via TCP and via UDP.The MODBUS/TCP protocol is a variation of the MODBUS protocol, whichwas optimized for communication via TCP/IP connections.This protocol was designed for data exchange in the field level (i.e. for theexchange of I/O data in the process image).All data packets are sent via a TCP connection with the port number 502.MODBUS/TCP segmentThe general MODBUS/TCP header is as follows:Byte: 0 1 2 3 4 5 6 7 8 - nIdentifier(entered byreceiver)Protocol-identifier(is always 0)Length field(High byte,Low byte)Unitidentifier(Slave address)MODBUSfunction codeDataFig. 4-12: MODBUS/TCP HeaderMore informationThe structure of a datagram is specific for the individual function. Refer to thedescriptions of the MODBUS Function Codes.For the MODBUS protocol 15 connections are made available over TCP.Thus it allows digital and analog output data to be directly read out at afieldbus node and special functions to be executed by way of simpleMODBUS function codes from 15 stations simultaneously.For this purpose a set of MODBUS functions from the OPEN MODBUS /TCPSPECIFICATION is realized.More informationMore information on the OPEN MODBUS / TCP SPECIFICATION you canfind in the Internet: www.modbus.org.Therefore the MODBUS protocol based essentially on the following basicdata types:Datatype Length DescriptionDiscrete Inputs 1 Bit Digital InputsCoils 1 Bit Digital OutputsInput Register 16 Bit Analog-Input dataHolding Register 16 Bit Analog-Output dataFor each basic data type one or more „FunctionCodes“ are defined.
