WAGO-I/O-SYSTEM 750 MODBUS/TCP 91758-874/000-131 WAGO-I/O-IPC-C6ManualVersion 1.0.0Pos: 46.1 /Serie 758 (Funk, IPC und PFC)/Feldbuskommunikation/Feldbuskommunikation 758-87x MODBUS allgemein @ 10\mod_1312456038415_21.doc @ 75904 @ 12233 @ 110 MODBUS/TCPThe modular concept of the 750 Series makes it possible to connect up to 250 (viainternal data bus extension) I/O modules to the I/O-IPC. This variableconstruction and the large number of different I/O modules prevent a staticassignment of input and output data to fixed MODBUS addresses, however. Theonly exceptions are the "digital" MODBUS services. For these, the MODBUSaddress is identical to the channel number; i.e., the 47th digital input can alwaysbe found at MODBUS address "46".By adding or removing I/O modules, the structure of the process images ischanged which also changes the MODBUS addresses of individual I/O modulechannels.MODBUS communication is performed via service calls, the MODBUS master(client) sending a request telegram to port 502 of the MODBUS slave (server).The MODBUS slave returns the result of the service call in a response telegram tothe MODBUS master.The most important elements of a MODBUS telegram are:Table 42: Elements of a MODBUS telegramTerm DescriptionUnitID Identification of which device is to be activated (FunctionCode (FC) Service identification: read or write operation in bits or wordsAddress Operation start addressCount Number of bits or words depending on the service[Data] Process dataThe service identification or "FunctionCode" (FC) first determines whether theservice is a read or write operation. It also determines the basic data type to whichthe operation is to be applied. Therefore, the meaning of the parameters "Address"and "Count" is also dependent on the function code. Thus "address :=3" can standfor a bit or a word in the input or output process image.The MODBUS/TCP protocol is largely based on the following basic data types:Table 43: Basic data types of MODBUSData type: Length DescriptionDiscrete Inputs 1 bits Digital InputsCoils 1 bits Digital outputs:Input Register 16 bits Analog input dataHolding Register 16 bits Analog output dataOne or more "FunctionCodes" are defined for each basic data type.
