WAGO-I/O-SYSTEM 750
MODBUS/TCP
91
758-874/000-131 WAGO-I/O-IPC-C6
Manual
Version 1.0.0
Pos: 46.1 /Serie 758 (Funk, IPC und PFC)/Feldbuskommunikation/Feldbuskommunikation 758-87x MODBUS allgemein @ 10\mod_1312456038415_21.doc @ 75904 @ 12233 @ 1
10 MODBUS/TCP
The modular concept of the 750 Series makes it possible to connect up to 250 (via
internal data bus extension) I/O modules to the I/O-IPC. This variable
construction and the large number of different I/O modules prevent a static
assignment of input and output data to fixed MODBUS addresses, however. The
only exceptions are the "digital" MODBUS services. For these, the MODBUS
address is identical to the channel number; i.e., the 47th digital input can always
be found at MODBUS address "46".
By adding or removing I/O modules, the structure of the process images is
changed which also changes the MODBUS addresses of individual I/O module
channels.
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 to
the MODBUS master.
The most important elements of a MODBUS telegram are:
Table 42: Elements of a MODBUS telegram
Term
Description
UnitID
Identification of which device is to be activated (<FF)
FunctionCode (FC) Service identification: read or write operation in bits or words
Address
Operation start address
Count
Number of bits or words depending on the service
[Data] Process
data
The service identification or "FunctionCode" (FC) first determines whether the
service is a read or write operation. It also determines the basic data type to which
the 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 stand
for 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 MODBUS
Data type:
Length
Description
Discrete Inputs
1 bits
Digital Inputs
Coils 1
bits
Digital
outputs:
Input Register
16 bits
Analog input data
Holding Register
16 bits
Analog output data
One or more "FunctionCodes" are defined for each basic data type.