Modbus/Tcp; Table 42: Elements Of A Modbus Telegram; Table 43: Basic Data Types Of Modbus - WAGO -I/O-IPC-C6 Manual

WAGO-I/O-SYSTEM 750
758-874/000-131 WAGO-I/O-IPC-C6
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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
UnitID
FunctionCode (FC) Service identification: read or write operation in bits or words
Address
Count
[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:
Discrete Inputs
Coils
Input Register
Holding Register
One or more "FunctionCodes" are defined for each basic data type.
Manual
Version 1.0.0
Description
Identification of which device is to be activated (<FF)
Operation start address
Number of bits or words depending on the service
Process data
Length Description
1 bits Digital Inputs
1 bits Digital outputs:
16 bits Analog input data
16 bits Analog output data
MODBUS/TCP 91