Ethernet; Modbus Protocol; Accessing The Process Image; Example - WAGO -I/O-SYSTEM 750 Series Manual

4.3 ETHERNET

4.3.1 Modbus protocol

4.3.1.1

Accessing the process image

The data in the AS-Interface master are accessed by means of the functions for
reading and writing registers. The registers can be read and written in blocks
or individually. One register contains two bytes.
The assignment of the registers to the module I/O data is dependent on the
order and the type of the modules.
The registers are written separately according to input and output data, in
increasing order, first with the data of the byte-orientated modules (analog and
special modules) and then subsequently with the data of the bit-orientated
modules (digital modules).
The first input or output register, which is assigned to an AS-Interface master
module, contains the status / control byte and one empty byte.
The registers for the permanently superimposed mailbox are attached to this.
If the mailbox is set up to be superimposed, these registers contain the
mailbox or process data.
The further registers associated with an AS-Interface master module contain
the remaining process data.
With blockwise access, the data are transmitted with one command (e.g. FC 3
– Read Multiple Registers, FC 16 – Write Multiple Registers or FC 23 –
Read/Write Multiple Registers). The start address and the number of registers
to be transmitted are specified in the function call. The individual data are then
accessed in the higher-level controller.
The command FC 6 (Write Single Register) can be used for direct access to
individual registers or the above commands can be used by setting the number
of registers to be transmitted to 1.
4.3.1.1.1

Example

Let a node contain the following modules with an input / output process
image:
3 x 750-402
1 x 750-452
1 x 750-655
1 x 750-550
1 x 750-452
1 x 750-550
1 x 750-452
1 x 750-504
WAGO-I/O-SYSTEM 750
I/O Modules
each with 4 bits of input data,
4 bytes of input data,
12 bytes of input data and 12 bytes of output data,
4 bytes of output data,
4 bytes of input data,
4 bytes of output data,
4 bytes of input data,
4 bits of output data.
Fieldbus-specific behavior
ETHERNET
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