Process Image - WAGO -I/O-SYSTEM 750 User's Installation And Configuration

56 • Fieldbus controller 750-842

Process image

3.2.4 Process image
The powered up controller recognizes all I/O modules connected in the node,
which are waiting for or transmitting data (data width/bit width > 0). In nodes
analog and digital I/O modules can be intermixed.
Note
For the number of input and output bits or bytes of the individual switched on
I/O modules please refer to the corresponding description of the I/O modules.
The controller generates an internal local process image from the data width
and the type of I/O module as well as the position of the I/O modules in the
node. This is divided into an input and an output area.
The data of the digital I/O modules are bit orientated, i.e. the data exchange is
made bit by bit. The analog I/O modules are representative for all I/O modules
which are byte orientated, in which the data exchange is also made byte by
byte. These I/O modules also include, for example, counter modules, I/O mod-
ules for angle and path measurement as well as communication modules.
The data of the I/O modules is separated for the local input and output process
image in the sequence of their position after the controller in the individual
process image.
First, all the byte-oriented bus modules are filed in the process image, then the
bit-oriented bus modules. The bits of the digital modules are grouped into
bytes. Once the number of digital I/O's exceeds 8 bit, the coupler automati-
cally starts another byte.
Note
If a node is changed, this may result in a new process image structure. In this
case the process data addresses also changes. In the event of adding modules,
the process data of all previous modules has to be taken into account.
The process image for the physical bus module data is identical with that of
the WAGO ETHERNET TCP/IP fieldbus coupler. The controller uses a mem-
ory space of 256 words (word 0 ... 255) for the phyical input and output data.
The controller is assigned an additional memory space for imaging the PFC
variables defined according to IEC 61131-3. This extended memory space
(word 256 ... 511 each) is used to map the PFC variables behind the physical
process image.
The division of the memory spaces and the access of the PLC functionality
(CPU) to the process data is identical with all WAGO fieldbus controllers. Ac-
cess is via an application related IEC 61131-3 program and independent on the
fieldbus system.
In contrast to the above, access from the fieldbus side is fieldbus specific.
For the ETHERNET TCP/IP fieldbus controller, a MODBUS/TCP master can
access the data via implemented MODBUS functions. Here, decimal and/or
hexadecimal MODBUS addresses are used.
Modular I/O System
ETHERNET TCP/IP