Process Image; General Structure - WAGO 750 Series Manual

46 • Fieldbus Controller 750-841

Process Image

3.1.5 Process Image

3.1.5.1 General Structure

The powered-up controller recognizes all I/O modules connected in the node
that are waiting to transmit or receive data (data width/bit width > 0). The
maximal length of a node is limited to 64 I/O modules.
Note
Use of the WAGO 750-628 Bus Extension Coupler Module and the 750-627
Extension End Module enables support of up to 250 I/O modules on the
750-841 controller.
Note!
Expansion to 250 I/O modules is enabled in the controllers with
softwareversion ≥ SW 9.
Note
For the number of input and output bits or words of an individual I/O module,
please refer to the corresponding module description later in this chapter.
The controller generates an internal local process image from the data width
and type of I/O modules, as well as the position of the I/O modules in the
node. This image is divided into an input and an output area.
The data of the digital I/O modules are bit-based (i.e., the data exchange is
made by bits). The analog I/O and most specialty modules (e.g., counter
modules, encoder modules, and communication modules) are byte-based, in
which the data exchange is made by bytes.
The process image is divided into an input and an output data area. Each I/O
module is assigned a location in the process image, based on the data
exchange type (i.e., bit-based or byte-based) and their position after the
controller.
All of the byte-based I/O modules are filled in the process image first, then the
bit-based modules. The bits of the digital modules are grouped into a word.
Once the number of digital I/Os exceeds 16 bits, the controller automatically
starts another word.
Note
Changing the physical layout of a node will result in a new structure of the
process image. Also, the addresses of the process data will change. When
adding or removing modules, the process data must be verified.
The process image for physical input and output data is stored in the first 256
words of memory (word 0 to 255). This memory actually consists of a
separate area for the input and output data, but both areas are referenced in a
PLC program with an index of 0 to 255 for word operations.
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