Process Image

After switching on, the head station identifies the inserted I/O modules that send or expect to receive data (data width > 0). The head station creates an internal local process image from the data width, the module type and the position of the I/O module in the node. This process image is divided into input and output data zones.

The head station provides one or more external process images that are used for data transmission via a fieldbus, for example. The structure of an external process image depends on:

• The selection and arrangement of the I/O modules in the node. Some I/O modules allow you to configure the arrangement and amount of the data they provide. Information on the process data provided by an I/O module is available in the & Product Manual of the I/O module.
• The properties of the external interface (e.g., of the fieldbus) and the properties of the head station. More information is available in the & Product Manual of the head station. Examples of process image properties that can differ among head stations:
• Arrangement of the process data in the order of the physical placement of the I/O modules
• Grouping of the process data so that analog field signals come before digital field signals
• Insertion of filler bits or bytes so that data of an I/O module starts at a word boundary
• Hiding of diagnostic bits from the process data of the I/O modules and/or mapping of them to fieldbus-specific diagnostic mechanisms
• Adaptation of the endianness to the properties of the fieldbus
• Limitation of the amount of data to packet sizes that can be processed by the fieldbus