Siemens SIMATIC ET 200AL, System Manual

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System diagnostics by means of the user program
5.2 System diagnostics using process image inputs
Diagnostics
Function Manual, 11/2019, A5E03735838-AF
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5.2 System diagnostics using process image inputs
Introduction
In addition to event-driven system diagnostics, the input and output modules of the SIMATIC
series provide diagnostics information using the process image input.
The system diagnostics described in the preceding sections is made available
asynchronously to the program processing. To ensure the correct processing of the input
and output data in the case of faults while reading the input and output data, some modules
offer the so-called value status (QI = Quality Information) for evaluation.
Requirement for evaluation of the value status
The diagnostics information about the process image input is transmitted synchronized with
the user data. Select the "Value status" check box in the properties of the I/O module in
STEP 7 if you want to evaluate the value status of the channel.
Figure 5-1 Enable value status