Siemens SIMATIC ET 200AL, System Manual

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What you should know about analog technology
2.15 Value status
Analog value processing
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Function Manual, 06/2014, A5E03461439-AC
2.15 Value status
Value status
In case of a faulty analog value, the analog module outputs the error values 0x7FFF (error
value for overflow and all other error states) or 0x8000 (error value for underflow) for the
affected channel. The user program is now able to detect and evaluate a channel error. The
user also has the option to evaluate the system diagnostics event-driven by means of
diagnostics alarms.
In addition to the error values 0x7FFF or 0x8000 and the configured diagnostic types, analog
input and output modules provide diagnostics information by means of the Process image
input (PII). It is referred to as Value status and transferred synchronously with the user data.
The value status (Quality Information = QI) provides a statement regarding the validity of the
input signal. A distinction is made between the quality level "Good" (signal valid = 1) and
"Bad" (signal invalid = 0).
Note
For analog input modules, the use of the value status is an alternative to the evaluation of
the channels by the error values 0x7FFF or 0x8000. Compared to the error values, the
binary evaluation of the value status (0 or 1) in the user program is simpler and more clearly
arranged.
For analog output modules, the value status informs you as to whether the written value can
be output by the respective channel. Using the value status you can respond to this
information without having to evaluate the diagnostics of the module in the first place.