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
An additional byte is occupied in the input address space of the respective module when you
enable the value status. Each bit in this byte is assigned to a channel and provides
information on, for example, whether the output value specified by the user program is
actually queued at the module terminal (0= value is invalid; 1= value is valid).
Error occurrence
If a wire break occurs at an analog input module, for example, the current signal state is
entered in the process image and the value status of the signal is set to "invalid". To trigger
an indicator light in case of an error, for example, evaluate the value status in the user
program.
Diagnostics alarms and value status
If you configure analog modules in a third-party product with the help of a GSD file and the
diagnostics alarms are not evaluated event-driven, it is best to use the value status.
Note
The Value status represents a group diagnostics which only gives the user a Good or Bad
information. The Value status is not suitable for a more specific identification of the cause of
the error (e.g., wire break, short circuit in the encoder cable or load voltage failure).