System diagnostics by means of the user program
5.2 System diagnostics using process image inputs
Diagnostics
Function Manual, 09/2016, A5E03735838-AD
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Evaluation of the value status
If you have enabled the value status for an I/O module, this module provides additional
information on the value status in addition to the user data. This information is directly
available in the process image input and can be called with simple binary operations.
Each channel is uniquely assigned one bit in the value status. The bit in the value status
indicates the validity of the read-in value in the user data.
Table 5- 2 Example for evaluation of the user datum in case of a wire break for a digital input module
Fault-free channel Logical 0 (actual value) 1 Valid
Influence of the value status
The value status of an active channel is "Invalid" when any
of the following factors is
given:
● Channel diagnostics pending (regardless of whether diagnostics, for example wire break,
is configured for this channel)
● For output channels: "Reaction to CPU STOP" feature is active (due to CPU STOP,
interrupted connection)
● For output channels of the digital on-board I/O of compact CPUs: If a channel is used for
technology functions, it returns the value status 0 ("Invalid"). It does not matter in this
context whether the output value is incorrect or not.
● PROFIenergy is active, i.e. hibernation is enabled (except in "Continue working mode")
When working with non-fail-safe modules, note that an invalid active channel also sets the
value status of all other active channels to "Invalid". We therefore recommend that you
disable all unconnected or unused channels.
Example - Evaluation of the value status for the input channel of an analog input module
The example below shows the basic evaluation of the value status for the input channel of
an analog input module in the user program. The diameter of a cylinder is measured by
means of a channel and its circumference is calculated.
● If the value status indicates the channel is error-free, the circumference is to be
calculated.
● If the value status indicates the channel is defective, the value "0" is to be output as
substitute value for the circumference.
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