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Data Quality is the measure of how reliable a particular tag's data is. If a tag's quality is not Good, the value generally should not be trusted. There are a wide variety of causes of bad data, from network disconnections to software failure, to invalid tag configuration. The quality is a property of the tag (Quality), and can be seen in the Tag Browser. Additionally, bad tag qualities will be reflected in components bound to tags through the quality overlay system.
The following table outlines the primary data qualities. There are more values, but these represent the most common:
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Good
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The data has met all criteria for being considered reliable.
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Bad
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The data is not reliable, further data isn't available.
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Stale
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The tag has not been evaluated within the expected time frame. There is likely a deeper problem with the tag provider.
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Config_Error
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There is a problem with the tag's configuration. The error log may provide more information as to the exact problem.
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Comm_Error
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There is a problem in communication somewhere between the tag and its data source.
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Tag_Exec_Error
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There was an error evaluating the tag.
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Expression_Eval_Error
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The expression in the tag generated an error during execution. The error log should provide more information on the error.
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Type_Conversion_Error
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The value of the tag could not be converted to the requested data type. Check the assigned data type of the tag.
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OPC_Not_Connected
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The OPC server driving the tag is not currently connected OR a value has not yet been received by the tag from the server.
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Not_Found
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The tag, or a tag referenced from inside of it, could not be found (incorrect reference path).
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Driver_Demo_Timeout
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The system driving the tag is operating in demo mode and has timed out.
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GW_Comm_Off
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When viewing tags in the designer, the tags will have this value if communication with the gateway is turned off from the toolbar.
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Access_Denied
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The tag permission settings do not allow the current user to view the tag.
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Disabled
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The tag's "enabled" property has been set to false.
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More information about Quality Overlays.
Tag Quality and Referenced Tags
When tags reference other tags, such as in expressions, they will often pass the worst sub-quality up as their own. For example, even though a particular tag's expression executes without problem, if the expression references a tag whose quality is "Bad", the expression tag will also report "Bad".
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