Tag Quality and Overlays
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 and an overlay is shown to make the bad quality visible. 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 called "Quality", and you can see it in the Tag Browser once you expand (click on the + icon) a tag. Additionally, bad tag qualities are reflected in components bound to tags through the quality overlay system.
The following table outlines the primary data qualities. The most important is Good, and that has a value of 192.There are more values, but these represent the most common:
Quality |
Value |
Meaning |
|
Good |
192 |
The data has met all criteria for being considered reliable. |
|
OPC_Bad_Data |
0 |
The data is not reliable, further data isn't available. |
|
OPC_Not_Connected |
8 |
The OPC server driving the tag is not currently connected OR a value has not yet been received by the tag from the server. |
|
Config_Error |
300 |
There is a problem with the tag's configuration. The error log may provide more information as to the exact problem. |
|
Comm_Error |
301 |
There is a problem in communication somewhere between the tag and its data source. |
|
Expression_Eval_Error |
310 |
The expression in the tag generated an error during execution. The error log should provide more information on the error. |
|
Tag_Exec_Error |
330 |
There was an error when evaluating the tag. |
|
Type_Conversion_Er |
340 |
The value of the tag could not be converted to the requested data type. Check the assigned data type of the tag. |
|
Driver_Demo_Timeout |
900 |
The system driving the tag is operating in demo mode and has timed out. |
|
Access_Denied |
403 |
The tag permission settings do not allow the current user to view the tag. |
|
Not_Found |
404 |
The tag, or a tag referenced from inside of it, could not be found (incorrect reference path). |
|
Disabled |
410 |
The tag's "enabled" property has been set to false. |
|
Stale |
500 |
The tag has not been evaluated within the expected time frame. There is likely a deeper problem with the tag provider. |
|
GW_Comm_Off |
901 |
When viewing Tags in the Designer, the tags will have this value if communication with the Gateway is turned off from the toolbar. |
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".
Quality Overlays
It is especially important for operators to be able to gauge the health and accuracy of what is displayed at a glance. In a highly distributed system like Ignition, it is especially important, as the client may be located at quite a distance (maybe across the world) from the physical process it is monitoring and controlling.
For these reasons, components will get visual overlays for various reasons to indicate that the data they are displaying is not good. Each data binding that drives a component is evaluated for quality. If any of these qualities becomes poor, the component will show an overlay. The different overlays can mean different things, denoting their underlying cause. What they indicate is based on the Quality properties of Tags.
