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Ignition has Quality built into Tags automatically. Data Quality is the measure of how reliable a particular Tag's data is. If a Tag's quality is not Good (192), the value generally should not be trusted, and an overlay is shown to make the Tag's bad quality visible. There are a wide variety of causes of bad data, from network disconnections, to software failure, to invalid Tag configurations.
Quality is extremely important and exists on every Tag, and is visible in the Designer, Perspective Sessions and Vision Clients through an Overlay system. There are a few of ways to check Tag Quality.
In the Tag Browser, find your Tag, expand it, and scroll down to the meta property called Quality. Here, you can check the quality of the Tag. This example shows a Good Quality Tag, meaning the Tag can be trusted.
One obvious indicator if the Tag is of bad quality is if there is a a red exclamation mark 
icon next to the Tag. Hover over the icon to see the error message and also expand the Tag to see the quality issue. This example shows the HOA Tag with with a Configuration Error. Ignition identifies the Quality issue to help you resolve the issue promptly. 
It is especially important in HMI screens 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, Perspective and Vision components display visual overlays for various reasons to indicate that the data they are displaying is not good, or pending a reply from the device. Each data binding that drives a component is evaluated for quality. If any of these qualities becomes poor, the Perspective or Vision 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.
Component overlays appear in the Designer workspace, Perspective Session, and Vision Client to let designers and operators know when there is a problem with one of the bindings on a component. What is cool about component overlays is that they not only tell you that there is a problem, but they also help diagnose the problem. Vision and Perspective overlay systems are similar, but each look a little bit different.
The sections below describe in detail Perspective and Vision overlays. Each module has its own Tag Quality Code Reference Table displaying the error codes and what they mean.
Perspective overlays give designers and operators a lot of information to help them diagnose and correct a problem. Component overlays look and behave slightly different in Designer Mode, Preview Mode, and in a Perspective Session. Each is described below.
In the following example, you see the LED Display component has an overlay with a red border around it and an exclamation mark 
icon. If you select the component, a red triangle will appear in upper right corner of the Property Editor giving the designer some information about the error. Click the red triangle to open the message box to identify the data quality error and cause . You'll also notice that the affected property (i.e., value) is highlighted in light red indicating this property is the one having a problem. If the error is binding related, click on the binding icon next to the property to open the Edit Binding window.
Under the Binding Preview area, you can see there is a Configuration Error. If the problem is with a binding type, chances are you might be able to correct it here, or you may need to go directly to the Tag, or check other resources like a database, OPC server, etc.
Switch from the Designer mode to the Preview Mode. To put your active view in Preview Mode, press the Preview / Designer mode 
icon in the top menubar. Components that have a problem will have a red overlay and exclamation mark around them. To see the error, click on the exclamation mark . This opens a message box that informs you that you have an error and identifies affected property.
Component overlays in a Perspective Session work the same way as they do in Preview Mode. The component with the problem will have a red overlay with an exclamation mark. Click on the exclamation mark to open the message box. It will identify the data quality error and affected property to help to identify the problem. To correct the problem, you'll have to go back to the Designer.
There are four primary quality codes which quickly inform the user of the quality of the Tag: Good, Uncertain, Bad, and Error.
Each quality code has a range of subcodes that provide more specific information about the value of the Tag. The following tables outline the primary data qualities. The most important is Good which has a value of 192 and does not have an overlay.
| Good Quality | Subcodes | Meaning |
|---|---|---|
| Good_Unspecified | 0 | A generic "good" code. Generally used in conjunction with a matching good quality subcode, (1,2, or 192). |
| Good_Provisional | 1 | Good data that should not be considered valid over long periods of time. Provisional values will not cache. |
| Good_WritePending | 2 | Used when a write is in progress. Generally, values use this code until the system knows the write through successfully, which would then result in a 192 code. |
| Good | 192 | This data has met all criteria for being considered reliable. |
Uncertain Quality | Subcodes 256 - 511 | Meaning |
| Uncertain | 256 | An unspecified degree of uncertainty exists in this value. |
| Uncertain_LastKnownValue | 257 | The current value is unavailable and represents the last known value. |
| Uncertain_InitialValue | 258 | Indicates that a subscription has been made and a good value should be arriving shortly. |
| Uncertain_DataSubNormal | 259 | Insufficient good-quality sources required for the derivation of this value. |
| Uncertain_EngineeringUnitsExceeded | 260 | Indicates that a value has gone beyond its configured engineering units. |
| Uncertain_IncompleteOperation | 261 | An async operation is currently pending and its result is unknown. |
Bad Quality | Subcodes 512 - 767 | Meaning |
| Bad | 512 | General quality for a bad quality. |
| Bad_Unauthorized | 513 | An unauthorized request was made for data that requires authorization. |
| Bad_AccessDenied | 514 | Data requested that requires credentials not held by the requesting user. |
| Bad_Disabled | 515 | Data source is currently not enabled. |
| Bad_Stale | 516 | Data is out-of-date based upon the requested refresh interval. |
| Bad_TrialExpired | 517 | The Trial Mode's timer expired. |
| Bad_LicenseExceeded | 518 | The license limit has been exceeded. |
| Bad_NotFound | 519 | Object requested was not found. |
| Bad_ReferenceNotFound | 520 | Derived or referenced value required an object which was not found. |
| Bad_AggregateNotFound | 521 | Requested aggregate was not found. |
| Bad_NotConnected | 522 | A conncection required for this valueis not currently connected. |
| Bad_GatewayCommOff | 523 | Connection to the Ignition Gateway is currently turned off. |
| Bad_OutofRange | 524 | This value exceeded its allowed range. |
| Bad_DatabaseNotConnected | 525 | A database connection required for this value is not connected. |
| Bad_ReadOnly | 526 | The target is not writable / read only. |
| Bad_Failure | 527 | A "failure" code was received from the underlying system. Additional details may be in the diagnostic message. This generally does not indicate an exception, which would be handled by Error_Exception, but instead a simple failure from a system that can return success or failure. |
| Bad_Unsupported | 528 | The operation is not supported by the target. |
Error Quality | Subcodes 768 - 1023 | Meaning |
| Error | 768 | An unexpected error occurred while retrieving or calculating this value. |
| Error_Configuration | 769 | The source of this value is not configured correctly. |
| Error_ExpressionEval | 770 | The source expression was unable to be executed. |
| Error_TagExecution | 771 | The source Tag could not be executed. |
| Error_TypeConversion | 772 | The actual value was not able to be coerced into the configured data type for the source of this value. |
| Error_DatabaseQuery | 773 | A database query required for this value caused an error upon execution. |
| Error_IO | 774 | An input/output error occurred while attempting to retrieve or calculate this value. |
| Error_TimeoutExpired | 775 | An async operation failed dure to a timeout. |
| Error_Exception | 776 | An exception was caught, and logged in the relevant system. |
| Error_InvalidPathSyntax | 777 | A path (i.e., Tag path, property path, etc.,) was not able to be parsed because the syntax is invalid. |
| Error_Formatting | 778 | Attempted formatting (i.e., numeric, date formatting) failed. |
| Error_ScriptEval | 779 | A script needed to create this value failed to execute. |
| Error_CycleDetected | 780 | Calculating the value involved an execution cycle. |
An overlay on a Vision component lets the operator know that they could be looking at a bad value for that Tag. When the overlay goes away and the values start coming in again, the operator knows that it's a valid Tag, and the values can be trusted.
In the following example, you see a red overlay with an icon in the top left corner of the selected LED Display component. The icon gives you a clue to the source of the problem. In this example, it is an SQL Database error. In the Vision Property Editor, the Quality property is highlighted and you'll notice there is a "Error_DatabaseQuery" error message.
The overlays table in the next section show all the possible Vision overlays and what they mean.
Let's switch from the Designer mode to the Preview Mode. To put your active view in Preview Mode, press the Preview / Designer mode icon in the top menubar. Components that have a problem will have a red overlay and an icon in the top left of the component overlay to indicate the problem. The overlay is identical to the overlay that is displayed in the Designer, but the component cannot be selected.
Component overlays in a Vision Client work the same way as they do in Preview Mode of the Designer. You have to look at the icon on the overlay to help you diagnose the problem. Go back to the Designer to correct the problem.
The quality codes for Vision are different from Perspective's quality codes. Each code has a description and a subcode. When you encounter a component overlay that you are unfamiliar with, check the following table because each quality code has a range of subcodes that provide more specific information about the value of the Tag. The most important is Good which has a value of 192.
The following tables outline the primary data qualities.
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 |
|---|---|---|
| Type_Conversion_Error | 340 | The value of the tag could not be converted to the requested data type. Check the assigned data type of the tag. |
| Tag_Exec_Error | 330 | There was an error when evaluating the tag. |
| Stale | 500 | The tag has not been evaluated within the expected time frame. There is likely a deeper problem with the tag provider. |
| 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. |
| OPC_Bad_Data | 0 | The data is not reliable, further data isn't available. |
| Not_Found | 404 | The tag, or a tag referenced from inside of it, could not be found (incorrect reference path). |
| 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. |
| Good (Provisional) | 320 | Temporary "Good" value. The value isn't cached because the underlying quality may differ than what appears on the Tag. |
| Good | 192 | The data has met all criteria for being considered reliable. |
| Expression_Eval_Error | 310 | The expression in the Tag generated an error during execution. The error log should provide more information on the error. |
| Driver_Demo_Timeout | 900 | The system driving the Tag is operating in demo mode and has timed out. |
| Disabled | 410 | The Tag's "enabled" property has been set to false. |
| 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. |
| Access_Denied | 403 | The Tag permission settings do not allow the current user to view the Tag. |
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."
Choosing the Overlay Opt-Out option will ignore the quality of the chosen Tag, making it have no effect on the component's quality overlay. The Overlay Opt-Out option is located in the Tag bindings for both Perspective and Vision components. If this option is enabled, the operator will not see any overlays and will have no indication that the underlying Tag quality is something other than good. A word of caution when you use the Opt-Out option because you always want to give the operator some indication that the values they are seeing on the screen can be trusted, and by opting out, you are removing that indicator for the operator. 
