Editing Scan Classes
Adding and Editing Scan Classes
You can add and edit scan classes by clicking on the Edit Scan Classes icon in the Tag Browser. The Scan Class Editor window is displayed showing a list of already configured Scan Classes on the left, and configuration settings on the right. To add a scan class, simply click the green + icon on the lower-left of the Scan Class Editor and then specify the name and the properties for it.
Scan Class Editor |
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Edit Selected Scan Class |
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Scan Class Name |
Unique name of the scan class. |
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Mode |
Direct |
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Slow Rate |
Base update rate, specified in milliseconds, at which tags will be executed. |
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Fast Rate |
Used by the Driven and Leased modes, this is the faster rate that the tags will be executed at when those modes are active. |
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Stale Timeout |
How long to wait before the tags in the scan class are determined to be stale (not running). This is calculated off of the last expected execution time of the scan class, and is particularly important for scan classes executed by other drivers through the external Tags provider. This property is not used by internal providers. |
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Driven PropertiesUsed by the driven mode to determine when the scan class should run at the fast rate. |
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Driving Tag |
Which tag will determine when the rate of the scan class should be changed. The Driving Tag should not use this same scan class. The Driving Tag should be on a separate scan class that runs at a faster rate than the Driven scan class. This means that changes to the Driving Tag will be recognized by the system faster, and the change in rate on the Driven scan class will occur faster. |
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Operator |
How the Value property should be compared to the Driving Tag's value. If the comparison is true, then the Fast Rate will be used by the Scan Class. Otherwise, the Slow Rate will be used. The Any Change operator works differently than the other operators: The scan class will execute immediately whenever the driving tag changes value. Using the Any Change operator means that the scan class no longer uses the Slow Rate or Fast Rate properties. |
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Value |
Used by the Operator property to determine if the Scan Class should execute at the slow or fast rate. |
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One-shot execution |
One-shot will execute once when the comparison condition is true, and not again until the condition become false, and subsequently true. |
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Advanced PropertiesSettings that subtly affect how the scan class operates. |
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OPC Data Mode |
This mode dictates how OPC values are obtained. The default mode, Subscription, is generally recommended. Subscribed Read |
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OPC Read After Write |
This feature is new in Ignition version 7.8.0 When enabled, a read request will be sent immediately after a write request. This means that the value on the tag will be updated much quicker to reflect the latest written value. Enabling this property is less efficient as a single write to a tag becomes two separate requests. This is especially helpful with slower scanclasses as it the tags will show the latest value quicker than the normal execution would allow. |
Historical Scan Classes
Historical scan classes are simply standard scan classes used by Tags to store history. By using separate scan classes for status and history, it's possible to maintain a tag's status at a fast rate, without storing large amounts of history unnecessarily.
Despite the fact that there is not a technical differentiation between standard and historical scan classes, it is recommended that you create separate scan classes for each purpose and name them in a manner that indicates their usage. It is common to modify scan classes in order to affect a large number of tags, and without a consistent distinction it may be possible to affect tag execution in unexpected ways.
OPC-UA Stale Threshold
Ignition uses the OPC-UA Stale Threshold setting to determine when updates from a driver have taken too long to complete. This time period will be calculated as the fastest sampling rate for that node multiplied by the Stale Threshold setting value.
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Go to the Configure section of the Gateway, and then go to OPC-UA > Settings.
The Settings page is displayed. -
Scroll down to the Other section and then to the Stale Threshold setting.
This is a multiplier by which the server determines that updates from a driver have become stale.