Leased Scan Class
The Leased scan class allows you to poll Tags at a fast rate when they are subscribed to and visible in the a Client window, or in the Designer. If the Tags are not subscribed to, or if the window in the Client is closed, or the folder containing the Tags in the Designer is not expanded to show the Tags, the scan class runs at the slow polling rate. Essentially, a Leased scan class polls Tags only when someone needs to look at the Tag values.
To Add a Leased Scan Class
Let's add a new Leased scan class that polls a Tag from the PLC at a 1 second rate when someone needs to view that Tag in the Client. If the Client is closed, or the Tags are not displayed in the Tag Browser of the Designer, the Tags will not poll at all.
Open the Designer. In the Tag Browser, click on the Timer (
) icon to open the Scan Class Editor.On the left side of the window, you can see all existing scan classes. To create new scan class, click on the green + icon to add a new scan class.
Enter the name of the scan class. For example, name it Leased.
- Set the Mode to Leased.
- Set the Slow Rate to 0, and the Fast Rate to 1000, or 1 second.
- Click OK.
Now, set the scan class on the Tags to use this new Leased scan class. Go to your Tag Browser, find some Tags you want to add to the scan class. This example uses three Sine Tags. Right click on the selected Tags, and click on Edit tag(s).
This opens the Tag Editor window. Select the Leased scan class from the dropdown, and click OK.
Now, you just created a new Leased scan class and added your Tags. These Tags are only polled when a component that is bound to it is showing in a Client, or when the Tags are showing in the Designer.
Remember that Tag History is stored based on the current Ignition Tag value. If your Leased scan class has a 0 slow rate, you will not get updated data to store for history.
Always make sure your scan class slow speed is at least as fast as your History scan class speed.