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The Driven Tag Group can be configured as a One-shot Tag Group. The One Shot Tag Group will not run at a rate, but instead it will be triggered by a change in the driving Tag's value. One-shot polling executes once when the condition becomes true, then waits for that condition to become false then true again. This makes it follow a 'rising edge' or 'falling edge' pattern. 

Let's make a Driven One-shot Tag Group that only updates once when a Memory Tag goes to true. We are using a Memory Tag so a user can change the value when they want to poll the PLC. This is actually a good example of how an operator can ask the PLC for new values whenever they want to, using a refresh button on the screen. 

Create a Memory Tag

  1. From Tag Browser, click on the New Tag   icon or right click on a folder and select New Standard Tag > Memory Tag.

  2. In the Tag Editor, enter the following: 
    1. Tag Name:  One Shot Trigger
    2. Data Type:  Boolean 
    3. Value:  false

  3. Click OK to add the One Shot Trigger Tag to the Tag Browser.

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Driven Scan Class - One shot

Create a Driven One-Shot Tag Group

Once you have your driving Tag created, add a Driven one-shot Tag Group that lets a user poll Tags manually.

  1. In the Tag Browser, click on the  Tag Group  icon to open the Tag Group Editor.

  2. Create a new Tag group by clicking on the Add  icon.
  3. In Tag Group Editor, enter the following:

    Name: Driven One Shot
    Mode: Driven
    Rate: 0
    Leased/Driven Rate: 1000ms  

  4. Set the driving Expression by clicking the Edit  icon on the right.
  5. Select the Tag  icon and choose the Tag you want to set the condition on (i.e., One Shot Trigger).
  6. Click OK and then click apply changes. Note: This Tag group executes one time when the One shot condition is set to true.

  7. While still in the Tag Group Editor, set the following values:
    Driving Comparison: =
    Comparison Value: 1
    One Shot: true

  8. Click OK to save the Tag Group. 

Apply the Driven One Shot Tag Group

Now, we have to apply our Driven One Shot Tag Group to the One Shot Trigger Tag we created above. 

  1. Select all the Tags that you want to run in this Tag Group. This example uses Pressure3 and Thickness3 Tags.
    Right click on one of the selected Tags, and click the Edit Tag  icon.

  2. The Tag Editor will open showing you are editing multiple Tags. From the Tag Group dropdown list, choose Driven One Shot, and click OK.  

  3. Let's test it out. In the Tag Browser, mark the checkbox for the One Shot Trigger Tag to set the value to true. You will see the Pressure3 and Thickness3 values update once, and then stop. 
    Uncheck the One Shot Trigger Tag, to set the value to false, and check it again to set it to true, and you will see the values update again. 

  4. You can also drag a Momentary Button component onto a Vision window, then drag your One Shot Trigger Tag onto that button.
  5. Put the Project into Preview mode  and then click the Momentary Button component. Now your operators can request new values and the button automatically resets the Tag.
  6. In the Tag Browser, you will see the Tag values update once, and then stop.

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