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The Direct scan class executes at a fixed rate, which is defined by the slow polling rate setting. Every Tag that uses the Direct scan class will poll the PLC at the Slow Rate setting at all times (24x7x365).

Ignition comes with two default Direct scan classes called Default and Default Historical. You can use any Tags in your system, including multiple Tags, for creating a Direct scan class. 

To Add a Direct Scan Class

  1. 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 the default scan classes: Default and Default Historical. They are both Direct scan classes. The Default scan class polls at a slow rate of 1 second, and Default Historical polls at a 10 second rate.

     
  2. To create a new scan class, click the green + icon on the bottom-left of the window.
     

  3. Enter the name of your new scan class: Direct 5 Second.
    From the Mode dropdown, select Direct.
    Set the Slow Rate (ms) to the number of milliseconds you want to poll at. This example sets the slow rate at 5,000 ms or 5 seconds.
    Click OK.


     

  4. Once you have your Direct scan class created, let's add multiple Tags to this scan class.
    Go to your Tag Browser, find some Tags you want to add to the scan class. This example uses several Ramp Tags.
    Right click on the selected Tags, and click on Edit tag(s).



  5. This opens the Tag Editor window. Select the Direct scan class from the dropdown, and click OK



    That’s it! You created a new Direct scan class and added your Tags. Just make sure you want to poll those values all the time (24/7) when you use a Direct scan class. 

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Direct Scan Class

 

 

 

 

Direct Mode - Properties

PropertyDescription
Common
Tag Group NameUnique name of the Tag Group.
Mode

The Tag group executes at a fixed rate, defined by the Rate setting.

RateBase update rate, specified in milliseconds, at which Tags will be executed.
Note: If the rate is set to 0, the Tag group will not be executed.
OPC Settings
Data Mode

This mode dictates how OPC values are obtained. The default mode, Subscription, is preferred because it is much more efficient than a read.

Subscribed
All OPC Tags in the scan class will be subscribed according to the scan class rate. Values will come in asynchronously as they change.

Polled
Tags will not be subscribed, but will instead be synchronously read each time the scan class executes. This operation is less efficient, but allows more precise control over when values are obtained. This mode is particularly useful when collecting data over a slow or expensive connection for display. When combined with the one-shot execution mode above, and a static Tag tied to a momentary button, it's easy to create a manual refresh button on a screen that pulls data on-demand.

Read After Write

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 scan classes as the Tags will show the latest value quicker than the normal execution would allow.

Optimistic Writes

If enabled, written values will be applied to the OPC Tag immediately. Normally, the system must receive confirmation that a write was successful from the device before the OPC Tag's value would change. This property changes the behavior by assuming the write went through unless the next read value or subscription update proves otherwise. Enabling this will make writes appear to execute much quicker.

Works in conjunction with the OPC Optimistic Write Timeout property below. If the OPC Tag does not receive confirmation that the new write was successful within the timeout, the Tag will fallback to the last known value. While in an ambiguous state, the Tag with have a quality of "Good (Provisional)".

This setting can be paired with the OPC Read After Write: the Tag will assume the newly written value, while an asynchronous read request is quickly sent out to confirm the write went through.

While the write is pending, values received from subscription activities will override the fallback value. Assuming an initial value of 0, if a write of 10 is applied to the OPC Tag, then the Tag will show a value of 10 until the system can confirm the new value. If a subscription update then returns a value of 5, the OPC Tag will fallback to 5.

Optimistic Write Timeout (MS)

The timeout period for Optimistic Writes. A value of 0 effectively disables the fallback functionality: the new value is maintained on the Tag until the next read or subscription activity.

History
Min Time Between Samples
Min Time Units
Max Time Between Samples
Max Time Units





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