Learn about the differences between Tag reference items and direct OPC items in groups.

Video recorded using: Ignition 8.1


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[00:00] As we've discussed in previous lessons a Transaction Group is fundamentally designed to link PLC data points in a database table. In this video series, we'll be expanding on that Transaction Group model by discussing the different ways that those PLC data points and in fact, other types of data too can be referenced inside of a group. In this lesson specifically, we'll be focused on the two primary ways of pulling in PLC data for a group to use and to demonstrate we have a basic historical Transaction Group pulled up already. The first way to reference PLC data is using OPC tags from our ignition system. So for example, we can find our Tag Browser on the left-hand side, find a tag we'd like to pull in and then drag and drop that tag into our basic OPC slash group item section like so. When we do this, we're telling the Transaction Group to use the tags value when the group executes, which seems very straightforward as our OPC tag is already configured the tag can be used across our system can have tag history and security and so on.

[01:05] There is one caveat here, though, remember that tags have their own update rate as determined by their tag group. Transaction Group executions are independent of the tag executions. So in some circumstances our group executions could become a little muddier. For example, if our group were configured to write to the tag here on execution but the tag only updates every 30 seconds it could take awhile for the PLC to receive the final value. If our group set up depends on very precise timing, this could cause trouble. For these situations, a second and more precise way of using OPC data is available. In the menu at the top of the Transaction Group Interface there is a Browse OPC Items icon. And if we click on that, it'll pull up our OPC browser. We can use this interface to add OPC Items directly provided our device supports browsing. In here, I'll expand ignition OPCUA server, Devices, GenSIM and then let's add sign one this time. So I'll find sign one and drag and drop into my OPC Group Items box.

[02:06] This group item is different from the previous one in that it is defined only in the group. We have a direct OPC Reference and no tags need to exist for the Transaction Group to run. As a result, we're working independently from the tag execution timings and tag group configurations won't impact our group in any way. Instead, we'll pull in the value at the rate required by the Transaction Group. And some settings inside the group can determine whether this involves asynchronous OPC subscriptions or synchronous OPC read operations. Now, when we configure OPC Items this way we'll get some special options in the items configuration. I can access that configuration menu by double clicking on the item or by right clicking on the item and selecting edit. From this menu, we'll see that in addition to configuring basic OPC Item info I can go to numeric on the left-hand side and find numeric scaling and dead band options and I can go to alarming and find an interface for configuring alarms, just like we would have on a tag.

[03:04] So in summary, when pulling data points from a device into a Transaction Group we can use either tags or OPC Items directly. Tags can be the better choice when we want to have the same data points and configuration in other places in ignition. While OPC Items can be the better choice when we're worried about complicated timing requirements or we want to avoid unnecessary delays in later videos in this series we'll look at some other possible item configurations in our Transaction Groups.

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