Learn about the differences between Tag reference items (Scan Class) and direct OPC items (Timer) in groups.

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[00:00] The data in groups comes fundamentally from two different sources. Either it comes from tags, defined in the system or it comes from items in the group, which are essentially tags defined only in the context of the group. So, for example, we can bring this tag over from our computer tag section and exist as a tag reference. This means that the tag continues to execute under its normal scan class. And the value is simply provided for use by the group. Alternatively, you might come over to the OPC Browser, browse, and bring data in directly in this manner. These tags are different in that they are defined only in the group. And they run under the parameters defined by the group, such as the specified update rate. These types of items support almost all of the features of normal tags, such as scaling, deadband, and alarming. In the context of the group, there's no history available because presumably that is what you might be doing with the group itself. The group additionally offers the ability to create expression items, which are something like a combination of memory, SQL query, and expression tags. They might be a static value if the expression type is set to none. They might be an expression. Or they might be a SQL query. Additional videos in this series will cover each of these modes more in depth. So why would you use one type of tag over another? As I already mentioned, tag references execute under their scan classes. In many times, tags may be used in different ways. They might be used on screens, they might be used to collect history to the tag history system, and then, you might want to use them also in a group to update a table or to pass just for our procedure. By using a tag reference, you are able to use the same value in multiple places. And so only one subscription needs to be created against the OPC server and thus against the device. On the other hand, many times there's data that should only be used inside of a group. And it's very important that the timing of this data aligns specifically with the operations of the group is performing. In those cases, you'll want to use direct OPC items or expressions defined in the group. These items will be subscribed or executed based on the group's rate as I already mentioned. Additionally, there are features in the group, such as subscribers versus read mode that can modify how the item actually behaves. In sort, items defined in the group are easier to understand because all of the settings are set directly around the group. Tag references can be a little tricky from a timing point of view because they operate on a completely different cycle. That is the cycle determined by their scan class.

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