Standard system items

System items provide you with easy access to Gateway Communication Driver’s status and diagnostics information. They are treated just like ordinary items with respect to the client. However, in most cases these items are not directly acquired via the communications layer. System item values are usually generated through internal calculations, measurements, and the tracking of the OI Engine.

System items, like ordinary items, are defined by the following properties:

• Group (client group/OPC group): The arbitrary collection of items, not correlated.

• Hierarchical location (link name/OPC path, the hierarchical node section of the fully qualified OPC item ID): The device the item is attached to.

• Device group (OPC access path/topic, or a Scan Group on a hierarchical branch): A collection of items on the same physical location with the same protocol update rate.

In the ArchestrA context, the device group plays the most important role of identifying the scope of any item. The device group defines the hierarchical location implicitly when using globally unique device-group names, which is required for DDE/SuiteLink compatibility.

All system items follow the same naming convention:

• All system items start with $SYS$.

• The OI Engine scans and parses the name for system items. Parsing of the name is case-insensitive.

All system items can be accessed through subscriptions to a Device Group. However, while some system items return data for that Device Group, others are gateway-wide.

For DDE/SuiteLink clients, you can access $SYS$Status always comes from the leaf level of the Gateway Communication Driver hierarchy branch, which is the destination data source.

For OPC clients, $SYS$Status can be accessed at all hierarchy levels. $SYS$Status at the root level of the whole hierarchy tree is always good, as it represents the quality status of the local computer itself. Hence, for practical application, OPC clients should reference $SYS$Status at any hierarchy levels other than the root. In the case of an ArchestrA data source, $SYS$Status is always good, even at the ArchestrA Group level.