Standard system items

System items provide easy access to the status and diagnostics information. Client applications can read data from them just like ordinary items.

However, in most cases the system item values are not directly acquired through the communications layer. System item values are usually generated through internal calculations, measurements, and tracking.

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 and the device the item is attached to.

• Device group (OPC access path/topic): A collection of items on the same physical location with the same protocol update rate.

For DDE/SuiteLink clients, $SYS$Status always comes from the leaf level of a Communication Driver hierarchy branch, which is the destination PLC node. 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. For practical application, OPC clients should reference $SYS$Status at any hierarchy levels other than the root.

All system items follow the same naming convention:

• All system items start with $SYS$.

• The Communication Drivers Core 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 server-wide.