OPC Data Access servers (DA servers) provide data from different data sources (for example, process control systems) that an OPC client (for example, SAP ODA) can read. At the same time, DA servers can receive data from an OPC client and process it. Thus, OPC servers enable accessing a large number of items from a large number of data sources (for example, process control systems).

Together with a DA server, an SAP R/3 System can use the following functions:

• Reading values from the DA server
• Writing values to the DA server
• Obtaining information about value changes in the DA server (Data Access subscription)

All data sources that a DA server provides are organized in a namespace, which can be flat or hierarchical.

In a flat namespace, the items are all on one level. In a hierarchical namespace, the nodes can, for example, represent the machines and devices in a production line. Below the nodes are the leaves that represent temperature sensors or regulators. The leaves are called items. Every item has an item ID and can provide values.

In order that the DA server knows which values it is to transfer to SAP ODA in a Data Access subscription, certain engineering units (for example, temperature) must be defined in the DA server for every process unit. The DA server uses the following data to determine an absolute range within which the value of the OPC item must at least change to be transferred to SAP ODA.

The following data must be defined:

• In the DA server:

• A lower limit up to which the value can still be shown exactly
• An upper limit up to which the value can still be shown exactly

• In the SAP R/3 System:

• Percentage deadband

Absolute deadband = (upper limit – lower limit) * percentage deadband

• Lower limit: -100
• Upper limit: +900
• Percentage deadband: 0.1%