OPC Data Access Servers (DA Servers)
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, the ERP 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)
See also:
Item Structuring
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.
Data Transfer for Value Changes
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 ERP system:
Percentage deadband
Calculation Formula:
Absolute deadband= (upper limit – lower limit) * percentage deadband
A temperature sensor has a range from -100°C to +900°C. A percentage deadband of 0.1% has been set up in the OPC client.
Lower limit: -100
Upper limit: +900
Percentage deadband: 0.1%
The absolute deadband is calculated as follows:
(900 - (-100)) * 0.1% = 1
This means that the temperature must change at least by ±1°C in order that the value is reported to the ERP system.