OPC item
The OPC item contains the actual data; it is the representation of the data point in the OPC environment. Each data point in WinCC OA, or each leaf, is assigned an OPC item via the peripheral address. The itemIDs are supplied by the server.
Each OPC item possesses quality information as well as its value. Each read transaction (Read, Update, Refresh) on one or more OPC items also supplies quality information (QI) in the form of a WORD for each of these items. The usage of the higher-value byte is left to the server manufacturer, while the usage of the lower-value byte is defined in OPC in three bit fields:
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QQSSSSLL
| Bit | Meaning |
|---|---|
| QQ: | Quality Bit Field (Good / Uncertain / Bad) |
| SSSS: | Substatus Bit Field (the meaning depends on the value of the Quality Bit Field) |
| LL: | Limit Bit Field (Not Limited / Low Limited / High Limited / Constant) |
Note that the item names may not contain any umlauts or other foreign language characters!
Note that items in nonpermanent groups can always be written no matter if they are subscribed or not!
If a server uses the higher-value byte, it should nonetheless set the value of the lower-value byte as precisely as possible.
See the chapter "OPC Server (OPC Client)" for the usage of the quality information in WinCC OA; consult the OPC Foundation's OPC Data Access Specification for the precise meaning of the individual status bits.
The server generates and sets the time stamp that is also sent with each read transaction. If the server was started as a remote server, problems can arise if the time stamp is seen by WinCC OA as coming from the future. This can be prevented with the config entry timeStampFromServer = "No". Otherwise it is necessary to synchronize the computers.
