An OPC Item representing a tag has three values that get updated whenever the tag value changes:
• Value
• Quality
• Timestamp
The timestamp is usually the time that the tag value changed, except in the case of input tags that retrieve timestamps along with values from I/O devices. In the latter case, it is the timestamp that corresponds to the current tag value.
VTS OPC Server items support the following qualities, where appropriate:
|
Quality value |
Name |
Description |
|
0x00 |
Bad |
The tag’s value is Invalid or the tag’s Quality value indicates Bad quality. |
|
0x04 |
Config Error |
The tag has been deleted or the server application has been stopped. |
|
0x40 |
Uncertain |
The tag is marked as Questionable. |
|
0xC0 |
Good |
Good quality |
|
0xD8 |
Good (Local Override) |
The tag has Manual Data specified. |