OPC DA Qualities
The OPC DA
OPC DA - OLE for Process Control Data Access Specification
Quality flags represent the quality state for a item's data value. The low 8 bits of the quality flags are currently defined in the form of three bit fields; Quality, Sub-status and Limit status. The 8 quality bits are arranged as follows:
QQSSSSLL
Quality bit fields
Good | The Quality of the value is Good |
Bad | Value is not useful for reasons indicated by the sub-status |
Uncertain | The quality of the value is uncertain for reasons indicated by the sub-status |
Sub-status bit fields
When the quality is "Good", the following sub-status may apply:
Non-specific | The value is good. There are no special conditions |
Local Override | The value has been Overridden. Typically this is means the input has been disconnected and a manually entered value has been ‘forced’ |
When the quality is "Bad", the following sub-status may apply:
Non-specific | The value is bad but no specific reason is known |
Configuration Error | There is some server specific problem with the configuration. For example the item is question has been deleted from the configuration |
Not Connected | The input is required to be logically connected to something but is not. This quality may reflect that no value is available at this time, for reasons like the value may have not been provided by the data source |
Device Failure | A device failure has been detected |
Sensor Failure | A sensor failure had been detected (the ’Limits’ field can provide additional diagnostic information in some situations.) |
Last Known Value | Communications have failed. However, the last known value is available |
Comm Failure | Communications have failed. There is no last known value is available |
Out of Service | The block is off scan or otherwise locked This quality is also used when the active state of the item or the group containing the item is InActive |
When the quality is Uncertain, the following sub-status may apply:
Non-specific | There is no specific reason why the value is uncertain |
Last Usable Value | Whatever was writing this value has stopped doing so |
Sensor Not Accurate | Either the value has ‘pegged’ at one of the sensor limits or the sensor is otherwise known to be out of calibration via some form of internal diagnostics |
Engineering Units Exceeded | The returned value is outside the limits defined for this parameter. Note that in this case (per the Fieldbus Specification) the ‘Limits’ field indicates which limit has been exceeded but does NOT necessarily imply that the value cannot move farther out of range |
Sub-Normal | The value is derived from multiple sources and has less than the required number of Good sources |
Limit bit field
The Limit Field is valid regardless of the Quality and sub-status. In some cases such as Sensor Failure it can provide useful diagnostic information. The following limits may occur:
Not Limited | The value is free to move up or down |
Low Limited | The value has ‘pegged’ at some lower limit |
High Limited | The value has ‘pegged’ at some high limit |
Constant | The value is a constant and cannot move |
See also
Apis Vendor Specific Item Qualities