OPC HDA Quality Values

Industrial Communication Toolbox™ software uses specific quality attributes defined by the OPC Foundation, based on a major quality value, a substatus for that major quality value, and a limit status indicating how the value is limited. This topic describes the standard quality attributes defined by the OPC Foundation that are used in the toolbox, and describes any special extensions that the toolbox uses.

OPC HDA quality values identify the quality or integrity of retrieved historical data. The quality is returned as a 32-bit number. The upper 16 bits relating specifically to HDA quality and the next 8 bits represent the vendor-specific quality information. The lower 8 bits are arranged in QQSSSSLL format, where QQ represents the major quality, SSSS represents the quality substatus, and LL represents the limit status.

Upper 16-bit HDA Quality Values

This table lists quality values represented by the upper 16 bits that relate to HDA quality.

Quality Values	Description	Mask Value	Associated DA Quality
`OPCHDA_EXTRADATA`	More than one piece of data that might be hidden exists at the same timestamp.	0x00010000	Good, Bad, Quest
`OPCHDA_INTERPOLATED`	Interpolated data value.	0x00020000	Good, Bad, Quest
`OPCHDA_RAW`	Raw data value.	0x00040000	Good, Bad, Quest
`OPCHDA_CALCULATED`	Calculated data value, as returned by `ReadProcessed`.	0x00080000	Good, Bad, Quest
`OPCHDA_NOBOUND`	No data found to provide upper or lower bound value.	0x00100000	Bad
`OPCHDA_NODATA`	No data collected; archiving not active (for item or all items).	0x00200000	Bad
`OPCHDA_DATALOST`	Collection started, stopped, or lost.	0x00400000	Bad
`OPCHDA_CONVERSION`	Scaling or conversion error.	0x00800000	Bad, Quest
`OPCHDA_PARTIAL`	Aggregate value is for an incomplete interval.	0x01000000	Good, Bad, Quest

Major Quality Values

This table lists the major quality values represented in bits 7 and 8 of the quality value.

Value	Quality Text	Description
0	`Bad`	The value is not useful for the reason indicated by the substatus. The table Bad Quality Substatus Values contains information about the substatus for bad quality.
1	`Uncertain`	The quality of the value is uncertain for reasons indicated by the substatus. The table Uncertain Quality Substatus Values contains information about the substatus for uncertain quality.
3	`Good`	The quality of the value is good. The table Good Quality Substatus Values contains information about the substatus for good quality.
N/A	`Repeat`	The value repeats a previously known value for this item. This value is specific to Industrial Communication Toolbox and occurs only when you request data in array formatted values.

Quality Substatus

Each major quality status has an additional substatus that describes the quality of the value in more detail. These tables describe the quality substatus for each major quality.

Good Quality Substatus Values

Value	Substatus Text	Description
0	`Non-specific`	The value is good. There are no special conditions.
6	`Local Override`	The value has been overridden. Typically, this means that the device has been disconnected from the OPC server (either physically, or through software) and a manually entered value has been forced.

Uncertain Quality Substatus Values

Value	Substatus Text	Description
0	`Non-Specific`	The server has not published a specific reason for why the value is uncertain.
1	`Last Usable Value`	Regard the value as stale because the data source has stopped writing the value. Unlike `'Bad: Last Known Value,'` which indicates a detectable communications error, `'Uncertain: Last Usable Value'` occurs when an external source fails to update the value within an acceptable period of time. You can examine the age of the value using the `TimeStamp` property associated with this quality.
4	`Sensor Not Accurate`	The value has either pegged at one of the sensor limits, or the sensor has been diagnosed as out of calibration.
5	`Engineering Units Exceeded`	The returned value is outside the limits defined for this value. This substatus does not imply that the value is pegged at some upper limit. The value may exceed the engineering units even further in future updates.
6	`Sub-Normal`	The value is derived from multiple sources and has less than the required number of good sources.

Bad Quality Substatus Values

Value	Substatus Text	Description
0	`Non-Specific`	The value is bad but no specific reason is known.
1	`Configuration Error`	There is some server-specific problem with the configuration. For example, the item in question is deleted from the running server configuration.
2	`Not Connected`	The input is required to be logically connected to something, but is not connected. This quality can reflect that no value is available at this time, possibly because the data source has not yet provided one.
3	`Device Failure`	A device failure has been detected.
4	`Sensor Failure`	A sensor failure has been detected.
5	`Last Known Value`	Communication between the device and the server has failed. However, the last known value is available. The age of the last known value can be determined from the `TimeStamp` property.
6	`Comm Failure`	Communication between the device and server has failed. No last-known value is available.
7	`Out of Service`	The `Active` state of the item or group containing the item is set to `off`. This quality is also used to indicate that the item is not being updated by the server for some reason.

Limit Status

The limit status represented by the lower 2 bits of the quality value is not dependent on the major quality and substatus value.

This table lists the limit status values and texts used in Industrial Communication Toolbox.

Value	Limit Status Text	Description
0	`Not Limited`	The value is free to move. When the limit status has this value, it is omitted from any quality attribute in the toolbox.
1	`Low Limited`	The value is fixed at some lower limit.
2	`High Limited`	The value is fixed at some upper limit.
3	`Constant`	The value is a constant and cannot change.