Understanding Quality Codes (version 21)
Canary provides the capability to write quality score as part of the tag timestamp and value whenever it is included from the data source. OPC standards are followed with an additional Canary quality code that signifies NoData.
For most the following four quality codes will handle 99% of your use cases.
Most Common Quality Codes
In addition to these quality codes, the standard OPC DA quality codes can be useful to have as a reference.
OPC DA Quality Codes
Here is the complete list of all OPC DA quality codes including the corresponding decimal (column 1) and hex value (column 2).
|4||0x00000004||Bad [Configuration Error]|
|8||0x00000008||Bad [Not Connected]|
|12||0x0000000c||Bad [Device Failure]|
|16||0x00000010||Bad [Sensor Failure]|
|20||0x00000014||Bad [Last Known Value]|
|24||0x00000018||Bad [Communication Failure]|
|28||0x0000001C||Bad [Out of Service]|
|65||0x00000041||Uncertain [Non-Specific] (Low Limited)|
|66||0x00000042||Uncertain [Non-Specific] (High Limited)|
|67||0x00000043||Uncertain [Non-Specific] (Constant)|
|68||0x00000044||Uncertain [Last Usable]|
|69||0x00000045||Uncertain [Last Usable] (Low Limited)|
|70||0x00000046||Uncertain [Last Usable] (High Limited)|
|71||0x00000047||Uncertain [Last Usable] (Constant)|
|80||0x00000050||Uncertain [Sensor Not Accurate]|
|81||0x00000051||Uncertain [Sensor Not Accurate] (Low Limited)|
|82||0x00000052||Uncertain [Sensor Not Accurate] (High Limited)|
|83||0x00000053||Uncertain [Sensor Not Accurate] (Constant)|
|84||0x00000054||Uncertain [EU Exceeded]|
|85||0x00000055||Uncertain [EU Exceeded] (Low Limited)|
|86||0x00000056||Uncertain [EU Exceeded] (High Limited)|
|87||0x00000057||Uncertain [EU Exceeded] (Constant)|
|89||0x00000059||Uncertain [Sub-Normal] (Low Limited)|
|90||0x0000005a||Uncertain [Sub-Normal] (High Limited)|
|91||0x0000005b||Uncertain [Sub-Normal] (Constant)|
|193||0x000000c1||Good [Non-Specific] (Low Limited)|
|194||0x000000c2||Good [Non-Specific] (High Limited)|
|195||0x000000c3||Good [Non-Specific] (Constant)|
|216||0x000000d8||Good [Local Override]|
|217||0x000000d9||Good [Local Override] (Low Limited)|
|218||0x000000da||Good [Local Override] (High Limited)|
|219||0x000000db||Good [Local Override] (Constant)|
For those that need or want to dive much deeper below is a complete guide to understanding the ins and outs of quality codes.
Historian Quality Bits
The low 8 bits of the quality are defined as follows in the OPC Specification. These flags represent the quality state for an item's data value. This is intended to be similar to but slightly simpler than the Fieldbus Data Quality Specification (section 4.4.1 in the H1 Final Specifications). This design makes it fairly easy for both servers and client applications to determine how much functionality they want to implement.
The low 8 bits of the Quality flags are currently defined in the form of three bit fields; Quality, Substatus and Limit status. The 8 Quality bits are arranged as follows:
The high 8 bits of the Quality Word are available for vendor specific use. If these bits are used, the standard OPC Quality bits must still be set as accurately as possible to indicate what assumptions the client can make about the returned data. In addition, it is the responsibility of any client interpreting vendor specific quality information to ensure that the server providing it uses the same 'rules' as the client. The details of such a negotiation are not specified in this standard although a QueryInterface call to the server for a vendor specific interface such as MyQualityDefinitions is a possible approach.
Details of the OPC standard quality bits follow:
The Quality Bit Field
|00SSSSLL||Bad||Value is not useful for reasons indicated by the Sub-status|
|1||01SSSSLL||Uncertain||The quality of the value is uncertain for reasons indicated by the Sub-status.|
|2||10SSSSLL||N/A||Not used by OPC|
|3||11SSSSLL||Good||The Quality of the value is Good.|
- A server which supports no quality information must return 3 (Good). It is also acceptable for a server to simply return Bad or Good (0x00 or 0xC0) and to always return 0 for sub-status and limit.
It is recommended that clients minimally check the Quality Bit field of all results (even if they do not check the sub-status or limit fields). Even when a 'BAD' value is indicated, the contents of the value field must still be a well defined VARIANT even though it does not contain an accurate value. This is to simplify error handling in client applications. For example, clients are always expected to call VariantClear() on the results of a Synchronous Read. Similarly the IAdviseSink needs to be able to interpret and 'unpack' the Value and Data included in the Stream even if that data is BAD.
If the server has no known value to return then some reasonable default should be returned such as a NUL string or a 0 numeric value.
The Sub-status Bit Field
The layout of this field depends on the value of the Quality Field.
Sub-status for BAD Quality:
|0||000000LL||Non-specific||The value is bad but no specific reason is known|
|1||000001LL||Configuration Error||There is some server specific problem with the configuration. For example the item is question has been deleted from the configuration.|
|2||000010LL||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.|
|3||000011LL||Device Failure||A device failure has been detected|
|4||000100LL||Sensor Failure||A sensor failure had been detected (the 'Limits' field can provide additional diagnostic information in some situations.)|
|5||000101LL||Last Known Value||Communications have failed. However, the last known value is available. Note that the 'age' of the value may be determined from the TIMESTAMP in the OPCITEMSTATE.|
|6||000110LL||Comm Failure||Communications have failed. There is no last known value is available.|
|7||000111LL||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.|
|8-15||N/A||Not used by OPC|
- Servers which do not support Sub-status should return 0.
- An 'old' value may be returned with the Quality set to BAD (0) and the Substatus set to 5. This is for consistency with the Fieldbus Specification. This is the only case in which a client may assume that a 'BAD' value is still usable by the application.
Sub-status for UNCERTAIN Quality:
|0||010000LL||Non-specific||There is no specific reason why the value is uncertain.|
|1||010001LL||Last Usable Value||Whatever was writing this value has stopped doing so. The returned value should be regarded as 'stale'. Note that this differs from a BAD value with Substatus 5 (Last Known Value). That status is associated specifically with a detectable communications error on a 'fetched' value. This error is associated with the failure of some external source to 'put' something into the value within an acceptable period of time. Note that the 'age' of the value can be determined from the TIMESTAMP in OPCITEMSTATE.|
|2-3||N/A||Not used by OPC|
|4||010100LL||Sensor Not Accurate||Either the value has 'pegged' at one of the sensor limits (in which case the limit field should be set to 1 or 2) or the sensor is otherwise known to be out of calibration via some form of internal diagnostics (in which case the limit field should be 0).|
|5||010101LL||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.|
|6||010110LL||Sub-Normal||The value is derived from multiple sources and has less than the required number of Good sources.|
|7-15||N/A||Not used by OPC|
- Servers which do not support Sub-status should return 0.
Sub-status for GOOD Quality:
|0||110000LL||Non-specific||The value is good. There are no special conditions|
|1-5||N/A||Not used by OPC|
|6||110110LL||Local Override||The value has been Overridden. Typically this is means the input has been disconnected and a manually entered value has been 'forced'.|
|7-15||N/A||Not used by OPC|
Servers which do not support Sub-status should return 0.
The 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.
|0||QQSSSS00||Not Limited||The value is free to move up or down|
|1||QQSSSS01||Low Limited||The value has 'pegged' at some lower limit|
|2||QQSSSS10||High Limited||The value has 'pegged' at some high limit.|
|3||QQSSSS11||Constant||The value is a constant and cannot move.|
- Servers which do not support Limit should return 0.
- Symbolic Equates are defined for values and masks for these BitFields in the "QUALITY" section of the OPC header files.
High 8 Bits of the Quality (Canary Defined Bits)
|0x8000||No Data (There can be other bits also)|
|0xE000||NoData (Edge of the Data - i.e. There is no data prior to this)|
|0x800||Reserved - currently unused|
|0x400||Reserved - currently unused|
|0x200||When this bit is set, the TVQ was inserted or replaced without audit information. This Quality can be passed|
|0x100||This is a reserved bit – used internally by the Historian – When this bit is set, it signifies that there is “Extra Data” associated with the TVQ such as (Audit info on inserts, replace, delete, such as when, who, what it changed from, and why information).|
Also sets some of the bits when computing the aggregates.
- The OPC bits are maintained in the low bits
|0x100||Partial (Doesn't cover the full aggregate interval)|
|0x200||Multiple values (when returning min, max, and average values for each aggregate interval)|
|0x400||Aggregate value is an interpolated value|
|0x800||This is a calculated value|