TiOPCItem.Quality

TiOPCItem

Used to get quality information about the current Data from this OPC Item.

property
Quality : Word;

Description

Use Quality to get the quality information about the current Data from this OPC Item. The quality data is encoded into the Quality property according to the OPC Foundation's OPC Specifications...

OPC Quality flags

These flags represent the quality state for a 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:

QQSSSSLL

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 insure 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 IMyQualityDefinitions is a possible approach.

Details of the OPC standard quality bits follow

The Quality BitField Values

QQ
BIT Value
Define
Description
0
00SSSSLL
Bad
Value is not useful for reasons indicated by the Substatus.

  1. SSSSLL
Uncertain
The quality of the value is uncertain for reasons indicated by the Substatus.

10SSSSLL
N/A
Not used by OPC

11SSSSLL
Good
The Quality of the value is Good.

Comment:

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 Substatus and limit.

It is recommended that clients minimally check the Quality Bit field of all results (even if they do not check the substatus 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 Sychronous 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 NULL string or a 0 numeric value.

The Substatus BitField

The layout of this field depends on the value of the Quality Field.

Substatus for BAD Quality:

QQ
BIT Value
Define
Description
0
000000LL
Non-specific
The value is bad but no specific reason is known

  1. LL
Configuration Error
There is some server specific problem with the configuration. For example the item is question has been deleted from the configuration.

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.

000011LL
Device Failure
A device failure has been detected

000100LL
Sensor Failure
A sensor failure had been detected (the 'Limits' field can provide additional diagnostic information in some situations.)

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.

000110LL
Comm Failure
Communications have failed. There is no last known value is available.

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.
  1. 15

N/A
Not used by OPC

Comment

Servers which do not support Substatus should return 0. Note that 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.

Substatus for UNCERTAIN Quality:

QQ
BIT Value
Define
Description
0
010000LL
Non-specific
There is no specific reason why the value is uncertain.

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.
  1. 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

Comment

Servers which do not support Substatus should return 0.

Substatus for GOOD Quality:

QQ
BIT Value
Define
Description
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

Comment

Servers which do not support Substatus should return 0.

The Limit BitField

The Limit Field is valid regardless of the Quality and Substatus. In some cases such as Sensor Failure it can provide useful diagnostic information.

QQ
BIT Value
Define
Description
0
QQSSSS00
Not Limited
The value is free to move up or down

QQSSSS01
Low Limited
The value has 'pegged' at some lower limit

QQSSSS10
High Limited
The value has 'pegged' at some high limit.

QQSSSS11
Constant
The value is a constant and cannot move.

Example

Delphi

Value := iComponent.OPCItem[0].Quality; //You will need to decode the value using the tables above

C++ Builder

Value = iComponent->OPCItem[0]->Quality; //You will need to decode the value using the tables above

Note: This property is Read Only.

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