Quality Flags

Mapping

Mapping means the projection of bits to WinCC OA Userbits.

• None: maps no quality to user bits

• Server specific bits: map only the server specific bits to the user
  bits 1-32.

• Standard bits: map only the OPC default bits to the user bits.

Depending of the combination all quality bits are used, or only quality and
substatus, or only substatus, or only limits. QQSSSSLL is mapped starting at QQ
respectively SSSS respectively LL:

Quality, Substatus and Limits: map all 8 bits to the userbits. QQ is set to
the user bits 1+2, SSSS to 3-6, LL to 7,8

Quality und Substatus: map only QQSSSS, starting at user bit X. That means, QQ
is set to X, X+1. SSSS to X+2-X+5

Substatus: maps only SSSS, starting at user bit X

Limits: map only LL to user bits X, X+1

Examples

The bits for “Good, local override” sets X, X+1 (Good), X+3, X+4 (Local override).
That means for userbit 2:

x
x

x
x

“Engineering units exceeded” sets X+1, X+3. For userbit:

x

x

Low Limited e.g. sets only userbit X+1

x

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: