PERFORMING A CONTROL OPERATION BASED ON
INACCURATE DATA.
When the tag is put into the override mode,
the default tag element value will be equal to the Override element
value instead of the Field element value. If the system is
configured in such a way that it does not give the operator any
indication that the tag is in override mode, the operator may
assume that he is seeing the Field element value (live data) and
consequently may operate the plant inappropriately.
Configure the system in such a way that it
provides a visual indication to the operator that a tag is in
override mode.
Failure to follow these instructions can
result in death, serious injury, or equipment damage.
To provide a visual indication to the operator
that a tag is in the override mode the following can be done:
Set any of the following citect.ini parameters in
such a way that override mode will be indicated by changing the
background color or overlaying the numeric or text graphics objects
and symbol set objects with a dithered pattern:
[Page]OverrideDitheringColor
[Page]OverrideDitheringDensity
[Page]OverrideTextBackgroundColor`
and set [Page]IgnoreValueQuality parameter to a
value of 0 or 2.
or
Use the Override Mode element value (0 if the tag
is not in the Override Mode or 1,2,3,4 otherwise)
or
Use the default or override element quality Tag
Status Override bit (1 if the tag is in the override mode or 0
otherwise)
Using Override Mode
If you determine that the tag value is incorrect
because of a sensor or communication interuption, the tag value may
need to be overridden. The Override tag element is used to store
the overridden tag value.
You can put the tag into the Override Mode which
will cause each unqualified tag reference (a tag referenced only by
the tag name, for example ‘MyTag’).in every I/O Data Consumer
(Control Client, Trend Server, Alarm server, etc) to return the
Override element.The quality of the tag will indicate that the tag
is in override mode.
Tag override is controlled by the Override and
Override Mode tag elements. The Override element contains the
override value and the Override Mode tag element is used to set the
specific override mode.
The following tables describe each of these
elements:
Override tag element
Reference
Syntax
CitectSCADA
Data type
Description
TagName.Override
INT
REAL
STRING
Implied value of
Override element
TagName.Override.V
INT
REAL
STRING
Value
TagName.Override.VT
TIMESTAMP
Timestamp of
when the value last changed
TagName.Override.Q
QUALITY
Quality. The
Override value has “Good” quality except when it is out of
range
TagName.Override.QT
TIMESTAMP
Timestamp of
when the quality last changed
TagName.Override.T
TIMESTAMP
Timestamp of
when the element was last updated
Note: The quality of tags referenced by
items, ex. Tag1.v or Tag1.Field.t, is constantly GOOD and its
timestamps are constantly 0 (INVALID TIMESTAMP). Therefore they
give no visual indication of any not good quality, error or change
in handling state such as control inhibit or override mode
regardless of the setting used for the [Page] IgnoreValueQuality
parameter.
Override Mode tag element
Reference
Syntax
CitectSCADA
Data type
Description
TagName.OverrideMode
INT
Value of
Override Mode element
TagName.OverrideMode.V
INT
Override Mode
Value. See the appropriate function references for the available
override mode values.
TagName.OverrideMode.VT
TIMESTAMP
Timestamp of
when the value last changed
TagName.OverrideMode.Q
QUALITY
The Override
Mode quality is QUAL_GOOD if the tag was put into the Override mode
on the primary server and it was propagated to any redundant
servers. Otherwise the general quality status will be QUAL_UNCR and
the extended substatus will be QE_NOT_REPLICATED to indicate that
some redundant servers are unaware of the fact that the tag has
been overridden.
TagName.OverrideMode.QT
TIMESTAMP
Timestamp of
when the quality last changed
TagName.OverrideMode.T
TIMESTAMP
Time when the
tag was put in or out of Override mode. (equal to 0 at start
up)
Override Modes
Override Mode
Value
Description
0
The Override
Mode is Off.
1
The tag is in
Static Override Mode and the Override value is initially set to the
value of the tag’s Field element.
2
The tag is in
Static Override Mode and the Override value is initially set to the
value of the tag’s Valid element.
3
The tag is in
Static Override Mode and the Override will remain at the previously
set value of the tag's Override element.
4
The tag is in
the Dynamic Override Mode and the Override value will continually
track the value of the tag's Valid element. The ability to write to
the value of the tag’s Override element is disabled in this
mode.
