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:
and set [Page]IgnoreValueQuality parameter to a value of 0 or 2.
Use the Override Mode element value (0 if the tag is not in the Override Mode or 1,2,3,4 otherwise)
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
|Implied value of Override element|
|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
|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 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.|