2.2. Data Item Expressions

Data item expressions are evaluated from class using object attributes or constants to derive the value of a data item field.

Note: The data items  become points when the object is instantiated.

  • Overview: Data item expressions.
  • Configuration: Data item expressions.

Overview: Data Item Expressions

As a result, the a point that is instantiated for different objects from a single data item can include different values in the same field, e.g. Alarm High or Setpoint Low. If a value was entered directly into a data item field, that value will be the same read-only value for every point instantiated from that data item.

Example

How the value of the Alarm High field for a single data item is determined depends on whether or not it was assigned an expression or a value.

  • Alarm High field assigned an expression.
  • A data item includes an expression for the Alarm High field.
  • The value for the Alarm High field is entered in the Object dialog box for three class objects.
  • The value in the Alarm High field for each instantiated point is as follows.
OBJECT1 5000
OBJECT2 2500
OBJECT3 15000
  • Alarm High data item field assigned a value.
  • The value entered in the  Alarm High field in a Data Item dialog box is 5000.
  • There is no configuration at the Object level for the Alarm High field.
  • The value in the Alarm High field for each instantiated point is as follows.
OBJECT1 5000
OBJECT2 5000
OBJECT3 5000
Note: A class attribute is a string. As a result the attribute cannot be entered in fields that require numeric entries, e.g. the Alarm Limits field on the data item's alarm tab.

Configuration: Data Item Expressions

2.2.1 Open an Expression dialog box.
2.2.2 CIMPLICITY Field ID's.
2.2.3 Field ID expression configuration.
2.2.4 Data item: Additional expression examples.