2.2.3. Field ID Expression Configuration

The Expression dialog box provides the fields either one operand or two operands connected by an operator.

  • Numeric entries.
  • Attribute entries.
  • Expression with two operands

Numeric Entries

An Expression for a field ID that is assigned to a data item can be a number. The number will be entered in the selected field for the data item's instantiated point in every object created from the class.

A Class: Expression configuration.
B Object: Object creation.
C Instantiation: Point properties result.
  1. Class: Expression Configuration

The number that should be assigned to the field is entered in the First operand>Number field in the Expression dialog box.


Every instantiated point for a data item, LEVELRES, in the TANKV class should have a 5 second alarm delay for the Warning Low (Lo) alarm.

5 is entered as the First operand for the $WARNING_LOW_DELAY field.

  1. Object: Object Creation

When a number only is used in the expression, object level entries for that field will not affect the points that instantiated from the selected data item.


An object STATION05 is created from the TANKV class.

  1. Instantiation: Point Properties Result

The number entered in the Expression dialog box displays in selected field in the instantiated point's Point Properties dialog box.


The STATION05.LEVELRES Point Properties dialog box has 5 seconds entered in the Delay alarm field for the Warning Low (Lo) alarm level on the Alarm Options tab.

Attribute Entries

An Expression for a field ID that is assigned to a data item can be an attribute. The value is entered at the object level each object can have a different value.

A Class: Expression configuration.
B Object: Field values entered.
C Instantiation: Point properties result.
  1. Class: Expression Configuration

All of the class analog attributes are listed in the Attribute ID fields' dropdown list. Any of those attributes can be applied to the selected field.


The Alarm High value for a data item, LEVEL, in the TANKN class may be different for each object.

In order to apply a value entered at the object level, the LEVEL Expressions list includes the $ALARM_HIGH field, which has the custom attribute LHIGHCRITICAL as the expression.

  1. Object: Field Values Entered

When an attribute is selected as a data item's field expression, the data item's field value is assigned at the object level

Object Created

Class objects are created from the class.


A class object created from the class TANKN is named: STATION01.

Field Expressions Assigned Values

Values that are entered in the Object dialog box fields will be applied to the instantiated points that have the attributes applied to field ID's.


5000 is entered in the HIGH CRITICAL  field, which is the prompt for the LHIGHCRITICAL attribute.  

  1. Instantiation: Point properties Result

The value entered for the attribute in the Object dialog box displays in the Point Properties dialog box associated field.


Expression with Two Operands

Expression versatility is expanded by using two operands instead of just one.

Operands can include:

  • Two numbers.
  • Two attributes.
  • One attribute and one number.

Operators that are available to create the expression are:

Operator Description
- Minus
* Times
/ Divided by
+ Plus
< Less an
<> Not equal to
= Equal to
> Greater than
none No operand (Disables the second operand)