2.2.4. Data Item: Additional Expression Examples

  • Example: Alarm Enable
  • Example: $DATA_ITEM_EXISTS

Example: Alarm Enable

Note: When the enable state fields are included for a data item, an object designer can enable/disable the included alarm levels for each object.

The instantiated analog point for a data item named LEVELEXP requires:

  • Four alarm states for some objects.
  • Alarm High and Alarm Low states only for other object.
A Class: Data item configuration.
B Object: Object defined.
C Instantiation: Point properties result.
  1. Class: Data Item Configuration

Two analog attributes are created that will display as fields in the Object dialog box.

The attributes are as follows.

Field ID Field Name for Attribute Dialog Box
A $WARNING_HIGH_ENABLED WARN HIGH ON
B $WARNING_LOW_ENABLED WARN LOW ON

Field ID's are assigned to the LEVELEXP data item.

Field ID Expression Example
A $WARNING_HIGH_ENABLED WARNHIGHON analog attribute.
B $WARNING_LOW_ENABLED WARNLOWON analog attribute.
  1. Object: Object Defined

Two objects are defined in the TANKV class.

STATION05

One object, STATION05, requires all alarm states.

A value that is greater than 0 is required to enable the alarm states.

1 is entered for each

STATION04

One object, STATION04, requires only the Alarm High and Alarm Low alarm states.

The value 0 is required to disable the alarm states.

0 is entered for each

  1. Instantiation: Point Properties Result

The alarm state values for the instantiated points reflect the entries in the Object dialog box.

STATION05

STATION05.LEVELEXP has four enabled alarm states.

STATION04

STATION04.LEVELEXP has Alarm High and Alarm Low only enabled.

A data item can be included or excluded from being instantiated into a point  based on the expression in an assigned $DATA_ITEM_EXISTS field ID.

Example: $DATA_ITEM_EXISTS

A class includes data items, that will be instantiated into points based on which control valve is selected for an object.

The data items require the following control valves.

Data Item CVALVE01 CVALVE02 CVALVE03
TANK01 X
TANK02 X
TANK03 X

One data item will be instantiated into a point for each object; the data item that is instantiated depends on which valve is selected for that object.

A Class: Data item configuration.
B Object: Object defined.
C Instantiation: Point properties result.
  1. Class: Data Item Configuration

In the class configuration data items will be assigned to the same attribute.

Attribute Created

An analog attribute, CVALVE, is created that will be used to identify the Valve that should be selected for each data item.

Note: The Prompt in the Attribute dialog box will be CONTROL VALVE.

Data Item Configuration

Three TANK data items are created.

The first data item, TANK01 is assigned $DATA_ITEM_EXISTS, with the following expression to associated Control Valve 1 with TANK01.

Field Example Value Assigned
First operand Attribute ID CVALVE
Operator =
Second operand Number 1

Two additional data items are assigned the field ID, $DATA_ITEM_EXISTS.

The three data items and expressions for $DATA_ITEM_EXISTS are as follows.

Data Item $DATA_ITEM_EXISTS Expression
TANK01 CVALVE=1
TANK02 CVALVE=2
TANK03 CVALVE=3
  1. Object: Object Defined

2 is entered in the CONTROL VALVE field for an object, STATION02.

  1. Instantiation: TANK Points Existing

TANK02 is instantiated for the STATION01 object.

TANK01 and TANK03 do not exist for that object.