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Expression Items are used for executing comparisons, simple math and querying additional database tables. They get their values from an expression made up of static values or other items, or from SQL Queries. They can have alerts and can be executed when the trigger is active or every time the group executes.
Expression Item Properties
General:
| • | Name - The name of the OPC item in the group. There cannot be duplicate names within a group. |
| • | Value - The static value of this Expression item. This will be overwritten by an Expression/SQL binding. |
| • | Datatype - The datatype values are stored as. |
| • | Property - Which property of the OPC item you want to use. |
| 2) | Quality - Quality code of the expression/SQL Query (192 = GOOD_DATA) |
| 3) | Timestamp - The last time the item value changed |
| 4) | Name - The SQLBridge Item Name property of this Item. |
| • | Mode - Options for displaying values based on the Item value. |
| 1) | Direct Value - Item value |
| 2) | Hour Meter - Record the amount of time the Item value is non-zero. This accumulation will reset to zero when the item value goes to zero. The datatype should be set to integer or float when using an Hour Meter regardless of the OPC Item type. |
| • | On Zero - Use a zero value to accumulate time instead of a non-zero value |
| • | Retentive - Retain the Hour Meter value when it is not accumulating. |
| • | Units - The time units to display. |
| 3) | Event Meter - Record the number or times the Item value is non-zero. The datatype should be set to integer when using an Event Meter regardless of the OPC Item type. |
| • | On Zero - Use a zero value to accumulate events instead of a non-zero value |
| • | evaluate at each group interval, before the trigger state is evaluated. |
| • | Target Type - This is the selection for what the Item will write to when the group executes. |
| 1) | None, read-only item - Do not write this value to the database. |
| 2) | Database field - Write the Item value to the specified column in the database table. |
| 3) | Other tag - Write the Expression Item's value back to an OPC item or SQLTag Reference. |
| • | Target Name - The name of the column in the database that this Item will write to when the group executes. The Target Name list will populate with all the OPC Item and SQLTag Reference names from this Group, or the column names from the Group's target table depending on the Target Type selected. |
Numeric:
Numeric properties for Expression Items. See SQLTags Numeric Properties for a full explanation.
Alerting:
Alerting settings for the OPC items. See SQLTags Alerting for a full explanation.
Expression:
Expression/SQLQuery options for Expression Items. See SQLTags Expression/SQL Properties for a full explanation.
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