String Data Point Tag

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Introduction

The String Data Point Tag String tag icon represents a variable length String Data Point Tag with a maximum length of 160 characters.

Configuration Properties

The String Data Point Tag supports the configuration properties of the Data Point Tag plus these properties:

Property Type Version Description
NormalColor STRING 3.1 The value of the Color property when the tag is good quality. See Color Values for the different ways you can configure a color. This property supports the color value 'Auto', which selects a contrast color to the current theme background color.
HistoricEnable BOOL 2.5 Whether the value of the tag is stored in the Data Historian.
HistoricDataSource USINT 3.59 The source of historical data samples. Three options are available:
  • Filtered tag value: The value of the tag after applying change filtering.
  • Raw tag value: The value of the tag without any change filtering.
  • Write commands: Write commands sent to the tag.
Available only if HistoricEnable is True. For more information see HistoricDataSource Property below.
HistoricOriginFilter USINT 3.73 A filter to choose which data is stored in the historian based on the data origin. Three options are available:
  • All data: All data is stored.
  • Timestamped data: Only data timestamped at source is stored.
  • Sampled data: Only sampled data is stored.
For more information see HistoricOriginFilter Property below.
HistoricSampleInterval TIME 2.5 The preferred maximum time interval between consecutive values recorded in the history. For information on how this configuration property works, see HistoricSampleInterval Property below.
HistoricSampleCount UDINT 2.5 The number of samples to retain in the Data Historian. Each historical record requires 174 bytes disk storage. To work out how much disk storage is required for historical data, multiply HistoricSampleCount by 174.
HistoricListRecordLimit UDINT 3.30 The number of records to show, when selecting Show Historic Data on the Standard Tag Menu. This value provides the row-count parameter to the FETCH FIRST clause for the SQL command. If HistoricListRecordLimit is set to less than 1, the driver rounds the value up to 1. Using very large values may cause the Show Historic Data command to take a long time to complete.

HistoricDataSource Property

The HistoricDataSource property defines where historical data samples are taken from. There are three options:

Option Description
Filtered tag value The historic samples are taken from the data point tag value after change filtering. Only changes to the data point tag value are stored in the historian.

This is the best option for real measurements.

Raw tag value The historical samples are taken directly from the data point tag value. Each update to the tag will create a new historical record. This option can generate a large amount of historical data. For example:
Write Commands The historical samples are taken from write commands to the data point tag. Each write command will create a new historical record. This option can generate a large amount of historical data. For example:

This is the best option for manually entered data.

HistoricOriginFilter Property

A filter to choose which data is stored in the historian based on the data origin. Three options are available:

Option Description
All data All data is stored.
Timestamped data Only data timestamped at source is stored.
Sampled data Only data sampled by Fernhill SCADA is stored.

The default value of HistoricOriginFilter is All data.

Most data sources provide either Timestamped data or Sampled data data but not both. You should only consider using this configuration property if:

For most applications you should use the default of All data.

The HistoricOriginFilter property is available when:

HistoricSampleInterval Property

The HistoricSampleInterval property adds extra samples to the history when a data point tag value does not change for a long period of time. The extra samples give confidence in the historical data when the source value is constant.

When a new value is obtained from the data source, a new record is added to the history if:

Note: To be effective, HistoricSampleInterval should longer than the interval between new samples from the data source.

The HistoricSampleInterval property is available when:

Data Properties

The String Data Point Tag supports the data properties of the Data Point Tag plus these properties:

Property Type Description
Value WSTRING[1024] The current value of the tag
Color DWORD Indicates the state of the tag using color
UpdateReason USINT A code to indicate the reason for the last update to the tag.
IsForced BOOL Indicates whether the tag has been forced (true), or not forced (false)
UnforcedValue WSTRING[1024] The current value of the tag when this tag has been forced to a fixed value
UnforcedQuality INT The quality of the current value when this tag has been forced to a fixed value
UnforcedTimestamp DATE_AND_TIME The timestamp of the last update when this tag has been forced to a fixed value
UnforcedColor DWORD The color of the tag when this tag has been forced to a fixed value
UnforcedUpdateReason USINT A code to indicate the reason for the last update to the tag when the tag is forced to a fix value.
TagErrorCode USINT The status of the last update to the tag. A non-zero value indicates an error.
DataSourceError UDINT A data source specific error code.

Color Property

The Color property provides an indication of the state of the tag using color:

Tag StateConditionColor Value
The tag is forced IsForced = True The ForcedColor defined in the Database Root Tag
The quality is bad Quality < 192 The BadQualityColor defined in the Database Root Tag
Otherwise The NormalColor defined above.

UpdateReason and UnforcedUpdateReason Properties

The UpdateReason and UnforcedUpdateReason properties give a reason for the most recent update to the String Data Point Tag:

ValueDescription
0 None: No update has been stored in the tag.
1 Forced Value: The data point tag is set to a fixed value.
2 Configuration error: There is an error in the configuration.
4 Scanned value: The value originates from an external source. The timestamp is provided by Fernhill SCADA.
5 Logged value: The value originates from an external source. The timestamp is provided by the external source.
6 Loaded value: The value was loaded from the last saved state of the tag database.
7 Initial value: The value was set by the initial value configuration.
8 WriteValue tag command: The value was set by the WriteValue Tag Command.
9 WriteValueQuality tag command: The value was set by the WriteValueQuality Tag Command.
11 Calculated value: The value is the result of a calculation.
12 Randomly generated value: The value is randomly generated.
13 Device state change: A device state change caused a change of quality.
14 Offline tag update: The value is an update to the tag when the data source is offline.

When the tag is forced to a fixed value: UpdateReason will be Forced value (1) and the actual update reason is stored in UnforcedUpdateReason.

TagErrorCode Property

When the Quality of the tag data is 'Bad', the TagErrorCode property indicates the reason for the error:

ErrorCodeDescription
0No error.
1The tag is not associated with a valid data source.
2The data source reference is valid, but the data source has a configuration error.
3The offset of the tag is outside the boundary of the data source.
4The data type is String, but the source value is not a valid string.
5There was an error converting the data.

Tag Commands

A tag command is an action you can trigger on a tag. The String Data Point Tag supports these tag commands:

Tag Command Version Description
ForceValue 1.2 Overrides the string data point tag to show a fixed constant string value.

The maximum length of string allowed is:

  • When historic storage is not enabled: 1024 UTF-16 characters.
  • When historic storage is enabled: 80 UTF-16 characters.

If the tag value is from a data source, updates from the data source are placed in a set-aside area. Note: The override is applied to the tag in the Fernhill SCADA database, and not to the data source.

RemoveForce 1.2 Removes any override currently active on the data point tag.

The actual value from the data source is restored from the set-aside area. If there is no override currently active, RemoveForce has no effect.

Access from IEC 61131-3 Code

To access a String Data Point Tag from IEC 61131-3 code, use this type of declaration:

VAR
    Variable AT %"Full Tag Name" : WSTRING[1024];
END_VAR

Relationships to Other Tag Types

The String Data Point Tag builds on the features of these tag types:

The String Data Point Tag is used as a building block for these tag types:

Further Information

Core Tag

To learn about properties common to all tags.

Data Point Tag

To learn about other types of data tag.

Glossary

For the meaning of terms used in Fernhill SCADA.