TagDirectWriteStringFB Function Block

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The TagDirectWriteStringFB Function Block writes a text string to an address in a data source associated with a tag.

The function block writes the text string by sending a DirectWriteString tag command to the tag. The function block provides status information to:

Functions and Function Blocks

The TagDirectWriteStringFB Function Block is intended for use in a Program Tag that runs on the Fernhill SCADA Server. A Program Tag uses a continuous execution model, where the program code runs at a fixed interval. The function block samples the state of the tag command as it executes.

To write a string to a Data Source from an Operator Action, use the TagDirectWriteString Function.


The TagDirectWriteStringFB Function Block has this syntax:

        ERRORMSG=>Variable-Reference );

Note: All parameters are optional.


The TagDirectWriteStringFB Function Block has these parameters:

Parameter Type Direction Description
REQ BOOL R_EDGE Input A rising edge starts the DirectWriteString Tag Command.
TAGNAME STRING Input The name of the tag to send the DirectWriteString Tag Command to. For more information see TAGNAME Parameter in the remarks section.
DATAADDRESS STRING Input The "Address" of the data within the data source. See Data Address Format to understand how to set the "Address".
VALUE ANY_STRING Input The text string to write to the data source.
BUSY BOOL Output Set to TRUE while the DirectWriteString Tag Command is in progress.
DONE BOOL Output Set to TRUE for 1 execution after the DirectWriteString Tag Command has completed.
ERROR BOOL Output Set to TRUE if the DirectWriteString Tag Command failed.
ERRORMSG WSTRING Output If ERROR is TRUE, the reason why the DirectWriteString Tag Command failed.


TAGNAME Parameter

The TAGNAME parameter is the full name of the tag that runs the DirectWriteString Tag Command. The value of this parameter can be an absolute tag name or a relative tag name. A relative name tag name is resolved relative to the program tag that runs the function block.

The tag should be one of the types listed in the Applies To list of the DirectWriteString Tag Command.

Timing Diagram

Once the DirectWriteString Tag Command begins, additional requests from the input signal REQ are ignored until the tag command completes.

These timing diagrams show the relationship of the REQ, BUSY, DONE, and ERROR signals processing two requests. The first request completes with an error, the second request completes normally:

        +--------+     +-+ +------+
  REQ   |        |     | | |      |
      --+        +-----+ +-+      +------

        +------+       +------+
 BUSY   |      |       |      |
      --+      +-------+      +----------

               ++             ++
 DONE          ||             ||

ERROR          |       |
      ---------+       +-----------------

The BUSY signal is TRUE while the tag command is in progress. The DONE signal is TRUE for 1 execution after the completion of the tag command.

Debug a Function Block

To monitor the state of a function block as it executes:

  1. Open the program in the Configuration Tool.
  2. Right click the program editor and select Watch Global Variables.
  3. Expand the variables on the left until the function block member variables are visible.

Use the ERROR and ERRORMSG member variables to check the tag command is executed correctly.


  1. Write a string value to a topic in an MQTT Broker:
    Program MAIN
            (* The function block declaration *)
            WriteStrFB : TagDirectWriteStringFB;
            (* The tag used to trigger the write operation
               Note: Uses a relative tag reference to 'Digital Trigger Tag' 
                     located in the same folder as this program *)
            TriggerTag AT %".Digital Trigger Tag" : BOOL;
        (* Run the TagDirectWriteStringFB function block *)
            TAGNAME:="MQTT Broker Tag",
            VALUE:="New Topic Value" );
        (* The DONE output is active for one cycle when the function block completes *)
        IF WriteStrFB.DONE THEN
            (* Was there an error ? *)
            IF WriteStrFB.ERROR THEN
                (* Handle any error *)
                (* Any additional work on successful completion *)

Further Information

DirectWriteString Tag Command

To learn about the DirectWriteString Tag Command.

Tag Command Function Blocks

To learn about other tag command function blocks.

IEC 61131-3 Expressions

To learn about the different types of expression to use as function parameters.

Elementary Data Types

To learn about the different types supported by Fernhill SCADA.

Common Elements

To learn about other IEC 61131-3 Common Elements.


For the meaning of terms used in Fernhill SCADA.