TagWriteValueFB Function Block

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The TagWriteValueFB Function Block writes a value to these tag types:

The function block writes the value by sending a WriteValue Tag Command to the tag. The function block provides status information to:

Functions and Function Blocks

The TagWriteValueFB 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 value from an Operator Action, use the TagWriteValue Function.


The TagWriteValueFB Function Block has this syntax:

        ERRORMSG=>Variable-Reference );

Note: All parameters are optional.


The TagWriteValueFB Function Block has these parameters:

Parameter Type Direction Description
REQ BOOL R_EDGE Input A rising edge starts the WriteValue Tag Command.
TAGNAME STRING Input The name of the tag to send the WriteValue Tag Command to. For more information see TAGNAME Parameter in the remarks section.
VALUE ANY Input The value to write to the tag.
BUSY BOOL Output Set to TRUE while the WriteValue Tag Command is in progress.
DONE BOOL Output Set to TRUE for 1 execution after the WriteValue Tag Command has completed.
ERROR BOOL Output Set to TRUE if the WriteValue Tag Command failed.
ERRORMSG WSTRING Output If ERROR is TRUE, the reason why the WriteValue Tag Command failed.


TAGNAME Parameter

The TAGNAME parameter is the full name of the tag that runs the WriteValue 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 WriteValue Tag Command.

Timing Diagram

Once the WriteValue 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.

Example Program

  1. Write a value to a data point tag when a digital data point tag changes to TRUE:
    Program MAIN
            (* The function block declaration *)
            WriteFB : TagWriteValueFB;
            (* The tag used to trigger the tag write
               Note: Uses a relative tag reference to 'Digital Trigger Tag' 
                     located in the same folder as this program *)
            TriggerTag AT %".Digital Trigger Tag" : BOOL;
        (* Call the TagWriteValueFB function block *)
            TAGNAME:="Tag to write the value to",
            VALUE:=37.5 );
        (* The DONE output is active for one cycle when the function block completes *)
        IF WriteFB.DONE THEN
            (* Was there an error ? *)
            IF WriteFB.ERROR THEN
                (* Handle any error *)
                (* Any additional work on successful completion *)

Further Information

WriteValue Tag Command

To learn about the WriteValue 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.