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. A program uses the function block to sample the state of the tag command as it executes.

If you want 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. This can be an absolute tag name, or a relative tag name. Relative tag names are resolved relative to the full name of the host program.
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.


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.

Example Program

The following IEC 61131-3 Program shows:

  1. A transition from FALSE to TRUE of a digital data point tag used to trigger a function block.
  2. The function block writing a value to another tag.
  3. Example logic to check the status of the function block.
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 *)

        (* 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.