TagWrite - Write tag command

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The TagWrite function block asks a tag to write its value to the data source.

The function block asks the tag to write its value by sending a Write tag command to the tag. The function block provides status information to:

Currently no tags support the Write write tag command.

Note: Early versions of Fernhill SCADA supported the concept of Write Register Blocks for bulk data transfer to PLCs. Write Register Blocks used this tag command to trigger the write operation. However Write Register Blocks were deprecated and replaced by the more flexible TagDirectWriteByte and TagDirectWriteWord function blocks.


The TagWrite function block has these parameters:

Parameter Type Direction Description
REQ BOOL R_EDGE Input A rising edge starts the Write command.
TAGNAME STRING Input The name of the tag to send the Write command to.
BUSY BOOL Output Set to true while the Write command is in progress.
DONE BOOL Output Set to true for 1 execution after the Write command has completed.
ERROR BOOL Output Set to TRUE if the Write command failed.
ERRORMSG WSTRING Output If ERROR is TRUE, the reason why the Write command failed.

Once the Write 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.

Further Information

Tag Command Function Blocks

To learn about other tag command function blocks.

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.