TagRefresh - Refresh tag command

Help Contents

Introduction

The TagRefresh function block asks a tag to refresh data from its data source.

The function block refreshes data by sending a Refresh tag command to the tag. The function block provides status information to:

Parameters

The TagRefresh function block has these parameters:

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

Once the Refresh 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 FALSE to TRUE transition of a digital data point tag used to trigger a function block.
  2. The function block requesting a tag to refresh its data.
  3. Example logic to check the status of the function block.
Program MAIN
    VAR 
        (* The function block declaration *)
        RefreshFB : TagRefresh;

        (* The tag used to trigger the data refresh
           Note: Uses a relative tag reference to 'Digital Trigger Tag' 
                 located in the same folder as this program *)
        TriggerTag AT %".Digital Trigger Tag" : BOOL;
    END_VAR

    (* Call the TagRefresh function block *)
    RefreshFB( 
        REQ:=TriggerTag,
        TAGNAME:="Tag to refresh data" );

    (* The DONE output is active for one cycle when the function block completes *)
    IF RefreshFB.DONE THEN

        (* Was there an error ? *)
        IF RefreshFB.ERROR THEN
            (* Handle any error *)
        ELSE
            (* Any additional work on successful completion *)
        END_IF;
    END_IF;

END_PROGRAM

Further Information

Refresh Tag Command

To learn about the Refresh tag command.

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.

Glossary

For the meaning of terms used in Fernhill SCADA.