dOPC Client Toolkit Help file
Example
procedure TForm1.dOPCGUI1Items0nChangeData(Control: TControl; var OPCValue: OleVariant; var Handled: Boolean);
begin
  if Control is TMemo then
  begin
    TMemo(Control).Lines.Add(OPCValue);
    Handled := true;  //no default work procedure for this item
  end;
end;

or

procedure TForm1.dOPCGUI1Items0nChangeData(Control: TControl; var OPCValue: OleVariant; var Handled: Boolean);
begin
  if OPCValue <> 0 then
    OPCValue := 1;
  Handled := false; //use default work procedure, but with new Value
end;
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