TYPE MGT_REQ : (* Management Request *)
STRUCT
    ID : DINT; (* 32-bit transaction ID *)
    CMD : MGT_CMD; (* Management Command per IEC 61499-1, Table 8 *)
    TYPE : MGT_TYPE; (* Management Object Type *)
    CONTENTS : USTRING; (* Management Object Contents *)
END_STRUCT;
END_TYPE

This Structured Data Type models the Request element defined in the FBMGT DTD and subclause 6.4 of the Compliance Profile for Feasibility Demonstrations as follows:

• The ID and Action variables represent the corresponding attributes of the Request element.
• The contained sub-element (FB, Connection, etc.) of the Request is represented by the combination of the TYPE and CONTENTS variables.

The use of a variable of this data type instead of an XML Request element encoded as a STRING variable has several advantages for the transmission of management requests to remote devices:

• Message length can be significantly shorter. For instance the STRING encoding of the following XML element would require a communication payload of 88 bytes, while the corresponding MGT_REQ value would require only 22 bytes:

  <Request ID="1" Action="CREATE" >
    <FB Name="FBName" Type="FBType" />
  </Request>

• Parsing of the CONTENTS field can be accomplished with simple String operations, which can provide significant advantages in both speed and memory usage vs. a general-purpose XML parser.
  • Click here for a more extensive comparison of encodings.
• UTF-8 provides an efficient encoding of the CONTENTS field for the widely used 7-bit US-ASCII character set, with a graceful upgrade path to larger character sets that is more efficient than the use of escape sequences that would be required in a STRING encoding.