Write a Custom MQTT Decoder

Three MQTT parsers are included with VTScada: JSONDec, XMLDec, and MQTTPassThruParser.

You specify which to use by naming it in the I/O address for the topic to be read. See: MQTT Client Addressing

 

If you require custom parsing, you can write your own decoder. As with any VTScada module, the code must be declared in the AppRoot.SRC file and file changes imported.

 

As a template for you to use, the code for MQTTPassThruParser.SRC is provided here. Note that it runs as a subroutine.

{========================== MQTTPassThruParser =============================}
{ Returns the payload as is without any parsing. Useful for examining the   }
{ full payload when used with a StringIO tag without any indexing.          }
{===========================================================================}
(
  Payload;  { required parameter }
)
[
]
Only [
  If Watch(1);
  [
    Return(Cast(Payload, \#VTypeText));
  ]
]
{ End of MQTTPassThruParser }

Payload is the value read from the topic address. Presumably, you will know the structure of the data you intend to read before attempting to write a custom parser. Within the body of the IF script block, you are free to manipulate it as needed before returning it. There is no requirement that the data be returned as type text.

 

A generalized example of a parser follows:

{=========================== MQTTExampleParser =============================}
{ Returns a processed value from the MQTT data                              }
{===========================================================================}
(
  Data  { required parameter };
)
[
  ReturnVal  { processed value };
  { Other local variables as needed };
]
Only [
  If Watch(1);
  [
    { ReturnVal = some set of steps to process the data }
    Return(ReturnVal);  { mandatory statement }
  ]
]
{ End of MQTTExampleParser }

 

If the payload carries a timestamp and we want VTScada to use this timestamp then the decoder must return a structure declared using exactly the following format:

SomeTimeValue Struct [
                       Value;
                       TimeStamp;
                     ];

Example using a timestamp:

{========================== DataParserX ======================================}
{ Returns a final value. No indexing needed since indexing is done here.      }
{ It expects that the payload is 13-byte: 4-byte for a single precision float,}
{ 1-byte for a char, 8-byte for an epoch timestamp.                           }
{ Returns a structure [Value Timestamp] that can be interpreted by VTSRead    }
{=============================================================================}
(
  Data;
)
[
  X; C; TS;
  Measurement STRUCT [
                Value;
                TimeStamp;
              ];
  M;
]

Only [
  If Watch(1);
  [
    BuffRead(Data, 0, "%c%3b%5b", C, X, TS); { "%3b" is for IEEE single precision float (4 bytes)  }
    M = Measurement();
    M\Value     = X;
    M\TimeStamp = TS;
    Return(M);
  ]
]

 

Write example using a custom encoder:

Code of the custom encoder:

{============================ DataEncoder1 =================================}
{ Returns a final value. No indexing needed since indexing is done here.    }
{===========================================================================}
(
  Data; { Unused in this example. Typically a value to be encoded.          }
)
[
  X;
  JsonDict;
  JsonStr = "{""V1"": 51.5, 
              ""K1"": {""K1_1"": {""K1_1_1"": 3, ""K1_1_2"": ""v1_1_2_49"", ""K1_1_3"": [13.50, 22.88, 36.09]}},
              ""K2"": 11.11, ""Timestamp"": 1554737681.6766405}";
]

Only [
  If Watch(1);
  [
    JsonDict = Dictionary();
    JsonDict["V1"] = 51.5; { static value for demonstration purposes only }
    JsonDict["K1"] = Dictionary();
    JsonDict["K1"]["K1_1"] = Dictionary();
    JsonDict["K1"]["K1_1"]["K1_1_1"] = 3;
    JsonDict["K1"]["K1_1"]["K1_1_2"] = "v1_1_2_49";
    JsonDict["K1"]["K1_1"]["K1_1_3"] = New(3); { More demonstration-only values }
    JsonDict["K1"]["K1_1"]["K1_1_3"][0] = 13.50;
    JsonDict["K1"]["K1_1"]["K1_1_3"][1] = 22.88;
    JsonDict["K1"]["K1_1"]["K1_1_3"][2] = 36.09;
    JsonDict["K2"] = Data;
    JsonDict["Timestamp"] = 1554737681.6766405; { Hard-coded timestamp }

    X = JSonEncode(JsonDict);

    Return(X);
  ]
]

This module is declared in AppRoot as:

    { Custom Encoders for MQTT data. }
  DataEncoder1        Module "DataEncoder1.SRC";

Finally, the Write address for the I/O tag is defined as:

VTSDemo/MyTopic+DataEncoder1++1

Where VTSDemo is the name used by the broker.