Twinson

Simple and lightweight JSON decoder for TwinCAT and CODESYS.

State of the implementation

• Continuous integration
• Parser
  • Strings
  • Numbers
  • Boolean
  • Null
  • Objects (including nested objects)
  • Arrays (including mixed array and nested arrays)
• Basic element access
• Monadic element access
• Unittests
• Performance improvements for accessing elements, use tree to store data instead of array
• Performance tuning
• Node access for "cached" element access

Comparison to Tc3_Json

Twinson is lightweight alternative to Beckhoff's Tc3_Json.FB_JsonDomParser. When comparing Twinson with the Tc3_Json (TC 3.1.4024.25), I discovered issues with Tc3_Json that might be addressed by Beckhoff in the future.

• null, as specified by the JSON format, is not parsed correctly.
• Parsing nested arrays is cumbersome, because there is no way (at least I didn't find any documented) to access an array element directly.

Benchmark

Twinson includes a seperate PLC for benchmarking the library against Tc3_Json. The benchmark uses several examples with increasing JSON complexity (In terms of how many "levels" the JSON objects have). We measure the duration it takes for parsing a JSON document and assigning all members of the JSON document to TwinCAT variables with direct element access.

• Example 1: Object with 3 key/value pairs
• Example 2: Object with 10 key/value pairs
• Example 3: Object with 3 arrays, each containing 10 strings
• Example 4: Object with 3 arrays, each containing 3 objects and each object has 10 key/value pairs
• Example 5: Nested object with 10 levels object1.object2.(...).object9.string1...string10)