Create Chirpstack Decoding Function

Decoding Function is not specific to Helium but standard in Chirpstack

You can use any decoding function created for Chirpstackbut you can only use a decoding function created for Chirpstack

Decoding functions are attached to Device Profile in your tenant setup

I found a decoding function, is that working with Chirpstack ?

A decoding function working with Chirpstack must have a function with the following structure and name:

  function decodeUplink(input) {
    return {
      data: {
        temp: 22.5
      }
    };
  }

The function must process the input field with the following structure:

{
   bytes : number [...], // Byte array containing the uplink payload, e.g. [255, 230, 255, 0]
   fPort : number,       // Uplink fPort.
   variables : { ... }   // Object containing the configured device variables.
}

If the decoding function you found does not extract the data from input.bytes[] field, it means it's not compatible with Chirpstack.

How to see if my decoding function works ?

• First, if your decoding function have a syntax error, you will see that in the event log in the event tab:

• If you had no error in the event log, you can check the decoding clicking on up event in the event log and have a look to the object entry, it should contain your decoded data:

• If you have error or the decoding is not as you are expecting it, the best approach is to test your function in an online javascript tool. As an example you can use playcode.io.

Let's take the following function (working)

function decodeUplink(input) { 
   var bytes = input.bytes;
  
   // Frame format  
   // 00..03 Base or BlueDay Night hours
   // 04..06 Total delta with previous transmission
  decoded = {
     baseKW : ((bytes[0] << 24) + (bytes[1] << 16) + (bytes[2] << 8) + bytes[3]) /1000.0,
     delta : (bytes[4] << 16) + (bytes[5] << 8) + bytes[6],
  };

  return { 
     data: decoded 
  };
}

Copy and paste in into playcode.io and add the following test function:

console.log(decodeUplink(
  {
    bytes: [0x02,0x67,0x01,0x04,0x03,0x02,0x01,0x48],
    fPort: 1,
    variables : {}
  }
))

The bytes field comes from the up event in the event log and reformated:

You should see the result in the console automatically:

If you have a syntax error, like in the following code where missing a ,

decoded = {
     baseKW : ((bytes[0] << 24) + (bytes[1] << 16) + (bytes[2] << 8) + bytes[3]) /1000.0
     delta : (bytes[4] << 16) + (bytes[5] << 8) + bytes[6],
  };

You may see the error in the console: