How Decoders Work

Decoders (also known as payload formatters) convert raw binary data from LoRaWAN devices into human-readable values.

Why Decoders Are Needed

LoRaWAN devices transmit data as bytes to minimize payload size and conserve battery life. For example:

Raw bytes: 01 67 01 88 05
Decoded data: {"temperature": 35.9, "humidity": 64.8}

Decoder Types

JavaScript Decoders

The most common format, used by Helium Console and ChirpStack:


function Decoder(bytes, port) {
  var temperature = ((bytes[0] << 8) | bytes[1]) / 10;
  var humidity = ((bytes[2] << 8) | bytes[3]) / 10;
 
  return {
    temperature: temperature,
    humidity: humidity,
    battery: bytes[4] / 10
  };
}

CayenneLPP

A standardized format for common sensor types:

Pre-defined data types
No custom decoder required
Limited flexibility

Writing a Decoder

Step 1: Understand the Payload Format

Refer to your device’s datasheet or manufacturer documentation for the payload structure.

Example: Dragino LHT65 sends 11 bytes:


Byte 0-1: Temperature (signed, divide by 100)
Byte 2-3: Humidity (unsigned, divide by 10)
Byte 4-5: Battery voltage (unsigned, in mV)
...

Step 2: Extract Values

Use bitwise operations to extract multi-byte values:


// Big-endian (most significant byte first)
var temp = (bytes[0] << 8) | bytes[1];
 
// Handle signed values
if (temp > 32767) {
  temp = temp - 65536;
}
temp = temp / 100;

Step 3: Return JSON Object


return {
  temperature: temp,
  humidity: humidity,
  battery: battery,
  timestamp: Date.now()
};

Common Pitfalls

Endianness

Big-endian: Most significant byte first [0x01, 0x67] = 359
Little-endian: Least significant byte first [0x67, 0x01] = 359

Signed vs Unsigned

Unsigned: 0 to 65535
Signed: -32768 to 32767

Data Types


// Correct: JavaScript handles division as floats
var value = bytes[0] / 10;
 
// Be explicit with bit operations for multi-byte values
var value = (bytes[0] << 8 | bytes[1]) / 100;

Testing Your Decoder

Obtain a sample payload from the device datasheet
Test with known values
Verify edge cases (e.g., negative temperatures, maximum values)
Check decimal precision

Example test:


// Test payload: 01 67 01 88 0F A0
var testBytes = [0x01, 0x67, 0x01, 0x88, 0x0F, 0xA0];
console.log(Decoder(testBytes, 1));
// Expected: {temperature: 35.9, humidity: 64.8, battery: 4000}

Finding Existing Decoders

Manufacturer documentation
The Things Network Device Repository
GitHub repositories (e.g., Helium Console Decoders )
Community forums