EM500-UDL.js

/**
 * Ursalink EM500-UDL Payload Decoder
 *
 * definition [channel-id] [channel-type] [channel-data]
 *
 * 01: battery      -> 0x01 0x75 [1 byte]  Unit: %
 * 03: distance     -> 0x03 0x82 [2 bytes] Unit: m
 * ------------------------------------------ EM500
 *
 * Example:
 *   base64: AXVaA4IeAA==
 *   bytes: 01 75 5A 03 82 1E 00
 *
 * After decode:
 * {
 *   "battery": 90,
 *   "distance": 30
 * }
 */
function Decoder(bytes, port) {
    var decoded = {};

    for (var i = 0; i < bytes.length; ) {
      var channel_id = bytes[i++];
      var channel_type = bytes[i++];
      // BATTERY
      if (channel_id === 0x01 && channel_type === 0x75) {
        decoded.battery = bytes[i];
        i += 1;
      }
      // DISTANCE
      else if (channel_id === 0x03 && channel_type === 0x82) {
        decoded.distance = readInt16LE(bytes.slice(i, i + 2))/1000;
        i += 2;
      } else {
        break;
      }
    }

    return decoded;
  }

  /*******************************************
   * bytes to number
   ********************************************/
  function readUInt16LE(bytes) {
    var value = (bytes[1] << 8) + bytes[0];
    return value & 0xffff;
  }

  function readInt16LE(bytes) {
    var ref = readUInt16LE(bytes);
    return ref > 0x7fff ? ref - 0x10000 : ref;
  }