Milesight UC100: Modbus RS485 to LoRaWAN Converter

function decodeUplink(input) {
  var bytes = input.bytes;
  var data = { modbus: {}, info: {} };

  for (var i = 0; i < bytes.length; ) {
    var channel = bytes[i++];
    var type = bytes[i++];

    // Modbus channel value (ff 19): channel id (1) + data length (1)
    //                              + data def (1) + value
    if (channel === 0xff && type === 0x19) {
      var chId = bytes[i++] + 1;            // 1-based channel index
      i++;                                  // skip data length byte
      var dataDef = bytes[i++];
      var sign = (dataDef >>> 7) & 0x01;    // high bit = signed
      var dataType = dataDef & 0x7f;        // low 7 bits = register type
      var read = readModbusValue(bytes, i, dataType, sign);
      data.modbus["chn" + chId] = read.value;
      i += read.size;
      continue;
    }

    // Modbus read error (ff 15): channel id (1)
    if (channel === 0xff && type === 0x15) {
      var errId = bytes[i++] + 1;
      data.modbus["chn" + errId + "_error"] = true;
      continue;
    }

    // Device info (join / power-on): version, SN, class, status
    if (channel === 0xff) { i += deviceInfoLen(type); continue; }

    // Unknown / config-specific segment: stop to avoid misreads
    break;
  }
  return { data: data };
}

function readModbusValue(b, i, dataType, sign) {
  // 0,1 coil/discrete -> 1 byte on/off; 2,3 16-bit; 4,6 32-bit;
  // 5,7 float; 8-11 16-bit value carried in a 4-byte slot
  if (dataType <= 1) {
    return { value: b[i] ? 1 : 0, size: 1 };
  }
  if (dataType === 2 || dataType === 3) {
    return { value: sign ? readInt16LE(b, i) : readUInt16LE(b, i), size: 2 };
  }
  if (dataType === 5 || dataType === 7) {
    return { value: readFloatLE(b, i), size: 4 };
  }
  if (dataType >= 8 && dataType <= 11) {
    return { value: sign ? readInt16LE(b, i) : readUInt16LE(b, i), size: 4 };
  }
  return { value: sign ? readInt32LE(b, i) : readUInt32LE(b, i), size: 4 };
}

function readUInt16LE(b, i) { return (b[i + 1] << 8) | b[i]; }
function readInt16LE(b, i) {
  var v = readUInt16LE(b, i);
  return v > 0x7fff ? v - 0x10000 : v;
}
function readUInt32LE(b, i) {
  return ((b[i + 3] << 24) | (b[i + 2] << 16) | (b[i + 1] << 8) | b[i]) >>> 0;
}
function readInt32LE(b, i) {
  var v = readUInt32LE(b, i);
  return v > 0x7fffffff ? v - 0x100000000 : v;
}
function readFloatLE(b, i) {
  var bits = readUInt32LE(b, i);
  var sign = (bits >>> 31) ? -1 : 1;
  var exp = (bits >>> 23) & 0xff;
  var frac = bits & 0x7fffff;
  if (exp === 0) { return sign * frac * Math.pow(2, -149); }
  if (exp === 0xff) { return frac ? NaN : sign * Infinity; }
  return sign * (1 + frac * Math.pow(2, -23)) * Math.pow(2, exp - 127);
}
function deviceInfoLen(type) {
  // 0xFF segment lengths are firmware-specific; tune per deployment
  if (type === 0x16) { return 8; } // serial number
  if (type === 0x09 || type === 0x0a || type === 0xff) { return 2; }
  return 1;
}