code: function _checkData(modbus, buf) {
      // check buffer size
      if (buf.length !== modbus._length && buf.length !== 5) return false;
  
      // calculate crc16
      var crcIn = buf.readUInt16LE(buf.length - 2);
  
      // check buffer unit-id, command and crc
      return (buf[0] === modbus._id &&
          (0x7f & buf[1]) === modbus._cmd &&
          crcIn === crc16(buf.slice(0, -2)));
  }
docs: Check if a buffer chunk can be a Modbus answer or modbus exception.
  
  @param {UdpPort} modbus
  @param {Buffer} buf the buffer to check.
  @return {boolean} if the buffer can be an answer
  @private