code: function _checkData(modbus, buf) {
      // check buffer size
      if (buf.length !== modbus._length && buf.length !== 5) {
          modbusSerialDebug({ action: "length error", recive: buf.length, expected: modbus._length });
  
          return false;
      }
  
      // check buffer unit-id and command
      return (buf[0] === modbus._id &&
          (0x7f & buf[1]) === modbus._cmd);
  }
docs: check if a buffer chunk can be a modbus answer
  or modbus exception
  
  @param {AsciiPort} modbus
  @param {Buffer} buf the buffer to check.
  @return {boolean} if the buffer can be an answer
  @private