create a frame with the following sequence: 0x01, 0x81, and 0x82 (representing the format byte, service ID, and checksum byte)
Putting the data into or pulling it out of a QByteArray seems to be the way to do this. QextSerialPort has a readAll() method that replaces the same method from the QIODevice base class. The base class has a QIODevice::write() method that uses a QByteArray argument.

Why aren't you using the built-in QSerialPort class from Qt? It seems to have nearly the same functionality and interface.