Have you looked at Chapter 14 of the manual GFG2222? It describes what you will need to do to set up COMMREQ instructions for serial I/O. You will need to execute one or more COMMREQs to set up the port, but ultimately you'll be using the "Write Bytes" function (code 4401) to send up to 125 words (250 bytes). You will also have to put the proper values into the consecutive data registers prior to executing the COMMREQ so that the receiving device can correctly interpret the data.