Abstract: An outbound frame state machine (OFSM) which generates data frames for transmission over a data link. The OFSM is microcode controlled and includes an outbound frame header buffer for containing information to be included in the header of the frame, a data buffer for storing data characters to be included in the data frame, a data generator register for providing special sequences of data characters, if required, and an outbound frame trace buffer for storing a trace log of all frames transmitted in the normal mode and to be used if the OFSM is operating in the simulated I/O mode or the diagnostic mode. In the normal mode of operation, the OFSM builds a data frame using header information from the frame heater bufferm and may include data from the data buffer, all as specified by control bits stored in a control register. The control register may also include a bit for causing special sequences to be transmitted.