Data processing system channel

An outbound frame state machine (OFSM) which generates data frames for transmission over a data link. The outbound frame state machine (OFSM) is microcode controlled and includes an outbound frame header buffer (60) for containing information to be included in the header of the frame, a data buffer (26) for storing data characters to be included in the data frame, a data generator register (98) 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 outbound frame state machine (OFSM) is operating in the simulated I/O mode or the diagnostic mode. In the normal mode of operation, the outbound frame state machine (OFSM) builds a data frame using header information from the frame header buffer, 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. In the simulation mode, frames are built by the outbound frame state machine (OFSM) and then wrapped back to the channel to be processed like frames from a control unit. In the diagnostic mode, the outbound frame state machine (OFSM) inserts various characters into the outbound data bit stream to create possible error situations on the data link.