Serial link in an i/o system

A link-level facility for managing the transmission of frames and continuous sequences of control characters over a link. The link-level facility includes a state machine which has a first state for allowing transmission of a frame over the link where a link-level facility at the other end of the link indicates that the link has a frame-transmitting status, and a second state for preventing the transmission of frames over the link when the link-level facility at the other end of the link has indicated that the link has a non-frame-transmitting status. The link-level facilities signal the status of the link by means of continuous sequences of special characters intermediate frames transmitted over the link such that synchronism is maintained. The first state includes an inactive state and a working state, and the second state includes a link-failure state, and connection-recovery state and an off-line state.