A computer network, method, and control unit provide two transmitters and two receivers connected to a server and workstations. The workstations are serially connected by transmit and receive paths. The transmitters are connected to opposing ends of the transmit path and the receivers are connected to opposing ends of the receive path. The first transmitter and receiver are enabled and the second transmitter and receiver are disabled during a first phase of a breakage diagnostic mode. The first transmitter transmits a first test sequence and the first receiver receives first acknowledgement sequences. The first transmitter and receiver are disabled and the second transmitter and receiver are enabled during a second phase. The second transmitter transmits a second test sequence and the second receiver receives a quantity of second acknowledgement sequences, which is compared to the quantity of first acknowledgement sequences to determine whether there is a discontinuity in the network.