Abstract: Apparatus is described for monitoring and diagnosing faults in the communication controller of a line termination unit in a program controlled data switching system. Individual devices detect faults and emit fault signals using safeguarding programs for the fault diagnosis. The communication controllers each contain a fault collection device which receives the individual fault signals and forms a fault word therefrom which describes the fault. Further, fault reaction signals are formed. A diagnosis routine device evaluates a diagnosis command read out from the diagnosis zone of a central storage unit. The diagnosis routine device contains a first decoding circuit for decoding the operations portion of the diagnosis command and for forming control signals therefrom.