A two-way CATV system having a substantial number of terminals incorporates a central processor and transmitter system which continually interrogates the terminals at high speed, monitoring changes of usage of premium programs and other data while also permitting central control of access to restricted premium programs. For inexpensive but reliable intercommunication, short, fixed word length digital messages are frequency multiplexed onto the cable in both upstream and downstream directions, and subscriber groups are controllably selected in turn, with each subscriber being scanned in what may be an arbitrary order by transmission of unique private messages which are recognized at the terminal and in response to which previously prepared fixed length messages are immediately transmitted in return. By employing both upstream and downstream messages which are initiated by a series of synchronizing bits and non-ambiguously related subsequent data sequences, and by the provision of means to initiate both downstream and upstream messages of short length without delay, rapid but reliable data interchange is effected despite differences in transmission and response times with different subscribers. Concurrently, terminal status and various special conditions including security alarms are remotely processed at each terminal, and short upstream messages are assembled containing priority selected data to be transmitted in response to an interrogation. The system incorporates additional means for monitoring power status, program selection and authorization and other operating conditions at each terminal effectively to provide accurate and reliable accounting information while preventing unauthorized or improper usage of restricted premium programs.