Abstract: A data communication subsystem for operation with a main host computer, the subsystem involving a plurality of Front-End Controllers (each of which handles data transfers for a particular type of peripheral terminal and type of transmission line), a Data Communication Processor which controls the activity of the Front-End Controllers, a local "autonomous" memory (sometimes called data communications memory) dedicated to storing instructions, control data, and information data primarily for data transfer operations, and a Basic Control Interface unit which ties together the autonomous memory, the Data Communications Processor, and the Front-End Controllers. The data communication subsystem includes means for sensing a halt or failure in the main host system and then operating in an "autonomous" mode to continuously provide for data transfer operations independent of the main system condition.