DATA TRANSFER SYSTEMS

A data transfer system has a plurality of peripheral units each of which may be required to receive data from, or transmit data to, another such unit. Each peripheral unit is connected to each of at least two data highways, and each highway is provided with its own highway controller, which operates to send a sequence of control instructions over the highway. Means are provided at each peripheral unit for checking the received control instructions against one another, and for acting upon the selected instruction. Data source peripheral units are arranged to send the selected instruction back over the highways to maintain the highway controllers in synchronism with one another.