A communications system for use in distributed computer control schemes is described. The requirements of such a system include data integrity, computer and data highway fault tolerance and the ability to insert or remove computers on-line. The performance of other communications systems in these respects is considered. The present scheme employs a broadcast serial double ring structure with equal access for all computers. Data are transmitted asynchronously in blocks with cyclic redundancy checking. Automatic reconfiguration takes place in the event of link failures, and the system recovers to the full double ring structure when links are repaired. The ring may be deliberately broken and computers inserted or removed on-line. The system has been developed and tested using S100-Bus microcomputers and the software has been written mainly in Fortran