Abstract: A digital data transfer system transfers digital data along a path in a ring topology. The digital data transfer system comprising a host controller and a digital data transfer subsystem. The host controller generates and receiving digital data and the digital data transfer subsystem receives the digital data from the host controller and transferring the digital data to one or more devices connectable thereto, and further receives digital data from the last of the devices connectable thereto for provision to the host controller, thereby to define the ring data transfer topology. The digital data transfer subsystem comprises an upstream port, a plurality of input/output ports and a port control. The upstream port transfers digital data from an external source to the input of the first input/output port in the series and transfers digital data received from the last input/output port in the series to the host controller.