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. Each input/output port includes a port input terminal, a port output terminal, a device output port, a device input terminal, and an output data selector. The input/output ports are connected in series such that the port input terminal of the first input/output port in the series receives data from the upstream port, the port output terminal of the last input/output port in the series provides data to the upstream port, and the port output terminal of each of the other input/output ports in the series provides data to the port input terminal of the next input/output port in the series. Each input/output port couples data from its port input terminal to its device output terminal thereby to provide data received at its port input terminal to a device which may be connected thereto, and coupling data from its device input terminal to its respective output data selector. The output data selector of each input/output port is further connected to receive data from the respective port input terminal, the output data selector selectively coupling data from one of the port input terminal or the device input terminal to the input/output port's output terminal. The port control controls the output data selector of each the input/output port in response to a determination as to whether a respective device is connected thereto.