SpaceWire provides full-duplex operation on four pairs of wires, allowing for equal data rates in each direction. In addition to data, the 'reverse' direction also provides flow control and control of link recovery after a transmission error. Some applications, however, do not need equal transmission rates in both directions - data from sensors or to actuators, for example. In this case the full-bandwidth reverse channel is seen as unnecessary and the value of the mass of the two pairs of wire for the reverse direction is open to question.An alternative approach is to use just two pairs of wires for both directions - half-duplex operation. The wires must be shared between both ends of the link and a mechanism for synchronizing transmission/reception introduced. The benefit of such an implementation is retention of flow control, error recovery and header configuration whilst the bandwidth may be almost entirely dedicated to data flow in one direction. As well as retaining the benefit of the reverse direction, all the other benefits of SpaceWire are retained, such as the modularity and simple mapping of other protocols (CCSDS, Ethernet, ...).This paper reports the performance and benefits that may be obtained by half-duplex operation of SpaceWire links.
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