Half-duplex SpaceWire: Reducing harness mass while retaining full compatibility with SpaceWire's modularity, configurability and adaptability
- Paper number
IAC-08.B4.7.11
- Author
Mr. Paul Walker, 4Links Limited, United Kingdom
- Coauthor
Dr. Barry Cook, 4Links Limited, United Kingdom
- Year
2008
- Abstract
SpaceWire is a symmetric point-to-point communications technology that has been shown to be capable of building modular, reconfigurable and adaptable systems. It is being used world-wide for many missions with many purposes, including Science (e.g. JWST, GAIA, BepiColombo...), Exploration (e.g. the European Drawer Rack (EDR) of the Columbus laboratory on the ISS, ...), Telecommunications (e.g. Inmarsat4, Hylas, ...), Small satellites (e.g. Argo, ...) and an extreme example of fast system design, build, integration test and flight (PnPSat). As defined, 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. One suggestion that has been made is to dispense with the reverse channel and run the link in simplex mode – without any flow control or simple error recovery mechanism. This lack of control or guaranteed recovery is not attractive, particularly when the requirements of modularity, reconfigurability and adaptability are considered. This would also prevent modules responding to requests such as identifying themselves – an important aspect of system verification and plug-and-play. 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, ...). And indeed SpaceWire networks can be built that use full-duplex where necessary and half duplex where appropriate. This paper reports the performance and benefits that may be obtained by half-duplex operation of SpaceWire links.
- Abstract document
- Manuscript document
IAC-08.B4.7.11.pdf (🔒 authorized access only).
To get the manuscript, please contact IAF Secretariat.