Manchester coding option for SpaceWire: Providing choices for system level design

This paper proposes an optional coding scheme for SpaceWire in lieu of the current Data Strobe scheme for three reasons. Firstly, to provide a straightforward method for electrical isolation of the interface; secondly, to provide ability to reduce the mass and bend radius of the SpaceWire cable; and thirdly, to provide a means for a common physical layer over which multiple spacecraft onboard data link protocols could operate for a wide range of data rates. The intent is to accomplish these goals without significant change to existing SpaceWire design investments. The ability to optionally use Manchester coding in place of the current Data Strobe coding provides the ability to DC balance signal transitions, unlike the SpaceWire Data Strobe coding; and therefore the ability to electrically isolate the interface without additional concerns. Additionally, because the Manchester coding scheme encodes the clock and data on the same signal, the number of wires in the existing SpaceWire cable could be reduced by 50%. This reduction could be an important consideration for many users of SpaceWire as indicated by the effort currently underway by the SpaceWire working group to reduce the cable mass and bend radius by elimination of shields. Reducing the signal count by half would provide even greater gains. It is proposed to restrict the data rate for the optional Manchester coding to a fixed data rate of 10 Megabits per second (Mbps) in order to simplify the necessary changes and still able to operate in existing radiation tolerant Field Programmable Gate Arrays (FPGAs). Even with this constraint, 10 Mbps will satisfy many applications where SpaceWire is used. These include command and control applications and instrumentation applications with moderate data rate requirements. For most NASA flight implementations, SpaceWire designs are implemented using rad-tolerant FPGAs and the desire to preserve the heritage design investment is important for cost and risk considerations. The Manchester coding option can be accommodated in existing designs with only changes to the FPGA.