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I just learned about SpaceWire in this great answer about the slip-ring rotating electrical connection between its solar panel and attached sun-pointing instrumentation, and the body of the craft with its Earth-pointing instrumentation.

The Wikipedia article suggests that SpaceWire should have nine conductors; differential pairs each for data in, data out, strobe in, strobe out, and a single ground.

It looks like a lot of thought has gone into SpaceWire, and it seems to have been developed specifically for space applications.

While Ethernet gets by on a single coaxial cable or (usually shielded) twisted pair, SpaceWire specifies nine conductors. Sometimes the weight of wires is considered a small but significant factor in spacecraft weight minimization. Why were nine wires considered optimal?

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  • $\begingroup$ A cable with nine conductors could have less weight per length than a coaxial cable. $\endgroup$ – Uwe May 19 '17 at 11:51
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    $\begingroup$ Do note that modern Ethernet transmits over Cat5 or Cat6 cabling, which has 8 conductors. It would have 9 if there was a desire for a common ground. Of course, it uses the wires very differently from how SpaceWire uses them. $\endgroup$ – Cort Ammon - Reinstate Monica May 19 '17 at 19:19
  • $\begingroup$ @CortAmmon looks like I went into orbit and ended up learning more about Ethernet instead! $\endgroup$ – uhoh May 20 '17 at 3:46
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Ethernet uses 2 or 4 pairs of wires (2 pairs for lower speeds: 1 pair in each direction; 4 pairs for gigabit Ethernet). SpaceWire uses Data Strobe encoding for higher reliability: instead of sending a single signal per bit, DSE sends 2 signals (Data and Strobe) over 2 wire pairs.

These have the property that either Data or Strobe changes its logical value in one clock cycle, but never both. This allows for easy clock recovery with a good jitter tolerance by XORing the two signal line values.

This makes the link more reliable:

The reason for using DS encoding is to improve the skew tolerance to almost 1-bit time, compared to 0.5 bit time for simple data and clock encoding.

The tradeoff of twisted pair vs. coax is complex. Twisted pair is simple and cheap, coax can transmit at higher frequencies but is more difficult to use (you have to terminate it correctly to get usable cable characteristics, it's less tolerant of bending than twisted-pair).

Coax links are more difficult to connect through a slip ring, because coax depends on the exact properties of the insulation between the core conductor and the shield.

Spacewire uses differential signalling: this means both wires of a pair carry a variable voltage. If you want to add shielding (and you do), you need another pin on the connectors, this is the 9th pin. Differential signalling is another technique used to get high reliability and high transfer speeds over twisted-pair links.

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  • $\begingroup$ For best results, twisted pairs should be terminated too. $\endgroup$ – Uwe May 19 '17 at 17:52
  • $\begingroup$ Am I wrong that Ethernet can (or at least could) be sent bidirectionally over a single coaxial cable or a single twisted pair, or just revealing my age? :) $\endgroup$ – uhoh May 20 '17 at 3:42
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    $\begingroup$ Coaxial cable was an option for 10T, but as speeds grew, the Ethernet standard shfited. 100T and 1000T (aka GigE) can only go over twisted pairs (or fiber). They do not have a coaxial variant. $\endgroup$ – Cort Ammon - Reinstate Monica May 20 '17 at 4:26
  • $\begingroup$ @CortAmmon Sorry, double-checking "...bidirectionally over a single..." $\endgroup$ – uhoh May 22 '17 at 3:30
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    $\begingroup$ "you need a separate ground reference" - That's wrong. The nice thing about differential signaling is that you do not need ground. (see e.g. Ethernet works over unshielded cables as well) $\endgroup$ – asdfex May 23 '17 at 16:08
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9 is a quite straight forward choice considering the features you need:

  1. all signals should be differential
  2. you need one data line for each direction
  3. you need to transport a clock along with the data
  4. a shield (GND) for the whole assembly

about 1) Differential data has advantages over single ended signals - they reject influences from external noise that couples into the wire. To some extent this can be achieved with coaxial cable as well.

about 3) There are ways to recover the clock from the data line, but this makes receiving (and sending) data more challenging with respect to the electronics needed on both sides (data encoder, clock recovery...).

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