[Soekris] Second Serial Port as RJ45?

Henry Spencer henry at spsystems.net
Wed Nov 27 00:42:49 UTC 2002


On Tue, 26 Nov 2002, Mark Smith wrote:
> The EIA standard.  Apparently, I was wrong.  You don't even get a Data/GND
> pair with EIA.  Can someone explain the logic behind this for me?

On the contrary, RxD is paired with GND.  Remember that the standard
pairing in a twisted-pair cable is 1-2, 3-6, 4-5, 7-8 -- the two middle
pairs are nested, rather than adjacent, in the connector.

As I understand it, the logic was to come as close as possible to
duplicating the functionality of the 9-pin D connector, while laying out
the signals to provide useful 4-wire and 6-wire subsets (recognizing that
there are differences of opinion about the most useful subsets), and
trying to pair the data signals with either GND or infrequently-changing
control signals.

Left-right symmetry, for cable reversal, was explicitly not an objective
in the end.  I'm not sure why, but my guess is that it's because it would
mean having two GNDs, i.e. one less signal, and merely having to throw *one*
signal overboard had caused extensive warfare over which one to sacrifice.

                                                          Henry Spencer
                                                       henry at spsystems.net




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