[Gllug] Ethernet crossover wiring

[John Winters]

On Thu, 2006-02-23 at 22:45 +0000, Russell Howe wrote:
> On Thu, Feb 23, 2006 at 10:32:18PM +0000, Dylan wrote:
> > Question is, if I put a socket in each room, can I configure the wiring 
> > between them to act as a crossover? It this likely to be stable, or 
> > should I tell him to shell out for a cheap hub?
> 
> This would violate the Principle of Least Astonishment.
> 
> If you're wiring sockets in, put straight wiring in place, and then if
> you want crossover wiring, use a crossover cable between the NIC and the
> socket.

It always amazes me how often hardware engineers make this fundamental
design mistake.  If you lay out a supposedly "straight" CAT5 patch lead
on a bench and look at it you will quickly realise that despite its name
it's really a crossover lead - like this:

1----------------\      /-8
2---------\    /--\----/--7
3----\  /--\--/----\--/---6
4-\/--\/----\/------\/----5
5-/\--/\----/\------/\----4
6----/  \--/--\----/--\---3
7---------/    \--/----\--2
8----------------/      \-1

The result of making the standard lead in this form is that the
appliances at each end of the lead have to be wired differently to make
things work.  Thus NICs have a different interface from hubs and
switches and when you want to connect to devices which are the same
together you have to use a special lead.  Then you have to keep track of
which leads are which, and eventually manufacturers start making the
devices auto-sensing so that they can pretend to be either type of
terminating device.

The same mistake was made in the design of 5 pin DIN audio leads, in
serial cables and I'm sure in other places.

If instead one recognises that a genuinely straight through lead needs
to be wired like this:

1-------------8
2-------------7
3-------------6
4-------------5
5-------------4
6-------------3
7-------------2
8-------------1

Then you need only one kind of lead, all appliances have their
interfaces them same and no messing about at all is needed.

Why do designers *keep* making this same mistake?

</rant>
John

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