[Gllug] Ethernet crossover wiring

[Ryan Cartwright]

John Winters wrote:
<snip>

> 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
> 

I take your point but if somebody described something with eight wires 
to be "straight through" *that* is exactly what I would expect.

<snip>
> 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.
> 

The problem is, as you have said, the design of the CAT-5 interface. It 
only uses pins 1,2,3 & 6 (even though we run all eight wires down the 
cable!) and thus a cable like that would not actually work between two 
(current) NICs. Unless, of course, all CAT-5 sockets used say pins 
1,2,7,8 or 3,4,5,6 instead.

I remember a place where I worked once where shared a phone and a PC on 
the same CAT-5 socket using standard CAT-5 splitters. The problem being 
that standard splitters split eight wires to two presenting to pins 
1,2,3,6 on the two sockets. Phones use 1,2,3,4 thus for each split 
connection I had to make up a cable that crossed 1,2,3,4 to 1,2,3,6 and 
re-solder the Mod-tap (to enable the phones to plug into the CAT-5 
socket). After doing that a hundered times the problems of cross-over 
CAT-5 cables seemed easy.

cheers
Ryan
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