Crimping cable fun

[Ken D'Ambrosio]

On 2020-03-05 16:03, Wynona Stacy Lockwood wrote:

> On Thu, Mar 5, 2020 at 1:32 PM Ken D'Ambrosio <ken at jots.org> wrote: 
> 
> On 2020-03-05 12:57, Drew Einhorn wrote: 
> It's been even longer since I crimped a crossover cable. Because back in the day I bought some adapters with male and female rj45 you can put on the end of any ethernet cable and use it as a crossover. Everybody should try a garden variety cable first and if it doesn't work they know what to google. 
> 
> One fun one that I had to crimp only a few years ago was a loopback cable:
> 1 <---> 2 
> 3 <---> 5 
> But *to itself*.  Basically, the RJ-45 and two wires going between those four pinouts.  Why?  Because there was something wonky about the machine, and if it didn't see a link, various stuff wouldn't happen, *even though we didn't care about the port or want it active*.  So we had the port give link to itself. Worked like a charm.   
> 
> (Hmmm.  Old memories are coming back -- it might have had to do with a licensing daemon.  Though I'm not sure.) 
> 
> -Ken

I didn't see it mentioned thus far, but if this is Gigabit Ethernet, it
will sense what the proper pinout is, thus eliminating the need for a
Xover cable at all. 

Actually, that's not a gigabit thing per se, that's an auto-MDIX thing,
which was mentioned.  auto-MDIX
(https://en.wikipedia.org/wiki/Medium-dependent_interface#Auto_MDI-X) is
the protocol used to figure out what's sending and what's receiving,
though it was baked into gigabit from the getgo, which does, I believe,
make gigabit the first Ethernet protocol that has it by definition, as
opposed to "it's fairly recent and *should* have it," which was true for
100-Mb (and maybe 10-Mb; I can't remember if anyone supported it back
then). 

$.02, 

-Ken
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