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I want to add a crossover to my network toolbox. Aside from an actual an actual crossover cable, I like the idea of an adapter (figure 1). There are crossover adapters available online, but I’m wondering about making one out of an Ethernet coupler (figure 2).

There are plenty of pages that show wiring diagrams for crossover cables, but it’s pretty confusing because there are several different wiring schemes depicted. Further, while it should only be needed with two 10/100 NICs, I want to make sure it doesn’t cause trouble if one or both of the NICs are GbE (I’m worried about damaging a NIC if the wires are not connected correctly).

Can anybody tell me whether there are any caveats about making a crossover from a coupler instead of a cable and which wires should be crossed?

Figure 1: Crossover adapter

Crossover adapter

Figure 2: Ethernet coupler

Ethernet coupler

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1  
Most GbE ports are auto-MDIX: en.wikipedia.org/wiki/Auto-MDIX#Auto-MDIX en.wikipedia.org/wiki/1000BASE-T#1000BASE-T so you don't need a crossover and can't damage anything. –  Andrew Jul 23 '12 at 0:14
    
That’s good, but what about with two 10/100 NICs? –  Synetech Jul 23 '12 at 0:17
    
Hit-and-run down-votres are so useless. –  Synetech Jul 23 '12 at 6:29

4 Answers 4

There are no caveats to doing this with 10/100 NICs. Most people would use 568B on one end, and 568A on the other, on a Cat5(e)/6 cable.

You seem to not be most people. You need to just make sure that these pins have continuity across the coupler:

1 <-> 3
2 <-> 6
3 <-> 1
6 <-> 2

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So the simple crossover is enough? I don’t have to bother with the brown/blue wires? I guess that would make sense since they are unused in 10/100 anyway, but what if one end is connected to a GbE? Do the blue/brown wires have to be crossed then? –  Synetech Jul 23 '12 at 1:03
    
The crossover is enough for 100Mb - you're sending the TX pair to the RX pair, and vice-versa. For 10/100/1000, it'll just dumb down to 100MB with that setup. If you need Gb crossover, the concept is the same - simple electric continuity from pin to pin. If you need to make it work on any standard, the wiki page shows the pins needed to setup the crossover. Choose one side one way, and the other side the other way. –  sjw Jul 23 '12 at 1:11

While I'm certain it's fun and easy to do this yourself... may I suggest you go after something that can do even more?

http://www.thinkgeek.com/product/c890/?srp=1

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That’s pretty cool. However it’s just not used often enough to make it worth the price. Couplers can be had on eBay for 1¢ and free shipping. ;-) –  Synetech Jul 23 '12 at 0:03
    
I think though when you take the coupler apart, crossing those wires might be a little fun... what's the inside look like? –  SpacemanSpiff Jul 23 '12 at 0:05
    
It depends on the coupler, but usually they are like RJ11 couplers: stiff wires sitting in/on little plastic slots. –  Synetech Jul 23 '12 at 0:17
    
@SpacemanSpiff - how did I not know that that network cable exists? I want... –  Mark Henderson Jul 23 '12 at 0:32
    
The Cat-5 'O Nine Tails is another useful tool for whipping one's network into submission too... (search ebay, there's one for sale there...) –  Red Tux Jul 23 '12 at 4:43

These can also be had in retail or wholesale quantities at monoprice, cablewholesale, etc... There are definitely non auto-MDI GE ports out there still, especially on network hardware. Keeping a dozen straight and another dozen or so crossover (in different colors!!) makes a lot of sense. What -also- makes a lot of sense is keeping a handful of 6" patch cables handy to both make using these guys easier as well as clearly showing where they are in the line for the next person coming along trying to figure out how X connects to Y.

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Crossover cables are useful enough to keep a bunch around.

We get ours in orange, and they're the only cables we get in that colour so they stand out. We get them all 6' long, which is enough to cross-connect ports in our wiring closets, and the few times we need a longer one, we use a straight coupler to connect a normal cable.

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Yes, but that’s not what I asked (or wanted). –  Synetech Jul 23 '12 at 6:28

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