Comment by rmunn | Hacker News Reader

Comment by rmunn | Hacker News Reader

That used to require a crossover cable; I've done precisely that (with a crossover cable) back before https://en.wikipedia.org/wiki/Medium-dependent_interface#Aut... became widespread. With a straight-through cable, you'd be connecting the Transmit (TX) pin of one adapter to the TX pin of the other one, and the Receive (RX) pin of one to the RX pin of the other — and neither device would "hear" the messages the other one was sending. A crossover cable flipped those wires, so each device's TX pin was connected to the RX pin on the other end, and both devices could "hear" each other.

But with auto MDI-X, each device would notice "hey, I'm sending but not receiving anything," and would try flipping its Transmit and Receive functions around (transmitting on the RX pin and receiving on the TX pin). Since each device waited a random period before doing that, it was very unlikely (nigh-impossible) that they would both flip at the exact same moment. And if they did, the second interval would most likely not be identical either.

I'm simplifying a bit in the explanation above, but that's the broad strokes. And that's how my carefully-labeled crossover cables started gathering dust. (And then I realized "hey wait, I can just use these as normal cables now", and pulled them back out of storage and mixed them with my normal patch cables).

Still very much a thing in the world of optical networking ("rolling the cable", i.e. switching the two sides at one end to match tx/rx up correctly, is a common first step in debugging a link that won't come up -- standard fiber patch cables are effectively crossover cables but when you're running through patch panels, have multiple cables in the path, etc it can end up mismatched). Some plants have primarily switched to bidirectional (bidi) transceivers that transmit and receive on two different wavelengths on the same strand, but that also requires matching optics at the other end with the tx/rx frequencies reversed.

Most optical cabling setups I encounter these days tend to be made so that you just use a straight through (A-B) cable, and it generally just works.

Although it is super easy to just pop the individual fibres out of the clip holding them together and flip them round.

The biggest polarity issue I see are with mtp/mpo cassettes, especially with multiple spans, you always seem to need to cross something there.

My first Ethernet cable was a crossover cable I used to link my original Xbox with my friend’s. We’d haul another tube TV down to the basement and LAN party all night.

I remember being at a tech-oriented summer camp for teenagers at a university and one of our classes was about networking and included practice making Ethernet cables so we made a bunch of different length crossover cables so we could connect the Xboxes a few of the kids brought to the dorms and play Halo.

Or there's also those 'gigabit crossover' cables, where they flipped pairs 3+4 as well as 1+2, which I only remember seeing for a short period of time.

They always seemed a bit pointless, and died out with crossovers I suspect.

Basically beat me to writing this, but I wouldn’t have made it so eloquent of an explanation.

That wasn't that long ago was it..?

/looks longingly out the window, and catch my aging face in the reflection

Yup ... still have some old 'xover' cables lying around - all of which are RED in colour to explicitly declare their usage

Interesting. I don't remember my crossover cables being marked that way; you had to read the dot-matrix printing along the side to know. Color would have been way more handy.

Well I have to add that was interesting too. Because I remember you had to hold the ends up next to each other and compare the order of the colored wires. Same order left-to-right, not cross-over. Anything else, cross-over.

I had quite a few cables with a yellow crimp "jackets" just behind the RJ45. IIRC they came with DSL modems / routers

I added labels (just a piece of paper under some clear tape) saying "CROSSOVER" at both ends of the cables, so that if I ever plugged a crossover cable into a place where I needed a straight-through, the label would be right there next to the plug.

I seem to recall crossover cables being yellow.

The ones at $dayjob were blue, but we used to tape post-it notes to the ends to make sure we didn't lose them.

Purple cables are faster.

Mine were mixed colours, the cable was grey but the "boot" and connector were black.

I believe a lot of NIC's could autodetect and adapt to the cable? I'm pretty sure I had 2 computers connected to each other using a regular network cable.

Yeah it died out because the hardware advanced and could just switch automatically.

With 1gig Ethernet it became a requirement, the definite end of existence for crossover cables.

Not just a requirement, 1gig ethernet transmit and receive at all of those 4 pairs simultaneously to get the throughput

> That wasn't that long ago was it..?

Last time I used a crossover cable I was still using dialup, probably about 20 years ago. Once I had DSL I also had a router, so the crossover cable became redundant.

I still have a null modem adapter somewhere, and a null modem cable (and they were REAL not the "fake" ones).

I have been struggling to get serial comms going with a TESmart KVM because the ethernet interface is stupid and I lost comms after a misconfiguration and there is no way to factory reset the device. That was not going any better and I ended up purchasing a null modem adapter (at an eye-wateringly high price) because of course I threw away a drawer full of serial adapters a decade ago.

Reply on news.ycombinator.com