RJ-45 Pinouts - What Went Wrong?

Some of the hard questions in life I've come to understand, or at least accept:

  • Why we use QWERTY keyboards to type when it is the worst layout imaginable,
  • Why Windows uses a backslash and Unix uses a slash, for the same thing (e.g. subdirectories), leading to untold instability, endless debugging and systemic confusion, ever since they were first introduced to each other on the WWW.
  • Why every single cell phone battery ever owned by my family (30+) is incompatible with every phone but one, even though looking and being rated much the same.

But why the 10Base-T RJ-45 jack pin assignments ended up so 'twisted' is still a mystery to me.

Specifically, what is the real reason that one of the pairs (the green in 568B) gets split around another, (the blue)? Making the task much more error prone and far more time consuming than need be.

Consider how much easier the task would be if each pair were assigned consecutively say orange 1-2, green 3-4, blue 5-6, brown 7-8. So much less untwisting would be required, pairs would stay together and would naturally end up on the right pin by default. Cross talk would be reduced.

Question 1: Wouldn't the time to terminate ends drop by half at least, if they were consecutively laid out?

Question 2: Is there a valid electrical reason for the present day arrangement?

Question 2: If not was there a valid procedural advantage to this arrangement?

Note: I understand some of the historical reason for this, namely the USOC convention of starting the pairs from the middle and working out, but it seems that was abandoned because the distance between pairs was too great for the last 2. But if they knew that why didn't they just lay them out all consecutively at that point? Also, I'm aware that they were just defining another standard, just like the many thousands already defined, and could not predict the future impact of their decision then.

Maybe a resident cabling God/Icon knows? :)

It goes back to phone wiring like you said. Line 1 was the middle 2 pins on an RJ11 Jack. Line 2 then become the two pins on either side those.

When the RJ45 pinouts were being standardized there was a conscious decision to make them compatible with the existing layout of RJ11 phone jack pinouts.

I don't really find it all that hard to layout the wires once you've done it a few times.


Do you think a lot of people use their 8P8C/RJ-45 panels for 6P2C/RJ11 or 6P4C/RJ14 phone connections as well as LAN?

Patch panels, no. Jacks yes. I did a significant amount of cabling work in the early 2000s that used 8P8C jacks everywhere, regardless of service. At the head end, though, the voice cables were often punched down on 66 or 110 blocks. Later on it started to flip to patch panels being used at both ends, but it always seemed to confuse people how to best patch things through.

It's not just that it was for two line phones, it was also common for phone systems back in the 80s/90s to use two pairs. The Lucent/Avaya partner systems I worked on all the time when I got my start used pair one for dialtone and pair two for signal. I remember at least 3-4 others that did the same.

Never knew the Lucent/Avaya bit, but I'll have to add it to my 'telelphone trivia' stockpile (that makes 3 factoids .. not quite a stand-up routine, but enough to be a boring dinner conversation starter!).

I have noticed that the pre-packaged RJ25 cables that come with some electronics often only have 2 conductors, while some have 4, very few sport the full complement of 6 wires. It's a cost-cutting world.

Right, was just reading, (and perhaps you were implying it), that the idea was to be able to have Ethernet and phone on the same cable at the same time, so there was no moving the assignments of pins 4-5, and so all solutions must split a pair.

Curious, from strip to crimp how long does it take you to do one plug?

To strip, prep wires, trim them, and crimp, about 30 seconds, at most. There's a trick to getting things in place and flat so you can slip them into the plug easily. Once you figure it out, it becomes extremely quick.

I found that those EZ Plug things which pull the wires through were actually slower, because the wires would inevitably end up jamming up or not passing through evenly, and you'd have to take them out and do it again anyway.

Really? We really like the EZ-RJ45 system. The biggest advantages we found are that it is much easier to check correct color placement before crimping and we can pull the wires coming out of the front to get nice, tight twists all the way to the IDC.

The only downside in my opinion is that occasionally the wire is not trimmed totally flush with the end so the plug doesn't go fully into the jack. My techs all have a set of very small flush cutters to fix that.

Top Tech Time: > 30 seconds or < 30 seconds per plug? Ethan has set the bar.

As long as we're talking about legacy telephone wiring, I ought to share this:

  • Q: Why, in telelphone wiring, is the polarity called 'tip' and 'ring' instead of positive/negative (or whatever else, as it's AC voltage)?
  • A: Because, way back when the switchboard ladies routed calls by moving an incoming plug into an outgoing jack on the wall of connections that they sat behind, the wiring to the tip of the 1/4" plug was the tip, while the other (connected to the ring on the plug) was the 'ring.
  • switchboard
  • Bonus trivia: The highly-technical sounding acronym "POTS" means 'plain old telephone system' ... and hearing it always makes me laugh.

Good to know. I always thought "POTS" meant "Piss Off, Tomorrow's Saturday!" (Australian slang phrase).

G'day Carl (that should give you a hint as to what's coming)

The Aussie slang to which you refer pertains to a Friday being POETS day - Piss Orf Early, Tomorrow's Satdee..

Cheers from Aus on this Australia Day long weekend. Drinkin' Beer and kickin' back at Barbies in our shorts and thongs,