Source
r/networking•
Mikernd

Untwisted pair length in gigabit Ethernet cables

I am a test system designer, not a network engineer, looking for some direction in where to find the appropriate standard.

I am working on a design in a test station that will require passing gigabit Ethernet through a non-standard connector (think similar to a D-sub style of pins).

I know there is a maximum length of untwisted pair allowed in a Ethernet cable run before the signal is significantly degraded to the point it can no longer link at the desired rate if at all.

What I am having trouble with is where to find this specification. I don't necessarily need an answer here to the maximum length, but pointing me in the direction of the appropriate spec or a commercial product spec referencing the industry standard would be all I need.

Posted Sep 30, 2019, 12:11 PM UTC

1744
ImaginaryCheetah•

you can technically get away with any connector you want.

it's the twist per foot in the cable itself that provides the EMI rejection, and thus the length limits.

so if you had 100' of cat6, but instead of terminating into a RJ45, you terminated into a 8 pin dsub, as long as you untwisted the pairs as little as possible, it would have very minimal impact on the length of run.

if you correctly terminate using RJ45, you're looking at like an inch and a half of untwisted wires. if you can make up an alternate connector with equivalent length of untwisted-ness for an connector, i really don't think it'd impact the 100m length of standard cat6 at all.

10
MikerndOP•

I am not too concerned about the connector, but more the amount of space in which the cable will be untwisted while going through the connection.

I know there is a limit, but I cannot seem to find a reference to it anywhere. The best I have seen is recommendations from technical sites that do not reference the spec they are getting their data from.

0
MY_FUCKING_USERNAME•

I'm sure there is a spec somewhere.

That said, I've seen outrageous wiring work just fine. In fact, I've never seen a cable experience an issue even with 8-10in untwist.

YMWV

13
ImaginaryCheetah•

hell, i've seen places that had their ethernet punched down on old telco 66 blocks.

stripped the jacket for the cat6 back to where it entered the 24x36 demarc can and had like 4 feet of bare twisted bares run over to the blocks.

they swear it worked just fine.

7
ThePersonalTachikoma•

You see that sometimes in adaptions from token ring cat5 on 66 blocks to Ethernet

2
listur65•

66 blocks can be Cat5e rated, and depending how homes are wired is used quite often to extend our dmarc to the jack the customer wants the router in.

Edit: Obviously it only matters if the wiring is done correctly, but was just saying.

2
zap_p25MTCRE•

My employer will utilize existing 66 blocks and wiring for older buildings. If the cable infrastructure doesn't work they pull actual runs with patch panels though. Coming from the two-way radio side of the communications industry, 66 blocks are I/O, serial and audio. 110's for IP and just rip System-D out and try again.

1
asdlkfesteemed fruit-loop•

You probably aren't actually concerned with the twist number itself; wrather, you should be concerned that the completed assembly passes Cat6a or Cat6 or Cat5e or Cat5 operational specifications for NEXT, FEXT, impedence, etc...

Specifically, you probably want your completed assembly to comply with TIA-568-C.2.

http://innovave.com/wp-content/uploads/2016/01/TIA-568-C.2.pdf

Specifically, sections 6.1, 6.2, and A4

6
RaydnJames•

I think the underlying question here is "Why do you need to switch to a non-standard ethernet connection"? What is it about this environment that makes you want to switch to a DB9 connector?

6
anomalous_cowherd•

The DB9 is insanely robust compared to RJ45 (which is actually an 8P8C connector for all the real pedants out there), I'd guess that for a test jig that needs to handle tens of thousands of insertions it's a better choice.

2
zanfar•

IMO: there are two ways to approach this.

The only Ethernet standards I'm aware of that approach what you are talking about are 1000BASE-KX and 1000-BASECX. KX is for PCB signalling, so while the signals may be differential, they won't be twisted. That spec will likely have SNR and other details in it. Also, the CX spec specifically lists the DE-9 subminiature connector as an approved device. While I don't think you actually want to use the signalling methods of CX, the spec might also have the details you want.

The other direction is to consider the connector part of the cable, and simply certify the entire assembly as CAT6 or better.

6
permitipanyany•

On this Wikipedia page: https://en.wikipedia.org/wiki/Category_5_cable

Under references, reference 3 is a PDF

"ANSI/TIA/EIA-568-B.1-2001

Approved: April 12, 2001"

Under section 10.2.3, which starts on page 34, is the text "When terminating category 5e and higher cables, the cable pair twists shall be maintained to within

13 mm (0.5 in) from the point of termination. When terminating category 3 cables, the cable pair twists

shall be maintained to within 75 mm (3 in) from the point of termination."

I'm not sure this is exactly what you need for your documentation (I think there are more recent references but I ran into paywalls) but I hope it at least helps you know where to look

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asdlkfesteemed fruit-loop•

http://innovave.com/wp-content/uploads/2016/01/TIA-568-C.2.pdf

1
MikerndOP•

That is the direction of what I need. I'll see if my company has a membership to let me get to the spec directly.

1
ImaginaryCheetah•

https://en.wikipedia.org/wiki/Category_6_cable

just look at the references at the bottom of the article.

4
MikerndOP•

Good point, I did dig through there and did not see a reference that covered the question at hand. I need to have a reference to point to in my design requirements for how long the assembler can untwist before it is "too much".

1
ImaginaryCheetah•

that's not how standards work.

the standard says "the twists should be x per foot, and this is the distance allowed".

the standard won't provide information about how much deviation is allowed, because any deviation from the posted standard makes the cable non compliant.

i'm sure folks have done tests though, just try googling "how far can you send ethernet through untwisted wires".

5
Blog_Pope•

The problem is you are looking at the specs for the cable, not the connectors.

He should track down the current version of this, the official standard, since he's basically talking about using a different connector than RJ45; though I doubt its going to give him a straightforward answer because its not as simple as "it can be untwisted for 2 inches"

2
ImaginaryCheetah•

i believe OP is asking about using non-standard connectors w/standard cable, they're just concerned about how much unjacketed separated pairs are allowed before the cable standard is breached.

which is why i posted the specs for the cable.

i posted the specs, because i couldn't find the standard, thanks for posting that link!

1
cantab314•

Best practice is untwist as little as reasonably possible for the purpose of connecting a plug or socket to the wire. That said ethernet over twisted pair is pretty resilient and there are many awful wiring jobs that still work fine. A lot's going to depend on what interference sources are around.

For your scenario I'd say try it and see. You can get the whole setup certified with testing equipment if you need to.

2
Plaidomatic•

One thing to consider is that the characteristic impedance change at your connectorization will degrade your signal thanks to reflection and loss. If your signal is already marginal (if you're at extended distances, or if your runs are impaired in some other way) the connector may take you into an unusable regime. Fortunately, ethernet's equalization and training are likely to overcome some difficult scenarios.

The correct answer to your question was already given elsewhere: There's no explicit mention of maximum untwisted length, because that's not the point of the standards. The standards give us FEXT, NEXT, capacitance, loss and characteristic impedance. If you can untwist a couple centimeters and still meet the requirements, you're compliant. If not, you're not. The real-world answer is to make the untwisted portion as short as possible, then test. In your case, you're going to have to test with your connectorization and see if you can still make it compliant with your relevant standard.

1
sarbuk•

Well if Cisco do proprietary connectors (with a similar pin style to DB9) on their industrial switches, I'm sure there's no reason you can't!

https://www.cisco.com/c/en/us/products/switches/catalyst-ie3400-heavy-duty-series/index.html

1
trueCABLE•

Ok. The maximum untwist for the conductors at the termination end of the cable is .5" from the end of the last twist to the actual electrical termination point per the ANSI/TIA 568 C.2 and D.2 standard. That said, many folks try and achieve 3/16" for best performance, and many RJ45s on the market will let you get there. Less untwist = less return loss. Fluke networks, FWIW, recommends the least amount of untwist possible, so they recommend going with less than .5" to get good performance and cables that will pass a true sine wave Certifier.

Your application is not familiar to me, but that is the best advice I can provide you. I work for a Ethernet cable company and test cabling and connector combinations literally all day.

1
MikerndOP•

That is helpful, I appreciate you taking the time to lay it out like that.

1
NiiWiiCamo•

That depends on the shielding, if you get good enough shielding between and to the outside, the only limit would be the signal degradation caused by the copper medium. That will be several hundred meters.

0
MikerndOP•

True, cutting down the potential noise sources would definitely help, and is a large reason for the twist in the first place, but I am still looking to be sure that I have every possible path covered.

3
NiiWiiCamo•

So what is your question? Look at the possible introduced noise and just go for it. I have seen badly installed Cat7 get screwed because of RF interference at <25m. I have also gotten gigabit speeds over a short length of dual ISDN flat cable...

1
ImaginaryCheetah•

several hundred meters may be ambitious considering the transmission voltage.

but you can easily do calcs for that part of the equation.

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NiiWiiCamo•

With regular cable definitely. But since we are already looking at special applications... 8AWG network cable anyone?

1
severach•

This should open up a whole new market of 10,000 watt POE devices.

4
ImaginaryCheetah•

do you need gbe speed?

my fire panels are good for up to 5000 feet on their network bus, which is untwisted 14/2.

but it's definitely not gbe speed. and it's between 9 and 24v dc, not sure what voltage ethernet is.

2
andrewpiroli(config)#no spanning-tree vlan 1-4094•

The spec says anywhere from -2 to 2 volts, but typically -1 to 1 volt. If you really need distance on copper it’s just better to go with a different protocol like DSL.

3
zap_p25MTCRE•

Most industrial PLCs can handle 24 VDC to about 9 VDC as much of that stuff is simple I/O contact closure type stuff. Ethernet, like analog telephony, is AC however and you are dealing with much lower signalling levels (+ 6 dBm which is about 0.049 VAC RMS on 600 ohm pair is about as hot as things get).

1
cheesesteaktits•

It’s 328ft. Also I think you mean UTP as in unshielded twisted pair

-9
ImaginaryCheetah•

that's the spec'd distance for standard cat cable, which is definitely not untwisted pair.

although you may be right about OP meaning unshielded, vs untwisted.

3
cheesesteaktits•

Well if you are referring to a copper Ethernet cable, I can assure you there is no untwisted pair cable. There would be too much interference to get a gig speed on it.

1
ImaginaryCheetah•

i think we're both trying to figure out what OP is referring to :)

for me, it sounds like they're talking about just some 22/8 access cable they're trying to use for ethernet.

hence the "untwisted" in the description. which, as you said, won't work for beans.

1
MikerndOP•

I am actually referring to the maximum allowed length of untwisted conductors along the entire link-length (i.e. you have to untwist some at each connector, and there is a maximum cumulative amount that is allowed along entire length of the run).

3
djweis•

It's half an inch. You'll need to derate that because of your connector choice though. With a short enough total length you'll likely get away with it.

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ImaginaryCheetah•

you got a spec for that half inch?

christ, if you can make up two ends of a cable untwisting only 1/4" per end, you're a wizard.

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