Talk:Twisted pair/Archive 2

Archive 1

Archive 2

Screen vs. shield

It is my understanding that, in the context of cable, "screen" and "shield" are synonyms. I think screen is used in British English and shield in American English. I would propose to apply WP:ENGVAR and settle on one or the other for the article? ~KvnG 15:15, 9 March 2015 (UTC)

The article doesn't have to settle on anything - it simply says that "screen" and "shield" are used inconsistently across the industry and some vendors use "shielded screened" for F/FTP cables, then provides a link to ISO/IEC 11801 Annex E, Acronyms for balanced cables (which refers to "screen" by the way). --Dmitry _{(talk•contibs)} 00:27, 12 September 2015 (UTC)

Also, the physics changes depending on electric or magnetic field, and high or low frequency. One word might be used in one context, another in a different context. Gah4 (talk) 01:19, 12 September 2015 (UTC)

As long as the section name reads "cable shielding", let's standardize on "shielding" in the list of cable types. I cannot really find any references for the original editor's claim [1] - "When shielding is applied to the collection of pairs, this is referred to as screening". --Dmitry _{(talk•contibs)} 10:40, 13 September 2015 (UTC)

Citation Needed tag on Disadvantages Section

I have noticed that there is a Citation Needed tag on the second paragraph regarding delay skew between different pairs. It is not clear which facts in the paragraph need a citation. My interpretation is that the tag applies only to the last sentence. That is, I regard the existence of skew and that it would be a problem for video if different pairs were used for different color channels to be non-controversial. However, I would like to see a citation saying that it is common or even occasional practice to compensate for skew by varying the length of the pairs. I have seen skew numbers on the order of 38nS per 100m. 38nS of twisted pair would be about 7m. I am dubious that it would be feasible to splice an extra 7m onto one or more pairs and lay them out inside termination box while still maintaining a good transmission line behavior and low cross talk. But maybe I'm assuming something. Constant314 (talk) 01:20, 11 August 2016 (UTC)

Seems to me that it is feasible, but not practical. I might do it myself, in a home installation, but would not expect it in a commercial installation. Most likely it violates some TIA specification for installations. I could imagine stripping the outer installation off the last 7m of a cable myself, but it must be done carefully. Also, it only works if installing it in a box, not directly to RJ45 connectors. A normal sized box wouldn't be big enough to easily fit it in for one, and even worse for two or more in the same box. And since low-skew cable is available, I would expect commercial installations to use that. (I presume they do this by varying the pitch of each pair along the cable, but I don't see it documented.) Gah4 (talk) 23:48, 11 August 2016 (UTC)

The Belden bonded twisted pairs likely keep their impedance (transmission line behavior), but you still have a splice with a likely reflection. For high resolution analog displays, with 100MHz pixel clock, you could notice 5 or so ns skew. For HDMI, https://e2e.ti.com/support/interface/high_speed_interface/f/138/t/267205 indicates 66ps intrapair, and 888ps interpair skew maximum. Belden makes a low skew VideoTwist cable: https://www.belden.com/docs/upload/NP212.pdf down to 2.2ns/100m. They also note that the broadcast standard is 40ns, but that seems a little high for HDTV or computer video displays. Gah4 (talk) 01:44, 11 August 2016 (UTC)

Seems that for DVI, you need two Cat5 cables: http://downloads.monoprice.com/files/manuals/4064_Manual_080103.pdf which better be close to the same length. This manual doesn't say anything about skew or using low-skew cabling. Gah4 (talk) 04:22, 11 August 2016 (UTC)

https://www.globalmediapro.com/att/a/2/e/r/a2er80/vs01.pdf is an adjustable skew correction device that goes after a VGA-CAT5-VGA link. It seems to adjust in units of 2ns, from 0ns to 62ns. My guess is delay lines in powers of two, and analog switches to select which ones go in the signal path. Using the low-skew cable, or adjusting the length in the box before termination might cost less than the $170 skew box. Gah4 (talk) 05:22, 11 August 2016 (UTC)

http://www.apantac.com/UserFiles/File/VGA-SKEW-EQ%20manual-SH.pdf gives two solutions to skew. Low skew cable, and electronic skew equalizers. Since the sell the latter it isn't surprising, but if you are installing new cable, it is probably cheaper to buy low-skew cable. They don't mention adjusting the cable length in the box, and I haven't seen anyone mention it. Given two other good solutions, I don't know why one would do that. Gah4 (talk) 06:29, 11 August 2016 (UTC)

I have an e-mail reply from someone selling VGA-Cat5-VGA box pairs, which suggests that I don't need to worry about skew. Seems that a $100 box pair has gain and peaking (skew) adjustments, which they claim work just fine. Maybe not as good as the $170 skew adjustment box, but close enough for ordinary users. I have found no evidence of anyone adjusting the lengths in the termination box, and suggest removing that paragraph. I also suggest adding a paragraph about low-skew cable, and one about skew correction boxes. Gah4 (talk) 01:06, 13 August 2016 (UTC)

By the way, there is another Citation Needed tag in the same section that I believe is important, and does need a reference. The impedance characteristics of the cable depend on the spacing between the wires of the pair. Some installation methods can cause the spacing to increase, For this reason, Belden (and maybe others) makes cable with the wires bonded together. Similar to lamp cord, the plastic covers both wires. It is more difficult to install. (Note: I have no connection to Belden, other than having used their cable.) I suspect that there is discussion about this on the belden.com web site, but didn't find them yet. This could be a problem in any installation, and gets more important at higher frequencies (bit rates). Gah4 (talk) 23:48, 11 August 2016 (UTC)

The new citation is helpful and does establish that pulling and bending can degrade performance, but it does not establish that pulling tension and bend radius are deterined by their effect on cable performance and not some other criteria like breaking the conductors. It also does not establish that adhearing to the tension and bend radius limits will preserve cable performance.Constant314 (talk) 14:30, 13 August 2016 (UTC)

Yes not perfect, but infinitely better than none at all. You never know, there could be a defective box of cable, with bad performance, but usually not. As I understand it, good installers test each one after installation. For home use, I usually don't. Gah4 (talk) 19:45, 13 August 2016 (UTC)

The article confuses the issue by starting with a discussion of balanced lines.

Twisted pairs are not balanced lines. Balanced lines require three conductors! Two phases and a return.

The purpose of twisting two conductors is so that the electrical loop which they form has an overall cross-sectional area of zero, and therefore is less susceptible to EMI.

This has nothing to do with balanced lines.

Twisting is a technique that is widely used not only for communication lines but for hookup wiring inside equipment, for all kinds of single-ended signals. For example, in any professionally assembled amplifier, the hookup wires to potentiometers, speaker terminals and such are twisted together.

The twisted pair technique may help with balanced lines also. In that case, the return can be split into two, so that there are two twisted pairs: one carries the positive phase, and return, and the other carries the inverted phase and a redundant return. Or else three conductors can be braided together, like ponytail. A sufficiently long section of braid also has a loop area of zero.

Balanced lines should make twisting redundant because even if there are exposed loops, whatever noise is induced in one loop will also be induced with an opposite voltage in the other loop. Twisting in balanced lines is just an extra measure. The balancing is the main protection mechanism, and is not related to the twisting.

192.139.122.42 (talk) 22:19, 20 November 2012 (UTC)

That is completely misguided. Twisted pair is a very common format used with balanced lines and was historically its original purpose. It is quite right that the article discusses balanced lines in depth. About the only thing you have right is that twisted pair is not a synonym for balanced line, but it is the case that a twisted pair line is guaranteed to maintain the balance of a balanced source. The idea that balanced lines require three conductors is completely wrong, the entire telephone network used to be (and the local ends still are) balanced two-wire circuits. I have never heard of balanced lines consisting of three braided conductors, perhaps you could point to a source of this information. I would be fascinated to read it. SpinningSpark 00:11, 21 November 2012 (UTC)

Shielded balance line, twisted pair (I suspect) within a braided shield is commonly used for microphone cables. But yes, the shield is not required for it to be balanced. In years past, 300 ohm twin-lead was commonly used for TV antenna signals, now more commonly done with 75 ohm coax. It is not twisted by default, but some twist it when installing it. One does have to be careful, as running it too close to metal surfaces can unbalance it. There used to be stand-offs to keep it a few inches away from other surfaces. I am not sure of the manufacturing tolerances, on how well balanced usual UTP cable is. It is pretty good, but not perfect. Gah4 (talk) 10:16, 13 September 2017 (UTC)

UTP cheaper than RG-58

The article has "UTP costs less per meter/foot than any other type of LAN cable". I removed it because I doubt that UTP is cheaper than RG-58. Please talkback. --Ysangkok (talk) 16:58, 21 January 2013 (UTC)

I expect everyone has forgotten about thinnet by now. Is anyone still using it? Can you even buy workstations with a coax LAN connection? It is so uncommon now that it is not realistic to construct a LAN like that any more. Anyway, checking a couple of suppliers there's not much in it, but RG-58 does seem to be marginally cheaper depending who you buy it from. Against that, BNC connectors are way more expensive than RJ45 plugs so your infrastructure is not likely to be cheaper overall. SpinningSpark 20:04, 21 January 2013 (UTC)

A fair comparison with RG-58 would be Cat-3 UTP, as both are used for 10 Mb/s ethernet. The price I see for bulk RG-58 is about USD 250/500ft. I believe you can do a lot better for non plenum-rated UTP. If you really want to get the price down, use two-pair UTP cable, as that is all 10baseT needs. Gah4 (talk) 17:43, 11 October 2017 (UTC)

Citations for Advantages/Disadvantages?

I tried searching for information on the fragility of twisted pairs when I saw they lacked citation, but most of the sites that turned up were copy-pastes of this Wiki article. I don't necessarily doubt that this is true, but it would be good to see some better sourcing. Breadblade (talk) 22:26, 24 April 2013 (UTC)

This search [2] turns up a good selection of book sources that seem to have the information. SpinningSpark 00:17, 25 April 2013 (UTC)

This can be useful for future cleanup of that section. Breadblade (talk) 03:29, 25 April 2013 (UTC)

The foremost reason for twisting the pairs is to prevent the signal power being radiated away. Long cables act as aerials when the signal becomes distributed along the length. (If the run is much shorter than 1/4 wave then the signal is lumped.) If the displacement currents are oppositely directed in two parallel aerials there is no radiation, just a Fresnel region. 1/4 wave at 30MHz is 2.5m. Excluding noise is a secondary reason for twisting. Low-impedance twisted line was historically called a current loop, typically using 20mA at 5V. Used on long R232 lines. It is not practical to twist 500kV power lines, so they are run close together, grouped in complete phase sets, so the net current is zero. An unacceptable amount of power is radiated otherwise. When a cable is shielded, the outer case automatically has a counter displacement current that exactly cancels any current imbalance. This stops any radiation, and interference.The draw back is that it makes the cable more bulky, for very lttle improvement in performance.203.219.80.17 (talk) 02:46, 16 July 2015 (UTC)

Over long enough distances, power cables are twisted. Every few km, there is a tower that exchanges the positions of the wires. For runs long enough to be a significant part of the wavelength, it is still needed. I have seen it driving along a desert highway. Balanced audio cables commonly have a shield. They might run close enough to power cables to pick up some noise. Also, they could pick up AM radio, through some non-linearity in the system, and extract the audio signal from that. Gah4 (talk) 17:52, 11 October 2017 (UTC)

There is a recent edit on Cat 3 cable, with the edit summary describing it as old fashioned.

There is a recent edit on Cat 3 cable, with the edit summary describing it as old fashioned. Seems to me that Cat 1 cable should be considered old fashioned, but then maybe I am older than you. I do remember seeing what I believe is Cat 1 cable. (One pair, individually cloth covered, twisted, and with no outer covering, and maybe 20 gauge.) There must be a picture somewhere. Gah4 (talk) 23:10, 29 March 2018 (UTC)

Are you talking about my edit? I didn't say Cat3 was old fashioned. I said old fashioned telephone cable (by which I meant GPO CW1308 type cable, not the early-twentieth century paper insulated lead sheathed kind - although come to think of it, that should be mentioned too) was roughly like Cat 3. That may or not be correct, but it's what the source says. Note that the words "old fashioned" only appear in the edit summary, not my contribution to the article. SpinningSpark 23:28, 29 March 2018 (UTC)

Yes, I said it was in the edit summary. The one I remember seeing, though I am not sure where, looks like this. I believe for indoor wiring only, and I suspect after the wax paper and before polyethylene. I am not sure which is actual Cat 1 cable. Gah4 (talk) 19:23, 30 March 2018 (UTC)

That is mainly electrical power use. I've seen a lot of it in old victorian houses as the drop cable for pendant lights. I don't think that can be counted as twisted pair as a transmission line. As far as I know, the only reason for twisting is to keep it neat and the conductors together. Cloth covered cable (as a sheath) is still in use as heat protection on irons and such. Remarkably, waxed paper insulation is still common in electrical power distribution.[3] SpinningSpark 19:50, 30 March 2018 (UTC)

I'm not sure what Cat 1 is either, but I've had hints that it may be the waxed paper. Whatever it's supposed to be, it's a backronym because it was never called that originally. SpinningSpark 19:53, 30 March 2018 (UTC)

Common mode

@Kvng: Why have you removed all mention of common mode and the link from the explanations section? It's an important concept here and a link would be useful. I agree that Common-mode rejection ratio was not the best link to point people to, Common-mode signal would be better. By the way, I think some merging needs to happen here, we also have Common-mode interference which makes two articles too many. SpinningSpark 09:13, 28 March 2018 (UTC)

Look carefully at the diff. I added a link to Common-mode signal as I removed the link to Common-mode rejection ratio. I'm sure you appreciate that the failing described in the second paragraph of the Explanation section is not the result of CMRR issues at the receiver but non-CM noise picked up by the transmission line.

Common-mode interference doesn't seem to be about differential signals so I don't think a merge with Common-mode signal is appropriate. ~Kvng (talk) 14:17, 28 March 2018 (UTC)

Ah, so you did. Sorry, I missed that, I had only seen the removal.

On the merging, what do you think of merging to a new title like common mode (electrical circuits)? The article common-mode interference is about differential signals (and interference thereto) except for the second 1037C def. I think that should be removed as it is a different topic, and by policy we have one topic per page. It is also a rather unusual use of the term. SpinningSpark 16:37, 28 March 2018 (UTC)

By my read the first part is about interference in single-ended signaling. Thes second part is the same noise affecting multiple otherwise unrelated signals. ~Kvng (talk) 23:32, 29 March 2018 (UTC)

The first definition reads "Interference that appears on both signal leads (signal and circuit return), or the terminals of a measuring circuit, and ground." "Appears on both signal leads (signal and circuit return)" can only mean common-mode interference on a pair of conductors. Similarly, "the terminals of a measuring circuit" is plural. They mean common-mode interference on both terminals. Perhaps you were misled by the final clause "and ground". The comma indicates that clause is applied to both parts. That is, the common-mode interfernce on both legs of the circuit is identical as measured to ground, not that single-ended ground return signalling is being considered. SpinningSpark 08:24, 30 March 2018 (UTC)

I think we've started splitting hairs here. In the end, there really is no such thing as single-ended signaling. All measurements are performed with respect to a reference so all signals are differential. But that's not how introductory sources generally treat things - I guess they don't want to risk blowing anyone's mind at the outset. Anyway, I'm reading that 1037C definition differently than you are and we'll have to leave it at that for now. ~Kvng (talk) 20:25, 30 March 2018 (UTC)

Differential signalling uses either a differential amplifier or transformer to convert to single ended (ground referenced) signal. Single ended means that the ground reference has low enough impedance to be a useful reference. In any case, remember that signals don't travel in wires, but in the electromagnetic field between wires. (That is most often used to explain split-pairs in ethernet cables, but is true in all signal cables.) In the early days of computers, transistors (or even vacuum tubes) were expensive, relative to wire. Systems used a lot of wire, to save on transistors. Now, we use transistors to save on wire. With a transformer, the differential signal can have a very large common-mode component. With a differential amplifier, not as large. (There are stories of running UTP ethernet with 220VAC common mode. Be careful where you put your hands.) Gah4 (talk) 21:19, 30 March 2018 (UTC)