This is an archive of past discussions. Do not edit the contents of this page. If you wish to start a new discussion or revive an old one, please do so on the current talk page. |
Archive 1 | Archive 2 |
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)
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)
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)
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)
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 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)
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)
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)
@ 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. Spinning Spark 09:13, 28 March 2018 (UTC)
This is an archive of past discussions. Do not edit the contents of this page. If you wish to start a new discussion or revive an old one, please do so on the current talk page. |
Archive 1 | Archive 2 |
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)
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)
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)
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)
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 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)
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)
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)
@ 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. Spinning Spark 09:13, 28 March 2018 (UTC)