Ethernet cable questions - Page 1
Forum section: electronics / beginners

author: sal_park
author_rank: Contributor
timestamp: March 03, 2019, 05:55:26 pm
content: Hi all,Background:I'm a software engineer but have some idea about EE stuff. I installed CAT6 cabling for my home network running 1G & 100M with PoE to various devices in my home. All cables run back to a patch panel and have work fine since I installed them 18 months ago. Last week I move the patch panel so had to pull all the cables out the back and the reattach them with the punch down tool in the new location. Long story short: none of the cables seem to work any more !To try and understand what's going one I'm testing a minimal setup:cat6socket -> cat6 cable (solid core) 10 meters long -> cat6 socket.Nothing is plugged into either socket.With this setup I'm seeing around 3 ohms between each twisted pair and around 150 ohms between different pairs. This was measured with my multimeter. Just a cheap VC99 which as far as I know is working.Given I was suspicious of the above result as it makes no sense I decided to measure current down one of the pairs. Connecting a PP3 9V batter straight across the multimeter I get reading of 1.5A (only held it on for a second or so!). I then connected the multimeter (amp setting), CAT6 pair and battery in series and got a reading of 1.1A. To my mind this says that the 3 ohms measurement on that pair is probably correct, although I'm not any the wiser in understand wtf is happening !So, does this make sense ? Any idea what's going on ? Help !TIA.sal

author: Nerull
author_rank: Frequent Contributor
timestamp: March 03, 2019, 06:02:05 pm
content: It sounds like you've somehow shorted pairs together. A cat6 cable consists of 8 wires twisted into 4 pairs. There should be no connections between any of the wires if all you have is a socket-cable-socket. There are no, as far as I am aware, termination resistors in a punchdown jack, it's just a straight connection from the pins in the jack to the punchdown terminals.

author: wraper
author_rank: Unknown
timestamp: March 03, 2019, 06:11:25 pm
content: Quote from: sal_park on March 03, 2019, 05:55:26 pmWhat are those sockets? A lot of sockets have built in transformers and termination resistors.

author: wraper
author_rank: Unknown
timestamp: March 03, 2019, 06:14:58 pm
content: I would expect that you fucked up when reconnecting the cables. If you plug a cable with PoE into regular socket, usually it would damage the device.

author: sal_park
author_rank: Contributor
timestamp: March 03, 2019, 06:15:13 pm
content: "It sounds like you've somehow shorted pairs together."I would agree with that, but am confused by the fact I'm getting 150ohms between different pairs. Can't make sense of that.Here are the readings I took:Green->Green/white: 3.2 ohmsBrown->Brown/white: 3.1 ohmsBlue->Blue/white: 3.2 ohmsOrange->Orange/white: 3.2 ohmsGreen->any brown/blue/orange: 150 ohmsBlue->any green/brown/organge: 150 ohms(didn't test any more after this).

author: sal_park
author_rank: Contributor
timestamp: March 03, 2019, 06:17:07 pm
content: "I would expect that you fucked up when reconnecting the cables"Not as far as I can tell. All the pairs are punched down on the correct terminal as far as I can see. (terminals have colour coding on the back)

author: sal_park
author_rank: Contributor
timestamp: March 03, 2019, 06:18:55 pm
content: "What are those sockets? A lot of sockets have built in transformers and termination resistors."Ah, didn't think of that. I'll check.

author: Bratster
author_rank: Frequent Contributor
timestamp: March 03, 2019, 08:38:06 pm
content: Quote from: sal_park on March 03, 2019, 06:15:13 pmWhen you did these tests, what was your set up?Was this just measuring at one ethernet Jack with your meter and nothing plugged into the other end of the cable / ethernet Jack?Or did you have a device plugged into the other end of the cable?Sent from my Fi Moto x4 using Tapatalk

author: sal_park
author_rank: Contributor
timestamp: March 03, 2019, 08:55:56 pm
content: Quote from: Bratster on March 03, 2019, 08:38:06 pmIn the patch panel end I've got 1/2 an ethernet cable plugged in with the wires striped. The multimeter readings are taken from here. The other end of the cable is wired up (punch connectors) to an ethernet socket with nothing plugged into it.

author: sal_park
author_rank: Contributor
timestamp: March 03, 2019, 08:57:10 pm
content: Quote from: wraper on March 03, 2019, 06:11:25 pmThey are just plain RJ45. No termination resistors as far as I can see.

author: sal_park
author_rank: Contributor
timestamp: March 03, 2019, 09:17:51 pm
content: So this is the back of the patch panel in question:https://photos.app.goo.gl/7BxfCT2NhPaPxWsm8And my geto 1/2 an ethernet cable:https://photos.app.goo.gl/kkRq6e8tyJY4Lq8MAAnd how each ethernet pair is terminated to test with the multimeter:https://photos.app.goo.gl/sirKY4ACEjXQJ1dWA

author: RobertHolcombe
author_rank: Frequent Contributor
timestamp: March 03, 2019, 11:29:29 pm
content: Seems pretty confusingProcess of elimination - you re-terminated the patch panel; disconnect what you punched down and measure the cable/wall socket without the patch panel connected. If it checks out terminate a pair on the patch panel, test again.

author: tooki
author_rank: Super Contributor
timestamp: March 05, 2019, 04:24:02 am
content: Quote from: wraper on March 03, 2019, 06:11:25 pmOnly the sockets for devices do this. The sockets for wall sockets and patch panels are pure connectors, since they aren’t endpoints.

author: LapTop006
author_rank: Unknown
timestamp: March 05, 2019, 07:02:59 am
content: Quote from: tooki on March 05, 2019, 04:24:02 amOne common exception is sockets intended for phone use. Even if they're 8p8c / RJ45 (insert argument about naming here). The patch panel shouldn't have any of that, but the wall socket might.

author: Bratster
author_rank: Frequent Contributor
timestamp: March 05, 2019, 07:27:49 am
content: Quote from: LapTop006 on March 05, 2019, 07:02:59 amDo you have some examples of that?I've worked on and installed 100 or so ethernet and telephone jacks and I've never come across any that actually have components.it's all just terminals to the Jack or punch down to the jack.Sent from my Fi Moto x4 using Tapatalk

author: Psi
author_rank: Super Contributor
timestamp: March 05, 2019, 09:01:51 am
content: Are you sure the 150 reading is 150ohm?If it was 150k i would expect that you are touching each meter probe to your finger while testing the wire

author: tooki
author_rank: Super Contributor
timestamp: March 05, 2019, 03:17:22 pm
content: Quote from: LapTop006 on March 05, 2019, 07:02:59 amNope. The jack is just a passive connector. An 8P8C jack only has a transformer in a NIC or network switch (i.e. in places where the signal is being send to a transceiver chip and from there onto wherever the decoded data goes). All the wiring in between is only wired straight through. A wall jack is literally a permanently-wired extension cord, nothing more.POTS phone jacks most definitely could not, and thus would not, have a transformer inline, since that would block the DC line voltage used to power the telephone itself.

author: tooki
author_rank: Super Contributor
timestamp: March 05, 2019, 03:29:12 pm
content: Quote from: LapTop006 on March 05, 2019, 07:02:59 amOK, I did a bit of googling and found some cases where the whole jacks (as in, the wall box) can contain a component: primarily ISDN jacks with built in terminators. ISDN does require terminator resistors on the last socket on a line. But in every example I saw, this was part of the whole wall box, not inside the actual connector itself. (E.g. it's a plain 8P8C connector, with punchdown blocks and the resistor, all on a PCB.) I also saw some British jack boxes with built in spark gaps or surge arrestors, resistors and ring capacitors. But again, on a PCB, not integrated into the connector itself. (I've never seen this anywhere else; I think elsewhere, it was normal to have these components inside the telephone itself.)But this thread is about Ethernet patch panels. The Cat 5/6/7 sockets used for wall jacks and in the patch panels MUST be purely passive, as I said. They're nothing but extension cords. The actual Ethernet endpoints are in the devices that are connected through the wall wiring.

author: Richard Crowley
author_rank: Super Contributor
timestamp: March 05, 2019, 03:46:30 pm
content: Such low resistances make me wonder whether you have your PoE injector (not identified) still in the circuit somewhere? Otherwise it is just completely bizarre.

author: Emo
author_rank: Regular Contributor
timestamp: March 05, 2019, 03:59:35 pm
content: Hi,You have mixed up all the colours. look at the bottom of the patch panel. It should have two color schemes a/b In your first picture one row should have only different coloured wires, the four pairs. As you did it there is only the same colour pairs per socket. So follow the colours on the bottom, preferable the /B setEricEdit; confusing in this situation is that there are roughly two systems. See attached images. Yours might be according to image 1. To be safe you could use a continuity tester to find out the correct pins/colors. Also no explanation for the 150 ohm resistance unless the POE is still attached somewhere a mentioned before