Open/short detection - Page 1
Forum section: electronics / projects

author: fredboivin
author_rank: Regular Contributor
timestamp: August 24, 2015, 12:54:01 pm
content: Hi,I looked around and didn't find enough info on that to start my new project with confidence. Do any of you know of a way to detect a short or an open between a pair of cable transporting high speed communication (DSL) without disrupting it?The most common way to do it seems to be TDR, but a pulse would disrupt the carrier and be difficult to differentiate from the communication's pulse train.Thank you all for the help!

author: retrolefty
author_rank: Super Contributor
timestamp: August 24, 2015, 01:38:20 pm
content: Quote from: fredboivin on August 24, 2015, 12:54:01 pmIf there is a open or short on a DSL link then there would be no need to be concerned about any existing data stream, it already has been disrupted!

author: fredboivin
author_rank: Regular Contributor
timestamp: August 24, 2015, 01:52:08 pm
content: One of the goal of this project is to up my game and start making my project meet standards. The one I chose has a requirement of detecting open/shorts on communication channels and in its convoluted writing, seems to suggest that the test has to be run periodically on the working channel to detect creeping problems. And another requirement states that the communication must never stop. Which lead to my question.But your comment made me realize something. What if I wait until the communication stops to detect a fault and then run a test to determine whether there is a short, an open or if it is only a software problem? I don't know if it's acceptable.What would be the simplest solution to do that? Injecting a voltage would easily detect a short, but an open would be more complicated without the two ends talking to each other.

author: Molenaar
author_rank: Regular Contributor
timestamp: August 24, 2015, 02:51:20 pm
content: Can you measure current? If so, define a threshold current under which you consider your circuit open.

author: Muttley Snickers
author_rank: Unknown
timestamp: August 24, 2015, 02:53:20 pm
content: I am not versed on how things work in your locality but down here the carriers or telcos are all geared up to remotely test the lines and confirm their integrity on their side of the boundary, on the customers side then the telcos can still test but if it's a cabling issue then we would as registered open cablers conduct our own tests with cable fault locators and TDRs, in regards to DSL/ ADSL then companies like Sunrise Telecoms, Anritsu, Fluke and others have some nice gear specifically for this type of analysis.If the fault exists with the network hardware then those brands I just metioned should identify it and then it's up to the computer network guy to sort it out.TDRs are better than cable fault locators because they dont need the far end to be bridged, nor do they need 3 or 4 wire to properly identify the distance to the fault which could be a saturated pit or corroded connection which will show up on a TDR and not on most locators.And as retrolefty pointed out if it's open or short then it will be as dead as a door nail, and you cant go injecting voltages and stuff onto live or active lines unless you are using approved and certified equipment and are licensed to do so.Muttley

author: Kalvin
author_rank: Super Contributor
timestamp: August 24, 2015, 03:18:15 pm
content: If the DSL is POTS-compatible, you might be able to use the test signals generated in the voiceband frequencies and analyzing the echo reflecting through the hybrid without affecting the DSL-connection. You might be able to tell difference between open line and the shorted line.Btw, the POTS will provide a small amount of DC-current which can be used for detecting cable faults (short/open). If the cable is private, you might be able to apply the test DC-current yourself.

author: fredboivin
author_rank: Regular Contributor
timestamp: August 28, 2015, 03:35:01 pm
content: Looks like I misinterpreted the standard. The fault should only be detected in a certain amount of time, not actively measured.Anyway, thanks all for your help! The amount of knowledgeable people on this forum is always astonishing.

author: DanielS
author_rank: Frequent Contributor
timestamp: August 28, 2015, 05:56:27 pm
content: Line monitoring is built directly into the DSL standard so from that point of view, any line with active DSL service on it is under continuous monitoring and a phone company can skim the line statistics from their DSLAMs for preventive maintenance planning.As for "waiting for communication to stop before testing," a DSL line never goes silent since the modem and DSLAM need to retain sync with each other and monitor the line for condition changes so they can adjust equalization coefficients and bit loading as needed to maintain optimal link performance and reliability.

author: NiHaoMike
author_rank: Super Contributor
timestamp: August 29, 2015, 01:37:03 am
content: It's possible to have a DC short or open that isn't a complete short or open at RF frequencies.