Newsgroups: comp.dcom.lans.ethernet Subject: Re: Basic link tester References: <[email protected]> [email protected] writes: > Ok, I've seen the keychain loopback plugs that'll light the link LED on > the switch. > > I need something to add to my toolbag so I know at the wall jack that I > can confirm I'm attached to the switch without having to go > upstairs/down the hall to see a light. > > Right now I carry around an old hub. Plug it in and see if it's link > light lights up, to confirm connectivity. I'd like to see if there was > something a bit smaller with an LED that lights up. I have seen plans to build this kind of device by using an external Ethernet transceiver module. Many transcievers with RJ-45 connectors are a size of "match box". Accroding some information on internet, you could take a suitable 9V battery and feed power to power pin. With that the transceiver LEDs shoudl work and tell you if connector works or not. > Could I build one? Not in eany very easy way. Enything can be built. The terter would nee practically the same electtronics as those Ethernet transceivers... meaning one special IC, RJ-45 connector, Ethernet m magnetics, and some passive components... Hard to make smaller than commercial transceivers. > Is there power coming over a couple of pairs that > could light a small LED on a homemade keychain loopback tester? I'm > not running power over ethernet. Normal Ethernet does not supply enough power to properly light up a LED from the power on the line. The power just when it leaves the Ethernet card on transmitting pair could just light a LED when you are transmitting all the time (we are talking about around 2V signal, 10-20 mA current), when tens of meters of cable has attenuated it then there is hardly enough power to do that.. When card/hub/switch is not transmitting, there is not much signal on the line on the line.. > How does the link light work? Is there a pair jumpered in each port of > the switch that provideds a circuit that the device on the other end > runs power over to power an LED? Does the device/switch at the other > end just complete a circuit to power the LED on your end? The link light on the Etherner card end is connected to Ethernet transceiver IC link output pin. The hub/switch end has some kind of transiceiver IC functionality in it (either separate IC or integrated as part of larger IC). The transceiver ICs know that there is another transciver on the line when they see some signal coming from each other. The signal coming other end could be acgtual data, and in case data is not coming there comes every now and then short link pulses on 10Base-T (100Base-T system uses continuous signal stream flowing when connection has established). -- Tomi Engdahl (http://www.iki.fi/then/) Take a look at my electronics web links and documents at http://www.epanorama.net/
