Basic link tester

Or, could I test for continuity with my Tempo AT8 tone generator on a particular pair???

Buster

That's pretty much your best bet. A couple of small batteries might eliminate your wall wart...

There's no (significant) power coming from a switch.

There's a set of pulses coming from the switch (and another set coming from your hub) that detail the capabilities of either end and let them negotiate a common set of connection details.

[Note that having link lights doesn't tell you if the wiring is OK, you really need a tester for that...]

I have heard that a transceiver, which is usually much smaller than a hub, can be run off a 9 volt battery. Probably not for very long, but long enough if you use a momentary switch that it should last for a reasonable number of tests.

I haven't tried it, though.

Otherwise a simple CMOS circuit could detect link pulses enough to light an LED, but I haven't done that, either.

-- glen

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.

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.

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..

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).

There is one web page describing such circuit at

I have not had spare Ethernet transceivers around, so I haven't tested this myself either.

Even if no packets are sent on a 10BASE-T cable, a pulse has to be sent periodically (called the "Normal Link Pulse" or "NLP"). It is used to keep the connection "alive". A pulse needs to be sent every

16ms or so.

The link pulse must have a width of 60 to 130 nsec with a repetition frequency of 42 to 125 Hz. Pulse amplitude should be 500 mV to 3V.

The NLP can also be replaced by a "Fast Link Pulse" (FLP) burst, during a process called "auto-negotiation". The FLP carries information about the capabilities of the sender, so that the hardware at both end of a cable can negotiate the link parameters, like the speed and the half/full duplex status.

I have not tested this particular device myself...

You can use continuety test for testing Ethertnet connections.

Take the transmitting and receiving pairs. In normal Ethernet connection there is a transformer coils (pretty low resistance) connected to transmitting and receiving pairs on both ends of connection. You could use a multimeter to check this continuety. I have used this several times for testing purposes.

Low resistance between two wires on the pair tells there is something attached on those lines on the other end of the cable (or cable has been damaged and shorted in the middle).

Thanks everyone. That tranceiver idea is a good one. I'll have to rummage around in the local electronics/computer store to see if they have one laying around. Ebay has them, but paying $1 for the item and then $20 for shipping just seems foolish.

We inherited this site, so a good chunk of the risers are in a state of disarray. Until I can go through all the risers and clean them up, I need a way to just verify the switch is connected in the closet when I'm at the desk. I do realize it doesn't actually test the wiring.

I tried the continuity test and it worked good. I feel a bit uneasy about sending 9v down the wire to the switch though.

Buster