Newsgroups: comp.dcom.lans.ethernet
Subject: Re: Basic link tester
References: <[email protected]> 
glen herrmannsfeldt  writes:

> [email protected] wrote:
> 
> > 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.
> 
> 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.

There is one web page describing such circuit at 
http://www.lausd.k12.ca.us/Jefferson_HS/td_ethtester.htm

I have not had spare Ethernet transceivers around, so I haven't  
tested this myself either. 
 
> Otherwise a simple CMOS circuit could detect link pulses enough to light
> an LED, but I haven't done that, 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.

-- 
Tomi Engdahl (http://www.iki.fi/then/)
Take a look at my electronics web links and documents at 
http://www.epanorama.net/