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From: David Brown <david.brown@hesbynett.no>
Newsgroups: sci.electronics.design
Subject: Re: Mid-span ethernet monitor
Date: Tue, 10 Jun 2025 08:55:14 +0200
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In-Reply-To: <10222of$37rr9$2@dont-email.me>

On 07/06/2025 21:09, Don Y wrote:
> On 6/7/2025 9:11 AM, Edward Rawde wrote:
>> "Don Y" <blockedofcourse@foo.invalid> wrote in message 
>> news:1020c8h$2oq18$1@dont-email.me...
>>> I need a device that can sit midspan (at a "splice point") and monitor
>>> an ethernet connection for integrity.  At the very least, that 
>>> continuity
>>> exists to both ends of the span.
>>>
>>> While these are PoE/PoE+ drops, I don't want to place any significant
>>> burden on the PSEs; an external power supply is therefor likely.
>>
>> In that case I'd just put an unmanaged four port non PoE switch at mid 
>> run and
>> look into getting access to the LEDs on the switch
>> which tell me what the link is doing.
>> The last design I did which had an ethernet port used KS8721BL
> 
> But that assumes the cables into and out of the switch are intact
> and correctly made.  (see my reply to JW)
> 

Some managed switches (and presumably therefore also managed switch 
chips, if you are making it yourself) have support for more detailed 
link monitoring and measurement.  As well as obvious thinks like 
connection rates (a lower than expected rate, or half duplex, indicates 
a very suspect electrical connection somewhere) and packet error 
counters, some also have measurement of link lengths.

I don't know how this would work with PoE passthrough as well.  If you 
make the device yourself, of course, it should be relatively 
straightforward.