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From: "Edward Rawde" <invalid@invalid.invalid>
Newsgroups: sci.electronics.design
Subject: Re: Mid-span ethernet monitor
Date: Sun, 8 Jun 2025 12:10:24 -0400
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"Theo" <theom+news@chiark.greenend.org.uk> wrote in message news:lKm*MvweA@news.chiark.greenend.org.uk...
> Edward Rawde <invalid@invalid.invalid> 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
>
> Won't that prevent PoE passing through the link?  This switch will terminate
> the PoE and won't pass power or PoE signalling through to the far end.

Yes you're right a PoE passthrough switch would be needed.
https://www.google.com/search?&q=poe+passthrough+switch&udm=2

>
> I suppose you could have some kind of relay box that normally passes the
> line through but interposes an ethernet switch (or something else) in the
> middle for testing purposes.  When you're in test mode the PoE drops, but
> maybe you don't mind that.  Or maybe you do a PoE receiver and PoE injector
> to let PoE pass the tester switch.  Some switches can do pair tests:
> https://www.engineerkhan.com/networking/testing-an-ethernet-cable-from-a-cisco-switch/
>
> Don't know what a relay would do to signal integrity though.
> (it probably has to be mechanical due to isolation requirements)
>
> Theo