Re: Mid-span ethernet monitor

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.

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:

Theo

Yes you're right a PoE passthrough switch would be needed.

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.