How does Passive PoE know to supply voltage

RouterOS Wireless Networking
1

Lots of posts on passive vs active PoE but I think this question is a little different. It’s a long time since I did Electrical Engineering and Electronics so whilst the basics might be there, they’re long buried.

I’ve read how active PoE works in negotiating the power required. The protocol is well documented. I’ve also discovered that the voltage on a non-PoE ethernet cable is very low, zero with no data, maybe up to 2V when data is transmitted.

I get that with passive PoE that there is no negotiation and the 24V (or whatever the PSU voltage is) is permanently applied to the wires assigned to PoE.

My question is what triggers the switch/router to turn on the 24V when a passive PoE device is connected when the setting is “auto”? I get the “off” and “forced-on” settings.

2

PSE checks PD resistance. PD resistance on spare pairs should have range from 3kΩ to 26.5kΩ.

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3

In RouterOS "auto" mode, the switch detects a passive PoE device by sending a low-voltage test pulse to check for a specific resistance (typically around 25k ohms) on the line.

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4

Thanks all for rapid response - so a little similar to active PoE initial probe but without the negotiation step?

5

A check is made for resistance on the cable.

A non-PoE device should have a resistance lower than 3 kOhm.

A passive PoE device should have it above between 3k Ohm and 26.5 kOhm (usually it is on the lower side of the range).

An active PoE device should have it betweeen 23.75 and 26.25 k Ohm.

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