Hello,

In the POE process (http://www.veracityglobal.com/resources/articles-and-white-papers/poe-explained-part-2.aspx) there is an initial step called - Signature detection. in this step the PSE (power sourcing equipment, POE network switch for example) is inducing a small voltage level on the PD's internal 25K ohm resistor and then it determines whether the connected PD (power device, IP camera for example) is a POE compliant or not.

I'm wondering how does this sensing is done by a MCU ? Is it a current sensing ? How can it be done ?

thanks

 

Well, I think there are two choices: Apply a voltage and measure current, or vice versa. Then use ohms law to validate whether the resistor is the right value.

The text hints at applying a voltage well under 48V but it's not definitive in my opinion, that it's a fixed voltage. That'd be my guess though. I'd test it or find a more detailed reference.

 

I'v read that in order to measure current, and let's assume the 25Kohm resistor is the load, you need to connect in series a shunt resistor, to amplify the voltage sampled on the shunt resistor and get this value into an ADC. Is that can be a possibility ?

 

It's generally done by a dedicated chip. LT has been pretty active in the development of POE. Here http://www.linear.com/product/LT4275 is probably one of their latest designs for the PD (or Powered Device). There is a two-step process depending on the power delivered.

The initial signature uses two voltages from 2.8 to 10 V and does a ΔV/ΔI to determine R.

POE is still messy in my opinion. Some early installs were done with only 4 conductor cables. There is some oddness of what a crossover gigabit cable really is because technically Gigabit doesn;t care how the cables are plugged in. I think, I'm right but a Gigabit xover cable technically does not cross the spare pairs. It can get away with it though.

So, if you have a 100 base T with only 4 wires, you have to be careful and you'll likely need a Gigabit compatable PD and PSE. Things are more rosey when all 8 conductors were run.

Then there's POE without any standard which is just power down the spare pairs and your just asking for trouble. Polarity doesn;t matter because the pairs are full wave rectified and combined.

Not sure if all of the pairs are POE detected. meaning, I don't know if you can put the highest classification over just 2 pairs.

The voltage did increase for POE from the earlier standards.