PM8805 PoE Classification issues

[LukeIma]

Hi everyone,

 

I'm working on a prototype, which is using a PM8805 to classify connected PoE types. 

However we are running into an issue, where neither T0, T1 or T2 are giving us any signals (giving always 1 on their open drain signals, making .the LEDs stay off).

 

Suspecting an issue with our prototype, we went on to get a STEVAL-POE001V1, which is showing the same scenario: With our different PoE, PoE+ and PoE++ injectors (Both Ubiquity and TP-Link brand), the T0, T1 and T2 LEDs are remaining off. The PGD LED is however on in those tests. We tested with and without a load on the output (for example 5kOhm) to prevent the injectors going into standby mode.

 

We checked and according to datasheets the TP-Link TL-POE160S is an active 802.3af/at compliant injector.

 

Is my understanding correct? With the CLS 1 resistor at 36.5Ohm and the CLS2 resistor at 51.1Ohm, the T0, T1 and T2 signals should, as an example, be 0, 1, 1 respectively for a Type 2 PSE-PD  (PoE+ according to  IEEE 802.3) as shown in Table 10 of the PM8805 Datasheet.

Any help would be greatly appreciated.

[Peter BENSCH]

Welcome @LukeIma, to the community!

Your understanding of the T0/T1/T2 encoding with CLS1 = 36.5Ω and CLS2 = 51.1Ω resistors is correct: for a Type 2 (PoE+) PSE the PM8805 should report 'T2:T1:T0 = 1:1:0', as shown in line 2 of table 10 of the data sheet.

The fact that PGD is asserted but all T0...T2 lines stay high (LEDs off on STEVAL-POE001V1 and your own board) suggests that the PM8805 does not detect the PSE as an IEEE‑802.3af/at/… compliant source, but rather as a legacy / passive injector or in a non‑standard mode.

To investigate further, could you please share:

• the exact test setup (cabling, injector mode/settings, power level),
• scope screenshots of VPORT and the CLS1/CLS2 pins during startup,
• confirmation that the injectors are set to 802.3af/at mode (not “passive PoE” or vendor‑specific mode)

Regards
/Peter

[LukeIma]

@Peter BENSCH Thank you for your reply!
After much investigation we found that both the Ubiquity and TP-Link brand injectors where in fact not always compliant.
After switching to a certified industrial injector it now works.

Thank you so much for your reply
Regards

[Peter BENSCH]

@LukeIma Is the question answered with that, or can you still provide the information?

Regards
/Peter

[LukeIma]

Yes my question was answered
Thank you very much