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no voltage on POE port
Just got this switch so I can use some POE devices.
Plugged in to one of the 4 POE ports but no voltage on the POE wires. Using a standard 8 wire CAT 5 cable. Is the switch faulty or do I need to configure something?
In due course I got this from Netgear tech support:
"During the detection phase the PSE relatively slowly (0.1 V/ms) ramps up the power from 0 to 10 V. During this time, the current allowed to flow is limited to 5 mA in case there’s a short or other problem. As the voltage rises, the PSE measures the current flow at two levels (at least 1-V apart) to determine the possible presence of the detection signature resistor in a PD at the other end. Two measurements are required to factor out the DC offset (cable loss) between the PSE and PD. If the PSE detects an open circuit (i.e. nothing or a non-PoE device plugged in), short circuit, or anything other than the specified detection signature resistance, it turns off the power.
Assuming that a valid PD is found (a valid detection resistance), the PSE continues ramping the voltage. As it passes between 15.5 and 20.5 V, the PSE and PD enter what’s known as the classification phase, in which the PD is given the opportunity to request a particular amount of power. Once again, the technique involves the PSE measuring the current flow, this time across a classification resistance. Note that from a PD’s perspective, the classification phase is optional with full power the default. However, a PD that volunteers to accept a lower power level will be welcomed by multiport PSE’s that must allocate their maximum power delivery capability across many ports. Knowing now that there is a PD requesting power (and how much it requires), the PSE ramps the voltage and current limit to their full specifications.
The PD must be able to accept power on either the data or spare pairs (but never both at the same time). The IEEE standard specifically disallows PDs that don’t accept either power-delivery option. "This was helpful in clarifying how the PSE is supposed to work. What is still not clear is why a 25K resistor across the spare pairs does not cause the GS308P to source the default 48 volts.
I conclude that it would be helpful is the product documentation stated exactly what the product does and how to test it. (Given that PDs have a range of behaviours just plugging one in would not prove much).
9 Replies
Hello paul_tanner,
Welcome to NETGEAR community!
The GS308P is an unmanaged switch which means you do not need to configure anything for it to work normally.
In the process of figuring out what is wrong, please try to answer the following:
Does everything else work? Lights come on as normal?
How did you establish that there is no PoE? I assume you used a PoE powered end device to see if it is supported or some PoE testing device. Could you please tell us what you used?
Thanks
The switch is functioning normally on the 4 non-POE ports.
Not wanting to hook it up to anything fragile without checking fiirst, I used a standard 4-pair patch cable and a multimeter to look at the voltage on the wires that should carry the POE (4/5 and 7/8). I expected to see 48 volts because I believe that's more or less standard. The multimeter showed no voltages at all.
Hi paul_tanner,
I'd like to know if you are getting green lights on the PoE ports when somethings is plugged in. If they are not green that means that PoE is not active. The PoE ports responds to the 802.3af standard, if they are not getting any request they will not be active.
Thanks,
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