I have a CRS112-8P-4S that is already powering three '“regular” at/af devices with the 48V PSU
When I want to power a Hex Refresh device, I get an error saying ‘detected poe-out status: low_voltage_too_low’
Is it normal that a passive PoE device like the Hex refresh doesn’t negotiate it’s power or that I can’t change the port settings on the CRS112 to act like a passive PoE port ?
None of MT PoE devices regulate voltage, all of them work as “voltage in - voltage out”. So it’s not possible to have mix of PoE clients connected at the same time.
Only a few rare MT devices accept two power inputs of different voltage and it is then possible to select which voltage to apply to which PoE-out port (those devices either have internal power supply which provides dual voltages or have two power inputs clearly marked with different voltage ranges). CRS112-8P is one of those, but you have to have both power adapters attached (and powered) at the same time.
BTW, PoE voltage is non-negotiable. The only thing that 802.3 af/at PoE devices do negotiate is max power so that if PSE (PoE switch) can’t provide the amount of power required by PD (powered device), the later can decide not to start/run .. or to optimize power consumption to the level provided by PSE (e.g. by running only a third of CPU cores or by running only single radio at one tenth of Tx power or by switching off all of those annoying ultra bright blue LEDs or some such)
Well, you neglect the most important information regarding the message you’re getting But I’ll give it a go.
This device can supply both low voltage (around 24V) and standard (48V+) devices, and the output is selectable. To be clear, the device doesn’t do voltage conversion, it only has basically switches (okay, with all sorts of sensing, but still just switches.) Again, to be clear, in order to support both types of output, two power supplies have to be connected, one at each voltage level.
The device is complaining, that the low voltage (24V-ish) power supply’s voltage is “too low”. Is such a power supply connected? Is it working? Is it sufficiently rated? Has it by any chance failed “softly” (in that it can support only limited and less-than-rated output current - this is a common failure mode of these sorts of power supplies)?
If uncertain, I would certainly attempt using the power supply you received with the Refresh to supply the “low voltage” input to the switch - if only temporarily for testing.
Interesting. So I could power it with the second 28V power adapter and that could provide the “passive” power to the Hex ? Second option would be to use a PoE injector. I’ll try both options later.
That’s why I thought providing both at/af and passive PoE would work with the single 48V PSU. But it doesn’t. Perhaps the sentence “unit will automatically detect and provide correct power to devices” only pertains to the at/af ones.
PS: my electrical knowledge is limited (read: non-existing), so perhaps that’s why I made the wrong assumption.
In any case, I put in a PoE injector. All working fine now.
To be clear: yes, you need to provide both voltages externally in such cases
I agree that the wording could be clearer (if many people are confused by this, please don’t hesitate to put in a support request for clarifying the docs - they usually do it, just not with a high priority) The wording is indeed very literal: it applies to power (Watts), not to voltage (Volts).
The CRS112-8P-4S has two power inputs — one for high voltage (48-57V) and one for low voltage (18-28V). It does not convert voltage internally, but simply passes through what’s supplied.
To power 24V passive PoE devices like the hEX, you need to connect a 24V PSU. The switch will then automatically detect and provide the correct voltage per port, as long as both 24V and 48V PSUs are connected.
So yes, the statement about automatic detection is correct — but only when both power supplies are used. No injector is needed.
But I’ll give it a go.