Duet 2 Ethernet questions

Danal

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A Former User

If using proper 802.3at or 802.3af PoE you will not damage non PoE equipment by plugging it into a PoE supply.

Problems can occur when using non compliant or passive PoE.

I couldn't find any reference to what this decide using but having 24v as an option is usually a sign that it is non compliant.

There are a range of external PoE splitters that can split PoE into separate network and power circuits, which would allow you to supply f.ex. 24v to the Duet without risking damage. These also come in 802.3at/af or passive varieties and has to be matched to the source (output voltage is usually fixed, but 5, 12, and 24v versions are common)

dc42

Please note, the Ethernet socket with magnetics used on the Duet Maestro and Duet Ethernet is not PoE compatible.

Danal

@dc42 said in Duet 2 Ethernet questions:

Please note, the Ethernet socket with magnetics used on the Duet Maestro and Duet Ethernet is not PoE compatible.

Interesting... I will edit or delete my post above.

dc42

My understanding is that PoE works by applying 48V between the two data pairs in the cable. Duets use the Hanrun HR911105A socket, whose schematic is here http://www.kosmodrom.com.ua/pdf/HR911105A.pdf. If this is connected to a powered PoE cable, the results would be 48V applied across two of the internal 75 ohm resistors in series. I think this would probably burn them out.

A Former User

@dc42 said in Duet 2 Ethernet questions:

My understanding is that PoE works by applying 48V between the two data pairs in the cable.

802.3af/at PoE only applies a voltage after handshaking as part of establishing the link (handshake will also control which pairs and what power limit to use). So using a standards compliant switch is safe even with non PoE devices, they'll just get a non PoE link.

Passive PoE has a constant voltage, usually on the unused pairs. This causes issues with devices not made for passive PoE. Which will make thinks a bit toasty indeed.

(Standards compliant PoE is limited to 15/30W so not suited for anything but stand by supply really)

dc42

@bearer said in Duet 2 Ethernet questions:

@dc42 said in Duet 2 Ethernet questions:

My understanding is that PoE works by applying 48V between the two data pairs in the cable.

802.3af/at PoE only applies a voltage after handshaking as part of establishing the link (handshake will also control which pairs and what power limit to use). So using a standards compliant switch is safe even with non PoE devices, they'll just get a non PoE link.

If the device is powered by PoE, how can it do a handshake if power is not provided until after the handshake? Does the switch supply a small current until after the handshake, enough to provide power for the handshake but not enough to destroy a non-PoE device?

Passive PoE has a constant voltage, usually on the unused pairs. This causes issues with devices not made for passive PoE. Which will make thinks a bit toasty indeed.

From what have read, for 100baseT Ethernet (as supported by Duets), the PoE standard defined two modes: mode A and mode B. Mode A uses the same two pairs of wires that carry the data, which would likely damage a Duet. Mode B uses the unused pairs, so it should be safe.

A Former User

@dc42 said in Duet 2 Ethernet questions:

Does the switch supply a small current until after the handshake, enough to provide power for the handshake but not enough to destroy a non-PoE device?

pretty much, it does a few pulses (not 48v) to detect a the impedance of a poe compliant device, it will not damage non-poe stuff.

you're right about the modes, pretty much all passive poe solutions will use mode B, compliant will negotiate, but still not damage a duet as it wouldn't get past the detection phase. a third mode is defined for 1000BaseT which basically has data and power on all pairs iirc.

dc42

@bearer, thanks for the clarification.

A Former User

Just to be very clear, the passive PoE solutions will not attempt to detect a supported device, they just supply a constant voltage. But there are few switches of this type, more common with inline power injectors.

(Also people using passive PoE in their products ought be taken out back and, uhm, be dealt with)

((compliant devices will also have just the PoE PD circuitry powered until handshaking is complete and the host device receives power))