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Connecting Non-PoE Devices to PoE Ports

PoE devices, like NETGEAR’s 5-Port adaptor (on Amazon), allow both information and electricity to travel along an ethernet cable, which is very convenient and easy to install. This leads to a quandary, however: what happens if a non-PoE device is connected to a PoE-enabled Ethernet port?

When using an Active PoE system, it safe to connect an ethernet device that doesn’t require PoE Power, and the data transfer component will work just fine. Connecting a non-PoE device to a Passive PoE system can damage equipment, though.

The great thing about standards is that there are so many to choose from. PoE is an evolving technology, and as such there can be variations in how it works. It’s important to look closely at your PoE sending and receiving devices and to make sure you understand both systems, Passive and Active PoE, and how to tell what your specific device requires.

How Auto-Sensing PoE Switches Manage Power and Data Transfer

Ethernet switch ports with PoE function closeup

In order to make sure that whatever receiving device you use will not be adversely affected by PoE, the best option is to use an Active PoE-transmitting device. Active, or auto-sensing, PoE switches are able to communicate with their attached devices to determine how much electricity can safely be delivered. With active PoE systems, the voltage that can be delivered generally has a maximum of 48V.

Part of the Power and Data management going on in an auto-sensing switch has to do with the different standards and modes of PoE Transmission. Let’s briefly review these more wonky details about the PoE standard which you’ll want to know about if you’re trying to manage a complex PoE setup.

3 Standards and Modes for PoE Transmission

There are three engineering standards established for wired Ethernet. The two that most commonly apply to active PoE devices are IEEE 802.3af and 802.3at. There is also an evolving standard called 802.3bt that takes advantage of Gigabit speeds and allows for increased levels of transmitted power. These IEEE standards are also commercially referred to as PoE, PoE+, and PoE++, respectively.

Additionally, there are three modes for PoE transmission, establishing which of the 4 pairs of wires within a typical ethernet cable transmit the power signal: Mode A, Mode B, and 4-pair. Active PoE devices, according to their adherence to IEEE standards, are able to utilize all three of these modes.

What Is PoE Handshaking?

When active PoE-compatible devices are connected and activated, they first determine if they can use the transmitted power. This is established via a Handshake: an initial communication signal between the receiving and transmitting devices that test for compatibility and protocol, and establishes what voltage level and transmission mode should be used.

Once the Handshake is transmitted and received, the PoE-transmitting device will begin sending the appropriate amount of voltage, along the associated paired wires, to the attached device. If the Handshake is not established, active PoE devices will simply not transmit power to that specific device. Because of this behavior, we can describe the system as one that “fails safe”–if it can’t figure out how much power to send, it just won’t send any.

So, a non-PoE device connected to an active PoE switch should work just fine. If you’re considering adding PoE to your ethernet system, it’s easy to see that using active PoE devices makes the decision-making process simpler. With so much to consider, and active PoE systems being able to handle all of these varying standards and variables with ease, why even consider passive PoE?

Advantages and Disadvantages of Passive PoE Systems

Power over ethernet (PoE) switch

Passive PoE systems have at least two things going for them. One is cost: passive systems being more simple, cost less. Secondly, since most PoE devices use a standard that accepts power from passive PoE transmitting switches, passive PoE works just fine most of the time.

Passive PoE devices utilize the most basic configuration standard established for ethernet-enabled devices, IEEE 802.3af. Unlike active PoE systems, they may only use the Mode B method of power transmission, which designates two of the four pairs of wires within typical ethernet cables for that purpose exclusively.

Passive PoE Devices Don’t Use a Communication Handshake

In further contrast with active PoE devices, passive PoE devices do not utilize a communication Handshake. A passive PoE device sends the maximum available voltage along whatever cable is plugged into their ports. The result is that the receiving device is essentially responsible for determining how the transmitted power gets handled.

For passive PoE devices, this generally works fine on both ends of the cable. The passive PoE receiving device is designed and capable of transforming down the received power to whatever voltage is required. With passive PoE systems, the transmitted voltage is generally 24V or less.

The primary concern is what happens when connecting a non-PoE device to a passive PoE transmitter. Without the two-way initial communication provided by the Handshake, the passive PoE system has no way of deactivating the undesired power transmission. If the resulting power surge is not accommodated by the receiving device’s design, the result could be damage to the device, or even overheating which causes a fire.

Passive PoE Receiving Devices Might Not Be Able to Handle Extra Power

Furthermore, there’s no good way to know if your non-PoE device can handle the unwanted power from a passive PoE system until after the smoke has cleared. More concerning, as the standards for PoE are evolving, some older passive PoE systems are not labeled as such. If you are planning on connecting a non-PoE device to a PoE system, be cautious and check the manufacturer’s guidance.

Using PoE Extractors

If you don’t have the option to change out your passive PoE switch for an active one, but need to add a non-PoE device, there is still hope. A device called a PoE Extractor, such as the Huacomm 5-Port (on Amazon), can be used to separate the unwanted power from a PoE cable on the receiving side, allowing data only to be transmitted along the ethernet cable.

 Additionally, an Extractor can take the power delivered by a PoE switch, and send it to a device via a more traditionally-shaped power connector. If do you use an Extractor to power your non-PoE device be aware there are many variations in quality, voltage, and adapter shape. Make sure to match the voltage of your non-PoE device with your Extractor voltage.