A Guide to Industrial Power over Ethernet

Power over Ethernet (PoE) carries both data and power over a single Ethernet cable, on RJ-45 or M12, to a device up to 100 metres away at 10, 100 or 1000 Mbps. A PoE port does everything a standard Ethernet port does and supplies power as well, which is what makes it useful at the edge: it powers cameras, sensors and computers in places where running a separate mains supply is awkward or expensive. Power levels are set by the IEEE 802.3af, 802.3at and 802.3bt standards.

This guide covers how PoE works, the standards and power levels, active versus passive PoE, the PSE and PD roles, and where a PoE computer fits, whether it sources power or draws it.

PoE uses the twisted pairs inside an Ethernet cable to carry power alongside data. Under 802.3af and 802.3at, two of the four pairs carry power and the data is unaffected; 802.3bt uses all four pairs to reach the highest power. The two signals coexist because the power is direct current while the data runs at a much higher frequency, in the tens to hundreds of megahertz (10 to 100 MHz).

Every link has two roles: the Power Sourcing Equipment (PSE) that supplies power, and the Powered Device (PD) that draws it. Most switches and some computers are the PSE; cameras, access points and VoIP phones are PDs. Where a device connects to a switch that has no PoE, a separate injector adds power to the cable.

A PoE computer can sit on either side of the link. As a source (PSE), a computer with several PoE+ ports powers and connects a set of cameras or sensors directly, so a vision or surveillance node needs no separate switch. As a powered device (PD), the computer itself runs from a single cable, which removes the need for a local power supply in a ceiling, on a pole or in a sealed cabinet.

Alongside the computers, our range includes PoE single board computers for board-level integration and industrial NVR systems for PoE camera recording. Which fits depends on how many ports you need, the power budget, and whether the computer is sourcing or drawing power. For applied examples, see how to size the PoE budget for a multi-camera edge AI build and how PoE powers IP cameras and recorders in surveillance systems.

Active PoE is the IEEE-standard kind: the source and the device perform a handshake to agree the voltage and power class before any power flows, typically at 48 V DC, so a device only receives what it can safely take. Passive PoE skips the handshake and applies a fixed voltage, often 24 V DC, to the line regardless of what is connected.

Because passive PoE does not negotiate, feeding it to a device that expects active PoE can damage that device. For interoperable, protected deployments, specify active, standards-compliant PoE end to end.

The PSE sources power and the PD draws it. Power can travel on the data pairs, Mode A (endspan, pins 1, 2, 3, 6), or on the spare pairs, Mode B (midspan, pins 4, 5, 7, 8); a compliant device must accept either, and 802.3bt uses all four pairs. These are choices for the source to make, and a standards-compliant device handles whichever it is given.

Things Embedded supplies industrial PoE computers that can act as the source for cameras and sensors, or be powered over PoE themselves. Tell us the device count, the power per port and the environment, and we will recommend the right configuration.