Using Power over Ethernet with NI Hardware

Publish Date: Nov 03, 2009 | 1 Ratings | 5.00 out of 5 | Print

Table of Contents

  1. PoE 101: Technology Overview
  2. Using PoE with NI Devices
  3. Selecting PoE Equipment

1. PoE 101: Technology Overview

The IEEE 802.3af standard defines a protocol for transmitting power and data signals over standard Ethernet 10/100BASE-T infrastructure commonly known as Power over Ethernet (PoE). A nominal 48 VDC at an Ethernet port of a PoE-enabled switch transmits power over unshielded twisted-pair wiring to end devices consuming 12.95 W or less. The newer IEEE 802.3at (PoE+) specification will increase the minimum amount of power available at an end device to as much as 59 W and enable transmission over faster 1000BASE-T gigabit Ethernet infrastructure.

With 802.3af, a PoE-enabled device can transmit or receive power over the four twisted-pair copper wires of a CAT 3 (minimum) Ethernet cable in two different configurations to carry power, data, or both. Power can be transmitted either on the two pairs of wires that conduct data (alternative A), or on the two unused pairs (alternative B). (Refer to Figure 1 below.) IEEE 802.3at requires a minimum of CAT 5e but allows for power to be transmitted on all four twisted pairs to increase the total amount of power available to end devices.

Figure 1. Alternative A uses the same twisted pairs for power and data, while alternative B uses the spare twisted pair to transmit power independently.

Specific terminology refers to which devices supply power and which receive:

Power Sourcing Equipment (PSE) is a device, such as an Ethernet switch or hub, that supplies power over the twisted pairs and controls the detection, classification, and shutdown activities of a powered device at the other end of the cable. Some switches may offer full power on every port, while others may only offer PoE capability on a subset of ports. Still others may offer reduced power levels on some or all ports to reduce equipment costs.

Powered Device (PD) is a device, such as a wireless access point, that draws power from a PSE. There are four classes of PDs defined by 802.3af, each with a different power rating (12.95 W being the maximum). IEEE 802.3at increases this limit up to 59 W if all four twisted pairs are in use.

Midspan is a type of PSE that acts as an intermediary between a non-PoE (traditional) switch and a PD by injecting power into the cable. Midspans provide an easy means of incorporating PoE into an existing network, because you do not need to replace any existing switches or infrastructure.

PoE Injector is a simple PSE that injects power into a single Ethernet cable. Whereas a midspan typically provides power to several Ethernet ports, a PoE injector is a one-off solution for providing power to a PD in a non-PoE network.

PoE Splitter is a simple PD that separates the data from the power at the edge of network. PoE splitters enable PoE functionality in traditional Ethernet equipment, so that you can take advantage of PoE technology even if your end device does not support it natively.

PoE devices are designed to coexist with traditional Ethernet infrastructure, so that you cannot damage a non-PoE device by connecting it to a PoE-enabled PSE. A PSE always tests for standards compliance before providing power to a PD. Each PD has a 25 kΩ resistor that serves as a PoE “flag” or “signature.” If the PSE does not detect this signature when connected to a PD, it will not energize the cable. Once a PSE establishes that a compliant PD is connected, it initiates a start-up sequence to determine its classification and power requirements, as demonstrated in Figure 2.

Figure 2. A PSE determines the class of a PD by presenting a voltage ramp to the PD, which rises from 2.5 to 10 V.

Finally, when a PD is disconnected, the PSE turns on the power at that port.

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2. Using PoE with NI Devices

Several National Instruments devices provide Ethernet connectivity for distributed or remote measurement and control applications. Some devices, such as NI CompactRIO programmable automation controllers (PACs), may have to wait for IEEE 802.3at (PoE+) ratification, but several others are within the 12.95 W power budget offered by the existing IEEE 802.3af standard. For example, NI ENET-9xxx Ethernet data acquisition devices and NI Smart Cameras lend themselves well to PoE networks.

  Input Voltage (V) Maximum Power (W) 802.3af Classification
NI ENET-9xxx 9 to 30 4.5 Class 2
NI Smart Camera 24 10.8 Class 3

Table 1. Power specifications for NI Ethernet DAQ and NI Smart Camera devices.

To take advantage of PoE with these devices, you must use a PoE splitter. For example, the MOXA SPL-24 industrial PoE splitter is IEEE 802.3af compliant and supports output power up to 12.95 W at 24 VDC. To set up your system, connect the energized Ethernet cable to the PoE input port on the splitter. Then connect the regular Ethernet output port and power out lines to your device.

Figure 3. Use a PoE splitter to provide data and power separately to an NI Ethernet DAQ device.

Alternatively, you can use a combination of a PoE injector and splitter to provide power and data connections over long distances (100 m) without the need for existing PoE infrastructure.

Figure 4. Use a PoE injector and splitter to provide data and power over long distances.

This approach is analogous to a long USB cable or USB extender, particularly for NI C Series data acquisition devices. For example, you could use a NI USB-9234 to reach up to 5 m from your PC or laptop. However, with PoE you can extend that reach to up to 100 m with a PoE injector/splitter combination and the NI ENET-9234.

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3. Selecting PoE Equipment

Many local electronic stores now carry PoE-enabled networking equipment, from routers and switches to injectors and splitters. It is important to check the product documentation for IEEE 802.3af or IEEE 802.3at compliance, as many manufacturers use proprietary techniques. For more rugged applications, both Advantech and MOXA offer a range of industrial-rated PoE networking hardware.

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