PoE — The Evolution Continues

Aug. 23, 2011
The IEEE PoE Standards Continue to Evolve, so What's the Impact of These "New" IEEE Standards on Industrial Applications
About the Author:
Ian Verhappen is an ISA Fellow, Certified Automation Professional, and a recognized authority on industrial communications technologies with 25+ years experience predominantly in the process control field.As I wrote here last year, Power over Ethernet (PoE) certainly removes the single-cable constraint to using Ethernet in the field. The challenge faced by PoE for automation applications is that the 802.3af standard specified a voltage of 48 Vdc, while this industry has standardized on 24 Vdc. At least one industrial Ethernet equipment supplier released a PoE splitter with a 24 Vdc output.

The IEEE PoE standards continue to evolve, so what's the impact of these "new" IEEE standards on industrial applications.

IEEE 802.3af specifies that the current delivered to each node is limited to 350 mA. The total amount of continuous power that can be delivered to each node, taking into account some power loss over the cable run, is 12.95 W.

The updated IEEE 802.3at-2009 PoE standard, also known as PoE+ or PoE plus, provides up to 25.5 W of power. The 2009 standard prohibits a powered device from using all four pairs for power. Some vendors announced products that claim to be compatible with the 802.3at standard and offer up to 51 W of power over a single cable by using all four pairs in the Cat. 5 cable. Table I summarizes some of the key parameter differences between the two PoE standards.

Table I: Standard PoE Parameters and Comparison
Property 802.3af (802.3at Type 1) 802.3at Type 2 
Power available at PD 12.95 W 25.50 W
Maximum power delivered by PSE 15.40 W 34.20 W
Maximum current 350 mA 600 mA per mode
Maximum cable resistance 20 Ω Cat. 3 12.5 Ω Cat. 5
Supported cabling Cat. 3 and Cat. 5 Cat. 5

Cat. 5 cable 24 AWG (0.205 mm²) has a resistance of ≤0.188 Ω/m—not very efficient in transmitting energy, which is why the power delivered by the power sourcing equipment (PSE) must be significantly higher than the power available to the powered device (PD).

Because there are devices non-compliant with PoE+ and legacy PoE solutions developed before the standard was written or ratified, you must be careful when using Powered Ethernet appliances. This should not be a problem in an industrial setting because we are used to verifying interoperability and in most cases purchase our equipment for a single facility from a limited number of suppliers.

[pullquote]With four-wire pairs in each Ethernet cable, there are two alternate methods of providing PoE capability. Alternative A allows the PSE to place power on the signal (data) pair, and Alternative B allows the PSE to place power directly on the spare pair.

802.3af-compliant PDs must be polarity-independent and able to draw power over either Alternative A or Alternative B. This is the only way to guarantee interoperability because the standard does not require the PSE to implement either alternative nor does it specify polarity using that alternative. However, because this is a dc current, polarity is required on data pairs. Because there is a risk that the power can be applied to a range of wire pairs, a diode bridge normally is used to manage polarity.

In addition, because it is not possible to know on which wire pair the voltage will be applied or even if the end device is PoE-capable, a low-voltage detection pulse is applied by the PSE upon connection of the device and PSE. The detection signature matches the impedance on the device end within the following constraints. The required impedance has a capacitance portion of less than 0.12–0.25 F and a resistance portion of 23.75-26.25 kΩ when measured at the PD end. For compliance, all that is really required is a 25 kΩ ±1% resistor across the power rails.

The detection logic determines which of the classes of power is supported by the PD. The IEEE 802.3af standard defined four classes of power and a fifth one was then used by the 802.3at standard. The classes are defined below in Table II.

Table II: Power Levels Available 
Class Usage Classification current
Power range
Class description
0 Default 0–4 0.44–12.94 Classification unimplemented
1 Optional 9–12 0.44–3.84 Very low power
2 Optional 17–20 3.84–6.49 Low power
3 Optional 26–30 6.49–12.95 Mid power
4 Valid for 802.3at (Type 2) devices, not allowed for 802.3af devices 36–44 12.95–25.50 High power

Class 4 can be used only by IEEE 802.3at (type 2) devices, requiring valid Class 2 and Mark 2 currents for the power-up stages. An 802.3af device presenting a Class 4 current is considered non-compliant and, instead, will be treated as a Class 0 device.

To manage PD currents, a PSE will start a timer that trips in 50–75 ms if the current does not drop back below the 15.4 W/Vport level. One consequence of this circuitry is that it limits the input capacitance of the dc/dc converter used in a PD. In addition, the IEEE specification says the maximum current a PD is allowed to draw is 450 mA. Once the power has come up in the PD, there is a requirement in the IEEE standard to current limit the PD to 400 mA during normal operation.

In addition to the work to increase the energy available on a Cat. 5 cable using PoE, efforts are underway to reduce the total energy demand as well. Energy Efficient Ethernet or IEEE 802.3az-2010, which was ratified Sept. 30, 2010, is targeted at saving energy in Ethernet networks for the popular 100BaseTX and 1000BaseT PHYs (transceivers), as well as emerging 10GBaseT technology and backplane interfaces, such as 10GBaseKR. The method of power savings currently planned for these PHYs is a technique known as Low Power Idle (LPI).

LPI provides for a lower-consumption energy state that can be employed during periods of low link use (high idle time). LPI allows for rapid transitions back to the active state for high-performance data transmission.

Remember, Ethernet is about the lower layers of the OSI model and does not define the protocol doing the actual communications between devices. Just like when we make a phone call, often over our PoE-enabled IP phone these days, everyone on the call must be speaking the same language in order for communications to take place. The same is true for industrial Ethernet and we have plenty of options available, each of them well-suited to the industry in which they are targeting — just be sure your selected hardware supports the software you plan to use.