PoE Plus speeds the transition to Ethernet based industrial control systems

Many of those involved installing networked devices for industrial or other data communications will have toiled long and hard working out how best way to install networked devices in some confined or hard-to-reach place. This need no longer be a problem for many applications because of the wide availability and technical acceptability of Power over Ethernet (PoE), where the same wires that carry the data to and from the connected equipment also carry the power.

 

PoE devices for industrial use must be capable of accommodating harsh operating environments, varying power systems and a management requirement. The latest ‘hardened’ industrial grade PoE devices meet such challenges and provide installation flexibility and versatile management facilities.

 

The IEEE 802.3af standard – also referred to as Data Terminal Equipment (DTE) Power via Media Dependent Interface (MDI) – was the first international standard to define the transmission of Power over Ethernet infrastructure. IEEE 802.3af uses a physical layer power classification method at device start up, based on five PD classes, which specify the maximum power level output at the PSE. 

 

 

Therefore, VoIP phones and other devices can be powered over existing Cat 5 (and above) Ethernet networks, allowing companies to organise their departments, conference and training rooms, call centres etc exactly as they want. Fast and simple relocation of those areas is also possible. However, 15.4W is quite limited; more was required for industrial use especially.

 

 

PoE Class 4 predicted later developments - in 2009, PoE+ was ratified under IEEE 802.3at. The rationale behind this updated standard was the higher power required by new generations of powered devices (PDs), of which an increasing number need power greater than that specified in IEEE 802.3af. 

 

Such PDs are typically IEEE 802.11n WiFi access points, Ethernet-connected LED display boards (LEDs demand substantially more power than the 15.4 W available over an 802.3af PoE line), high-performance wireless devices that provide wider coverage (their high gain antennas and multiple RFs demand much more power), motorised pan-tilt-zoom (PTZ) security cameras, netbooks and IP telephones having advanced features such as video conferencing. Cameras ruggedised for outdoors use, especially in harsh conditions, also need more power for their integral heating units. Such PDs all benefit from IEEE P802.3at.

 

Hardened Gigabit Ethernet infrastructures using PoE Plus also provide an ideal backbone for ITS networks. Long-distance fibre allows sufficient range to cover, for example, a complete grid of traffic lights and related systems, such as, intelligent lights, traffic cameras, sensors and embedded computers, while PoE Plus devices can be used anywhere that Ethernet cables go, even where there is no power supply. 

 

Industry’s growing standardisation on CAT5 cabling helps because its reduced cable resistance allows significantly higher current to be carried without risk of overheating. Also, PoE Plus requires Cat 5 (eight-wire) instead of Cat 3 (four-wire), because more power can be transmitted over two four-wire cables. IEEE 802.3at, therefore, allows power to be raised to 34.2W at the power sourcing equipment (PSE), 25.5W at the PD, with currents up to 600mA. Voltage is raised to 50-57V (42.5-57V at the PD). See the Table for standard PoE parameters compared

 

This is ample to power most of today’s higher-powered typical ‘edge of network’ devices, but the race is on to see just who can squeeze the most power out of a PoE cable. Manufacturers are already claiming 60W, and some say they can push this even higher, although these products would not conform to the 802.3at specification. The limiting factor is not the PSE, but the CAT5 cable, which - even at 34.4W - often requires temperature derating to avoid overheating. 

 

 

IEEE P802.3at makes it easier to combine other services in a single infrastructure, but PoE Plus is also more efficient, because IEEE 802.3at defines a new method of communication and identification between PSEs and PDs. With IEEE 802.3af, the hardware layer (Layer 1) classification is optional. For IEEE 802.3at however, it is mandatory, and an extra data-layer (Layer 2) classification - the Link Layer Discovery Protocol (LLDP) is also added. LLDP allows the PSE to repeatedly request status information from the PD and dynamically allocate power in reply.

 

Therefore, PDs use power more efficiently, only requesting maximum power when required, so saving significant money. This feature of IEEE 802.3at is especially suited to those applications that must be careful with their power consumption, such as remote systems powered by batteries or solar PV cells. 

 

 

The PSE is the start of any PoE Plus network, as it delivers power to the PDs. Typically, unmanaged switches supply power through four IEEE 802.3af/at compliant PoE/Ethernet combo ports. These may feature, as does – for example - Moxa equipment, 10/100M, full/half-duplex, and MDI/MDI-X auto-sensing. Such a specification provides an economic PoE Plus system foundation. It also allows future-proofing of an existing PoE system to support more PDs. A suitable PoE managed switch allows advanced management and protocol support, including SNMP security, multicast filtering, quality of service and automatic warning. 

 

Another benefit of PoE Plus is that it allows two PD interfaces on a single RJ45 connector, with one operating in Mode A and the other in Mode B. This allows higher installation flexibility – as long as Mode B limitations are understood in terms of whether the PD is capable of accepting PoE. Therefore, despite the benefits offered by PoE Plus, much of today’s focus is on making the most of PoE flexibility. This often means use of industrially hardened PSEs that meet the performance requirements of the PDs - as distinct from the power requirements.

 

Industrial Ethernet users rely on ‘plug and play’ installation simplicity, and a degree of ruggedness unheard of in typical IT applications. Best of breed industrial Ethernet switches are hardened to offer extended temperature operation from -50°C to +95°C. They offer absolute EMI/RFI interference rejection and protection as standard, and can be conformally coated for protection against high humidity, corrosive chemicals and salt spray.

 

Industrial PoE switches must also possess the bandwidth and management facilities to deal with applications that use huge amounts of bandwidth at high data rates. Such multicast traffic has to be effectively managed, to prevent unnecessary data flooding the entire network. Best of breed infrastructure components also offer the benefits of IGMP software as standard, which provides a means to manage this traffic.

 

 

What of the future for PoE and PoE Plus? Siemens Industry Automation has said: “For industrial automation applications, PoE will move in the future more and more into end devices and remote I/O. It is not there yet because the power that can be transmitted is too low. PoE Plus helps by providing up to 30W and more, which is needed to the latest requirements. However, to operate many new access points and most I/O will need more power, and to drive electric motors will need much more again.

 

The company had, even three years ago, switches with PoE functionality, as well as access points, but no remote I/O with PoE because of the power problem. But for the future, one way of helping to solve this difficulty is to reduce the energy requirement of devices. For example, more efficient chipsets will consume less power.

 

Where variable speed drives and motors are concerned, at the moment PoE, or even PoE Plus, cannot be used as the power requirement is far too high – the need is there, but this is some time ahead; perhaps in the next few years. However, Siemens has had a pilot installation in a monorail application where wireless communication using wireless power supplies provides more than 100W. This ‘wireless PoAir’ works fine, but the problem is that when you take power from the air, by induction of whatever means, people feel that it is not safe, even though it is. Siemens commented: “They are not worried about high power over wire, only over wireless! We are looking into this aspect.”

 

 

Ethernet over Power (or powerline networking) is another take on the 'no new wires' concept for networks. It uses the existing power wiring in a building or facility to create the network, and so keeps installation costs low. Anywhere there is a power outlet is a potential network socket. 

 

Various Ethernet switches have appeared on the market - adaptors that contain the Ethernet bridge circuitry required to transmit data over the mains powerline.

 

Early versions were based on the Passport specification developed by Intelogis. By today's standards, the performance was very slow – sub-megabit - and even that was dependent on the condition of the wiring and the actual power usage. More recently, devices have come to market based on the PowerPacket specification developed by Intellon. This is much faster, is not affected by disruptions in the powerline, is far better at maintaining connections and speeds, and suffers no signal degradation related to wiring condition.

 

Powerline cables are not ideal for handling data traffic, but PowerPacket uses Orthogonal Frequency Division Multiplexing (OFDM) to provide reliable communications. OFDM can detect changes within the powerline to maintain network communication even during power fluctuations.

 

OFDM is a variation of the frequency-division multiplexing (FDM) used in phone-line networking. FDM puts data on separate frequencies from the voice signals being carried by the phone line, separating the extra signal space on a typical phone line into distinct data channels by splitting it into uniform chunks of bandwidth. 

 

With OFDM, the available range of frequencies on the electrical subsystem is split into 84 separate carriers. OFDM sends packets of data simultaneously along several of the carrier frequencies, allowing for increased speed and reliability. If noise or a surge in power usage disrupts one of the frequencies, the PowerPacket chip will sense it and switch that data to another carrier. This rate-adaptive design allows PowerPacket to maintain an Ethernet-class connection throughout the power-line network without losing any data.

 

How reliable and effective is this in mission critical industrial applications? In fact, there are problems. The first is related to transmission speed: 200Mbit/sec is generally quoted as the maximum speed, although 500Mbit/sec has also been quoted, but even this is some way below the Gigabit speeds that many of today's industrial networks rely on. 

 

Further, there is substantial noise and frequency dependent signal attenuation on every powerline, and this will tend to increase error rates with increasing transmission speeds. So the higher the communication speed, the less reliable the connection will tend to be. A much higher degree of error detection and general communications robustness has to be built into the network from the outset.

 

A final issue is that, to date, the various EoP solutions that have been brought to market have been a long way from the ruggedised products demanded by real-world industrial applications.

 

 

Since its introduction in 2003 as IEEE 802.3af, and its update in 2009 as IEEE 802.3at, PoE has made a major contribution to flexible, low cost, reliable industrial networking. Not only can PoE and PoE Plus allow a wide range of network devices to be powered in remote locations where access to power is unavailable, but PoE in particular will continue to drive forward major changes for industrial applications. The advantages of blending signal and power in one Ethernet cable connection will contribute to the already-rapid transition to Ethernet based industrial control systems.

 

With modern, hardened PSEs, plus a growing range of PDs, PoE, and PoE Plus, are providing the ideal solution for applications where large data rates are involved and where flexible distribution of power is needed. Factory floor installations, particularly those with field devices at the edge of the network, are being simplified. 

 

Also, with the maximum voltage being under the limit defined for high voltage applications, and with the current and fault protection built into the standard, PoE and PoE Plus are inherently safe power technologies, so compliant industrial switches should meet Underwriters Laboratories (UL) Safety Extra Low Voltage (SELV) classification for a safe working environment – which will lead to further applications in industry. 

 

There is also the development of Power over Wireless Ethernet (PoWE), combining PoE with IEEE 802.11g wireless Ethernet. As yet, this has not emerged from the laboratory, where it is still limited to low powers, but there will soon be sufficient power to drive many of today’s PDs. PoWE is likely to redefine flexibility in industrial applications, and provide a new impetus for PoE in safety rated applications.

 

To see a large schematic showing PoE switches networking a process vision system at a stainless steel plant, please use the link below. To see a table comparing the performances of IEEE 802.3af with IEEE 802.3at standards, please use the second link below.