High-power PoE strengthens Industrial Ethernet as the leading network

Industrial Embedded Systems — January 11, 2008

6The Industrial Ethernet community has made great strides promoting Ethernet use for industrial networks and factory automation. The challenge now is to further enhance this technology to become the leading industrial networking standard.

Introducing PoE over industrial Ethernet lines creates new opportunities to directly power industrial equipment, drastically reduces overall installation and maintenance costs, improves overall reliability, and increases configuration flexibility. Combining power and data over the same medium allows Industrial Ethernet to compete with Fieldbus. This article discusses the market drivers for Industrial Ethernet and how PoE can meet the expanding needs.

Ethernet migration from enterprise to the factory

Ethernet is a very popular communication standard widely deployed in enterprise applications. Driven by IP convergence, Ethernet is migrating from the enterprise level to the factory floor in the form of Industrial Ethernet. Adopting a common standard is facilitating integration and communication between machines and equipment. Sharing data from the machine level to the enterprise resource planner platform becomes possible over a single integrated network. This integrated network – where data on every machine can be accessed easily – simplifies logistic process control, including tracking inventory, work-in-process control, and production planning.

Ethernet everywhere has many advantages. For example, when reconfiguring a production system, a common communication system supports adaptations and extensions in a flexible way. Adding a module is plug-and-play. Security and reliability are improved by relying on the common Ethernet standard, which is well proven and robust. Installation and replacement costs are significantly lower with a standard than with a proprietary interface. Furthermore, Ethernet enjoys widespread usage and is available worldwide, which means that Industrial Ethernet can take advantage of COTS technology and equipment.

Initially, Ethernet connectivity appeared in industrial devices relatively high in the industrial network hierarchy and closer to the enterprise network. The hierarchy starts with the enterprise server network and expands toward the factory server and zone controllers. Each zone has Programmable Logic Controller (PLC) devices controlling several submodules. Today, migration has reached the PLC level as shown in Figure 1.

21
Figure 1: Industrial network migration has reached the PLC level

Indeed, IP convergence is driving Industrial Ethernet in the market today. Worldwide shipments of industrial I/O modules with Ethernet-enabled connectivity comprised 14.7 percent of the total market in 2005 and are expected to account for 20.6 percent of the total by 2010[1, 2].

Industrial Ethernet protocols and competitors

The Ethernet standard addresses the physical and data link layers of communication protocols. These layers are the two bottom layers of the seven-layer OSI model as shown in Figure 2. The Ethernet protocol defines the Ethernet 10/100BASE-T physical media over which data is sent and the CSMA/CD access protocol for different devices connected to the media.

22
Figure 2: The Ethernet standard addresses the physical and data link layers of the seven-layer OSI model

Industrial Ethernet competes with several other communication protocols, particularly at the physical and data link layers as shown in Figure 3. For example, other physical layers include RS-485 and RS-232. Different data link protocols include point-to-point or CAN.

23
Figure 3: Industrial Ethernet competes with other communication protocols at the physical and data link layers

All Industrial Ethernet protocols use the same physical layer but add some proprietary features on the data link layer to enable real-time applications. Several other protocol stacks can be added on the data link and higher layers, which are responsible for the network, transport, and application. Applications such as security management and real-time control can be embedded, resulting in a variety of industrial standards supported by different vendors as shown in Table 1.

Some of these protocols, such as Ethernet POWERLINK and Modbus, can run on different physical and data layers like 10/100BASE-T and RS-485. Several of these protocols can be supported on different network topologies like bus, tree, and star networks.

Because of the variety of Industrial Ethernet flavors, itís clear that Ethernet is gaining widespread acceptance by equipment manufacturers. It is important to mention that in the enterprise environment, Ethernet is primarily implemented in a star configuration. However, some Industrial Ethernet protocols allow bus and tree topologies. From a security and reliability standpoint, star configurations provide some advantages since they do not affect other devices on the star in case of failure. For bus and tree topologies, this is not always the case. In the industrial world, bus topologies are historically quite popular. Therefore, some protocols like EtherCAT and PROFINET enable bus configurations on top of the Ethernet protocol.

PoE could power industrial soon

PoE is becoming widely available on enterprise networks where it remotely powers the edge of the network close to applications. Today, it powers VoIP phones and WLAN access points and is becoming popular in industrial security applications, such as security cameras, access control systems, and RFID access points.

For industrial networks, the network edge in every node at the end of the industrial network is close to the manufacturing line or process. In parallel with adoption in the enterprise world, industrial applications are becoming PoE-enabled. Some Industrial Ethernet switches are available with PoE-enabled ports. A few WLAN access points for industrial networks and small industrial cameras powered over the Industrial Ethernet network are currently available on the market.

This is only a first step; clearly there are more opportunities for PoE on Industrial Ethernet. Other possible elements to supply with PoE networks are gateways, sensors, and interfacing devices such as HMI and, potentially, parts of PLCs, drives, and I/O modules.

PoE depends on two devices, one at each end of the cable. The Power Sourcing Equipment (PSE) device is normally located in an Industrial Ethernet switch or zone controller and distributes the power on the cable. On the consumer side, such as WLAN access points, sensors, I/Os, or PLCs, the Powered Device (PD) accepts the power and converts it to the voltage level the application needs as shown in Figure 4.

24
Figure 4: PoE depends on two devices at each end of the cable the Power Sourcing Equipment (PSE) device and the Powered Device (PD)

Is PoE ready for industrial adoption?

PoE has proven to be advantageous on the enterprise network, but what can it bring to the industrial network? Several criteria drive adoption decisions for new technologies in the industrial world. Flexibility, safety, integration, support, price, compliancy, and robustness are the dominant ones. By itself, Industrial Ethernet aligns well with these criteria, and adding PoE on the network only helps to further satisfy these requirements.

Industrial equipment must be easily programmable and flexible for changes in configuration. Bringing power on Industrial Ethernet can remove power plugs and converters, reduce cabling cost, and simplify reconfiguration or new equipment additions.

Robustness and reliability are key requirements for industrial equipment. PoE in combination with an uninterruptible power supply on the Ethernet switch can significantly improve plant safety in the event of a power failure. When power drops out, all equipment on the Ethernet cable stays active. This provides the ability to monitor critical functions and trigger alarms, as well as safely manage the power restoration process.

PoE network reliability may be a concern, particularly in harsh industrial environments where high temperatures and destructive transient ESD events often occur. Fortunately, PoE devices designed to survive under these conditions are available on the market.

Other benefits of PoE include the size reduction that can be realized by removing AC power converters on the equipment. As previously mentioned, installation costs can be reduced since using multiple power cables and conduits can be avoided, increasing the ease of installation.

When interconnecting equipment across different industrial network standards, extensive vendor support is often required. However, using a standard network with standardized power opens the door for product competition, which improves support availability and equipment diversification, ultimately reducing the overall cost. Some of these issues are driving the industrial world to adopt the Ethernet standard. The PoE IEEE 802.3af and the preliminary 802.3at standards are in place to ensure compliance between devices receiving PoE.

Evaluating these business drivers demonstrates how PoE brings substantial value to Industrial Ethernet. The next question is: Where will this new technology penetrate the industrial network, and what are the challenges?

Bringing value to industrial networks

Today, Industrial Ethernet is penetrating the industrial network core but has not yet migrated to the edge of the network. It started with the enterprise network and reached the controller level. Below that level, Fieldbus is the most common communication standard. Small sensor devices, such as push buttons, valve control blocks, and other control or sensing devices are connected via Fieldbus as shown in Figure 1. So where does PoE fit and how does it add value?

Ethernet can connect devices currently communicating via Fieldbus. Complex sensors such as level measurement or optical sensors, HMI, and other interfacing devices will move to Ethernet. Basic sensors such as pressure and temperature will continue to use Fieldbus, but the Fieldbus network will be connected to a gateway with Ethernet as the backbone (Figure 5). The reason why some sensors will move to Ethernet and others will not is mainly a trade-off between criteria, such as required data rate, Ethernet connectivity cost, and sensor real estate.

25
Figure 5: Basic sensors are expected to use Fieldbus connected to a gateway with Ethernet as the backbone

To succeed in replacing Fieldbus on complex sensors and gateways, all the available Fieldbus features must also be available in Industrial Ethernet. Fieldbusí first and most important feature is its capability to deliver power over the network cable. PoE on Industrial Ethernet addresses this requirement.

Another feature of Fieldbus is that it can be wired in a bus or tree topology instead of a star. By adding additional data link or application stacks, some of the Industrial Ethernet protocols support line and tree network topologies. With this feature, the Ethernet network can be configured as a star, bus, or tree.

How can PoE devices be combined to support a bus configuration? Adding a single port PSE device combined with a PD in every node allows power to be received and forwarded in every node. Of course, the length of the line and number of devices on the line are limited by the maximum power injected at the beginning of the line.

PoE adds robustness, security, and redundancy

Currently, many PLCs, I/O modules, and drives have Industrial Ethernet connections. However, their power requirements do not allow them to be powered over the Ethernet cable. Digital output modules can drive up to 500 mA per port at 24 V resulting in 12 W per output. Instead of bringing power to the complete module, it makes sense to only supply the communication and control hardware block via PoE. The rest of the module is powered over the 24 V industrial power line. PoE ensures communication between the modules if the 24 V or AC main power drops out (Figure 6). The control part of the module is always available and ensures correct module shutdown. In this way, PoE guarantees a secure operation mode and secured storage of process data on the industrial system.

26
Figure 6: PoE ensures communication between modules if the power line drops out

While the current standard IEEE 802.3af limits the power level to 13 W. The new IEEE 802.3at standard brings the power level to 30 W. This evolution makes it possible to bring PoE to a higher level of industrial devices. With more power available, high-end devices such as sensors, HMI, industrial cameras, and RFID access points will be supported. Also, a longer bus or tree of PLC, I/O, or drive module communication blocks can be supplied. As depicted in Figure 6, a combination of PSE devices and PDs can support bus and tree network topologies. With more power available, PoE could support longer buses or trees.

PoE as defined in the IEEE 802.3 standard is very closely linked with the physical layer and, in particular, with the cable media. Typically, for the 10/100BASE-T physical media, Cat3 up to Cat5e cables are used. As shown in the Industrial Ethernet standards overview in Figure 4, it is clear that all flavors of Industrial Ethernet use the same physical layer. Theoretically, PoE can be implemented on all of these standards.

The standard is to use Cat5 cable for IEEE 802.3af, and it has been proposed that Cat5e or higher be used for IEEE 802.3at. The main argument for defining a different cable for higher power is the cable resistance. Cat5e and Cat6 have a 100 m loop resistance of 25 W compared to 40 W for standard Cat3-Cat5 cable. This requirement reduces the voltage drop and power dissipation in the cable. Industrial Ethernet uses Cat5-6 cables and, therefore, does not add any restriction to the PoE technology. Industrial Ethernet is currently using RJ-45 and M12 connectors (besides optical connectors). The RJ-45 has four pair connected while M12 connectors have only two pair. Both PoE standards, IEEE 802.3af and IEEE 802.3at, will only use two pair for distributing the power; thus, the different connectors will not restrict PoE adoption.

Ready to power the network

IP convergence will drive Industrial Ethernet from the enterprise to the factory floor. PoE is ready to be introduced across the complete Industrial Ethernet. PoE+ removes the power restriction on Industrial Ethernet controllers and network topologies. In the future, Industrial Ethernet will migrate farther down to the Fieldbus device level. PoE can only strengthen Industrial Ethernet, positioning it to become the leading industrial network standard.

References

[1] VDC Report, Industrial Distributed/Remote I/O: Global Market Demand Analysis And User Requirements Analysis, 4th Edition, June 2006 reference from www.fieldbus.org

[2] ARC Advisory Group, Industrial Ethernet Market Analysis and Forecast Through 2009, April 2005.

Koen Geirnaert is senior marketing engineer at AMI Semiconductor in Belgium, where he is responsible for defining and marketing high-voltage communication products. From 2000-2006 he was project leader at AMI in the product development department, where he was responsible for mixed signal IC designs. Prior to AMI, he held R&D positions at Alcatel Microelectronics and Inverto, a spin-off company of the University of Ghent. He holds an MSEE from the University of Ghent and an MBA from Flanders Business School.

AMI Semiconductor

32-2-752-69-51

koen_Geirnaert@amis.com

www.amis.com