Power over Ethernet: Myths, Mystery, and the Facts

Transcription

1 Power over Ethernet: Myths, Mystery, and the Facts Written by: Paul Wacker, Advantech Corporation, Industrial Automation Group Field level wiring used to be very simple. A sensor was either powered or passive. If it was powered, it needed two, three, or four wires. If it was passive, it needed only two. Sensors were all analog, and were powered by 12, 24, 36 or 48 VDC or 110 or 220 VAC. Field controllers had power supplies for every sensor and every sensor was wired to its associated controller. Even controllers used to be analog, and so plant wiring increased. Even the PLC and PAC didn t improve the situation in the field. But when networking of PLCs and PACs into the control system network became both practical and popular, engineers started thinking about how to provide power over the same set of wires as the network connection whether to a PAC, a network device like an Ethernet switch, or to a device on the plant or factory floor. This became simplified when Ethernet and TCP/IP became the de-facto standard for all networks, whether on the plant floor, in the control room, the plant office, or the entire enterprise connectivity level. And so the concept of Power over Ethernet (PoE) was born. PoE Just the facts PoE is a technology that permits a standard Ethernet connection to transmit both data and power over a single Cat5 cable. It provides for a nominal 48 VDC over conventional twisted-pair Cat5 cable, and was originally created by Cisco Systems long before the current standard for PoE was released: IEEE 802.3af The standard describes two types of devices that use PoE. The first is Power Sourcing Equipment (PSE). They are primarily Midspan adapters and Ethernet switches, which supply power (and network connectivity) to attached PoE devices. The second type of device is called a Powered Device (PD), which can be anything from a wireless access point to a VOIP telephone, to a security camera or other digital device. Figure-1 VOIP Telephone and WiFi Access Point

2 There are two modes of operation specified by the 802.3af standard: Type A, where the power and the data are transmitted over the same data pairs as 10/100Base-T Ethernet, and Type B mode, where the power is transmitted over unused pairs. The choice of Type A or Type B rests with the PSE, not the Powered Device (PD). Further, the standard says that the PSE can use either mode, but not both simultaneously. Figure-2 PoE Modes of Operation The PSE tests for standards-compliance before providing power to the PD, since there are currently so many instances of non-compliant devices being provided. If the PD is non-compliant, the PSE does not supply power to it, automatically. This feature, incorporated into every 802.3af compliant device, protects non-compliant devices from failure when Power over Ethernet is incorrectly applied. Figure-3 Powered Device (PD) Auto-Detection The PoE device has a 25k Ohm resistor that serves as a PoE signature. When the switch or other PSE detects the signature resistor, it auto-detects the device connecting to it, and power is only applied after a successful auto-detection. When the device is disconnected, the PSE turns off PoE power.

3 Why PoE? Providing power over and Ethernet data over the same cable means a dramatic reduction in the amount of wiring required industrial environments. Wiring cost, and wiring maintenance, in the typical manufacturing plant environment are non-trivial costs. Depending on the location in the plant and environment, wiring costs vary between 10 s to 100 s of dollars per foot. Any reduction in the cost of wiring is therefore highly interesting to engineers and designers. Networking equipment and even many field devices, require DC power to operate. Running DC power over long distances is impractical, so AC to DC power supplies must be provided at reasonably close range to the device. This generally means the engineer or designer has to provide an AC power source as well as the signal or data wiring. All too often, even in the plant floor environment, this consists of a duplex convenience receptacle with a wall wart power adapter. From a maintenance perspective, providing Ethernet devices with power over the Cat5 cable is a significant improvement. Not only are the costs of rewiring reduced, but since power and data come on the same set of cables, maintenance and troubleshooting are simplified, and it becomes significantly easier, not to mention faster, to repair connectivity when a cable is broken or otherwise damaged. Implementing PoE systems Figure-4 Wall Mount Power Adapter PoE network systems are designed the same way as non-poe networks are. The designer lays out the physical network of routers, switches and so on. At the head end of the network, power is injected using a UPS into the master network switch, and additional power can be injected into midpoint PSE devices by additional UPS systems. Devices at the outer edges of the network are called endpoint devices. Power needs to be injected into the system approximately every 328 feet (100 meters), the same maximum distance allowed for an Ethernet connection. Figure-5 Conventional Network Design with PoE

4 The use of Midpoint PoE Power Injectors has diminished, with the availability of PoE enabled Ethernet Switches. Today, compact and ruggedized models are available for industrial environments. While PoE was designed for telecoms and IT networking, the solution is so elegant that as devices become smarter, and Ethernet connectivity becomes even more ubiquitous, most Ethernet connected sensors and field devices will be PoE enabled. 10/100 Ethernet Dual 12 to 48 VDC Power Inputs 10/100 Ethernet plus PoE 4 ports Some best practices for Designing PoE systems Figure-6 Ethernet Switch with Integrated PoE Support In the industrial environment, PoE enabled switches and other devices need to be ruggedized and designed specifically for use in the plant and factory floor applications. Designers need to make sure that the selected devices have a proper enclosure design and temperature and vibration specification for use in harsh environments. Often, industrial PoE switches must also be DIN-rail or panel mounted. Finally, selected units should use available power, like 24 VDC, so no wall warts will ever be required, and where possible, redundant power inputs will ensure continuous operation in the event of an external power supply failure. PoE Power Status Indicators 10/100Base-TX Ethernet / Uplink 10/100Base-TX Ethernet ports PoE Powered Dual Power Inputs (12 to 48 VDC) Reverse Power Polarity Protection Input Power Surge Protection (3000V) Fault Relay for external PLC/Controller IP30 Metal Chassis Wide Operating Temp (-10 to 60 C) IEC60068 Shock, Vibration & Freefall Magnetically Isolated Ethernet (1500V) Ethernet ESD Protection (4000V) Figure-5 Hardware Overview of PoE Switch The designer must also be aware of non-standard PoE devices. As PoE evolved, many different types of designs were introduced. Cisco Systems for example, supports several different PoE standards, including pre-standard Inline Power and standard 802.3af PoE. There are also a wide

5 range of homebrew methods, utilizing various techniques and power supply voltages, to be wary of. Designers producing PoE-enabled networks should insist, as a best practice, on only devices that adhere to the 802.3af standard. Connecting non-poe devices Many Ethernet networks connect to devices such as VDTs and embedded computing devices. These devices are typically not PoE-enabled devices, but require power. Rather than supplying a separate power supply for the endpoint device, the designer can use a PoE splitter device which divides the combined power-and-data signal and provides a separate output for power and one for data. PoE Splitter 24 VDC Power Ethernet 3.5 Touch Panel Computer Ethernet + PoE Figure-6 PoE Splitter The future of PoE Power over Ethernet is a technology that has far to go before it is fully implemented, and it provides tremendous opportunity for connectivity in many applications. The advantages of a single Cat5 cable for both power and network, combined with centralized power management, and the ability to provide safe low-voltage DC power, means that PoE is an important enabling technology for the future. Currently, industrial PoE applications are limited mainly to wireless access points, video surveillance cameras, and machine vision systems, but sensor applications are not far off. As industrial controls manufacturers continue to embrace Power over Ethernet, more devices will be designed to be PoE compliant. For example, several companies now make Ethernet-enabled sensors. These sensors do not require analog transmitters, and can connect directly to a control network. Several industrial Fieldbus networks are considering conformance with the 802.3af standard which will further expand the range of industrial PoE applications in the future. Figure b/g Wireless Access Point ###