[ 3 pages—2,293 words]

Industrial Ethernet as a Fieldbus

Industrial Ethernet, as a fieldbus replacement technology, lacks two attributes found in fieldbuses. The first is bus topology. The second is power sourced from the network cable to energize field devices. DeviceNet is a good example of a fieldbus that can accomplish both. DeviceNet can be wired in a bus topology while providing 24-volt power in the cable for powering field devices such as photo-eyes, push-button stations, and limit switches. Higher-powered actuators usually have their own power sources. Industrial Ethernet only supports star topology and, until now, could not provide power over the cable without implementing a non-standard approach. With the approval of IEEE 802.3af in 2003, the power-sourcing problem has been solved with the Power Over Ethernet (PoE) standard. PoE not only provides a safe and effective way of applying power to modern devices, but also utilizes its star topology to its advantage by controlling the amount of power each connected device receives while protecting non-powered devices from harm. DeviceNet is incapable of doing the same with its bus topology.

Twisted-pair Cable Carries the Power and Data

PoE was not designed for the industrial or building automation markets but for a much larger information technology market incorporating IP phones, IP cameras, and wireless access points. An IP phone, which utilizes Voice over IP (VoIP) standards, should look and feel like an ordinary telephone. A telephone is powered from its data connection and works when the power is lost because its 48-volt power source operates from batteries. These same attributes are attractive in industrial and building automation when it is inconvenient or expensive to run higher voltage power in the field or when it is desirable to backup the entire automation system from one power source. Most automation systems operate from 24 volts and not 48 volts so the 802.3af standard must be examined for applicability to the automation industry

Modern Ethernet cabling, complying with the 10 Mbps 10BASE-T and 100 Mbps 100BASE-TX standards, consists of four twisted-pairs of which only two pairs are employed. The Gigabit standard, 1000BASE-T, uses all four pairs. Pre-802.3af compliant networks attempted to utilize the unused pairs for carrying power by simply applying 24 or 48 volts to the unused pairs assuming they were not being employed for any other purpose. Clearly this would not work for Gigabit Ethernet since all pairs are used for data. The other approach is to incorporate the data lines for carrying power. This would allow Gigabit devices to be powered as well. The 802.3af standard provided a unified approach to powering devices using either method while addressing the issue of not damaging non-POE compliant devices that are not expecting power. The standard is much more extensive than simply applying power to unused pairs, as we shall find out.

PSE and PD

The 802.3af standard identifies two types of devices. The Power Sourcing Equipment (PSE) provides the required power — just like its name implies. The Powered Device (PD) is the receiver of the power. The two are connected via the Ethernet communications cable. Understanding Ethernet's star topology, the PSE would naturally be assumed to be a port on a hub or switch while the PD would be an end station or node on the network. There are two ways of connecting power as shown in Figures 1 and 2. The first method, Alternative A, is via the data lines while the second method, Alternative B, is via the unused pairs. Using both approaches simultaneously is currently not allowed. The standard is being reviewed to determine if by using both approaches, more power could be transferred to the PD. When using the unused pairs, the wires within the pair are connected together in order to increase the cable's current carrying capacity. The same trick can be used with the data lines, but in this case the power must be fed through a center-tapped transformer. If the DC-current flow in each wire of the pair is equal, there will be no DC bias established in the winding of the transformer so this method could be very effective for carrying power and data simultaneously. In fact, this method is the only one possible for using Gigabit Ethernet since no spare pairs exist. A PSE can be designed to source power either through the data lines or through the unused pairs. The PD must be able to do either, making the PD design a bit more complex.

Figure 1 — Alternative A. The signal pairs carry both data and power. Polarity is indeterminate.

Figure 2 — Alternative B. The unused pairs carry power. Polarity is as shown.

The power being supplied by the PSE is a nominal 48 volts (44 to 57 volts). When using the spare pairs for power, the voltage polarity is defined. When using the data pairs for power, the voltage polarity is undefined — requiring the PD to be able to accept either polarity and still function. The reason being that a crossover cable could be between the PSE and the PD. Normally, a straight-through cable is used to connect a switch to an end-device (see Figure 1). The end device is wired as an MDI while the switch is wired as an MDIX. The "X" means that the switch implements an internal crossover function by having transmitters on the end device. If two MDI-compliant devices need to communicate to one another (an end device to another end device), then a crossover cable is required to make the equivalent connection. Modern switches have a feature called Auto-MDIX where the switch port will adjust to either a straight-through cable or a crossover cable and still function with an end device or another switch port. A crossover cable matches up data pairs by having pins 1 and 2 on one side connect to 3 and 6 on the other. Likewise, pins 3 and 6 are matched up with 1 and 2. Because of this crossover situation, the polarity of the voltage on the data pairs will reverse at the PD. To guard against this reverse voltage condition, an auto-polarity circuit should be used at the PD. Crossover cables do not affect the spare pairs. Therefore, the PD will experience no polarity change. To be safe, it is best to incorporate auto-polarity on the spare pairs as well.

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