In process automation systems, important parameters such as temperature, pressure, flow rate, humidity, and many others must be monitored and measured. In the era of Industry 4.0, Ethernet is a popular communication standard. Because Ethernet is wired and transmitters and sensors typically require a power supply, the question arises: why not use the Ethernet cable for both data transmission and supply? This article describes how Ethernet devices can use the cable simultaneously for transmitting data and for supplying power. Power over Ethernet (PoE) systems are widely used in the industry and will play an important role in the future.
The supply of power via a Cat-5 cable is defined in the IEEE 802.3 af Power over Ethernet standard. With PoE, Ethernet devices can be supplied with power at the same time as the actual data transmission takes place via an RJ45 cable. The PoE standards used to be limited to a few watts, but newer PoE technologies enable even higher power.
"PSE devices have a signature process while powering up to protect incompatible devices from damage when they are connected. This involves first checking the signature resistance of the PD"
For example, PoE+ allows power up to 25 W per port and PoE++ (a four-pair Power over Ethernet system) ranges from 70 W to 100 W by using all of the wires of the existing cable. To extend the benefits of the PoE standard and give OEMs greater flexibility, PoE manufacturers are developing parallel standards that provide a more optimal approach for different applications. For example, Analog Devices has defined the LTPoE++ standard which supports up to 90 W powered device (PD) power (Table 1) LTPoE++ reduces the technical complexity of the PoE system in relation to comparable solutions. Plug and play capability, easy implementation, and a safe, robust power supply are further features of LTPoE++. Moreover, LTPoE++ is interoperable and backwardcompatible with the standard PoE specifications of the IEEE. However, the usable power is somewhat lower than the specified PD power because of losses in the system as well as cable losses, as is also the case with PoE+ and PoE++.
Essentially two components are necessary to supply devices over the Ethernet cable: the powered device and the power sourcing equipment (PSE).
Figure 1. Block diagram showing the main components of a PoE system.
The PSE has the task of delivering the power like a power supply, whereas the PD receives the power and uses it (load). PSE devices have a signature process while powering up to protect incompatible devices from damage when they are connected.