Field level wiring used to be very simple, a sensor was either powered or passive and were all analogue, field controllers had power supplies for every sensor and every sensor had an associated controller. It was only when the idea of networking these devices into a control system network became popular and practical that engineer’s began questioning how else power can be provided over the same set of wires used for the network connection. The idea became much simpler when Ethernet and TCP/IP became the de-facto standard for all networks and so Power over Ethernet (PoE) was born.

So What is Power over Ethernet? (PoE)

PoE permits a standard Ethernet connection to transmit data and power over a single Cat5 cable. It was originally produced by Cisco Systems well before the current IEEE 802.3af-2008 standard was released.  The standard covers 2 types of devices, Power Sourcing Equipment (PSE) which are usually Ethernet Switches and adaptors to supply power and connectivity to attached PoE devices. The second type is for Powered Devices (PD), this entails anything from a wireless access point right through to a security camera. There are 2 type of operations specified in the 802.af standard with Type A transmitting power and data over the same data pairs as 10/100Base-T Ethernet whilst Type B utilises unused pairs. The PSE can use either mode but not both simultaneously, whilst the Power Sourcing Equipment (PSE) will also test for standards-compliance before providing power to the Powered Device (PD).

Why Power over Ethernet? (PoE)

Well why not? Providing power and Ethernet data over a single cable results in a dramatic reduction in the amount of wiring required, therefore initial cost and long-term maintenance in manufacturing and industrial environments are minimal costs. Running DC power over long distance...CONTINUE READING HERE