Can PoE become a reality for Ethernet power supply?

In the past, on-site cabling was very simple. The sensor can be either power-supply or passive. The power supply type requires 2, 3, or 4 cables, and the passive type requires only 2 cables. At the same time, the sensors are both analog-type and support 12, 24, 36, 48VDC, or 110, 220VAC voltages. The field controller provides power for each sensor, and each sensor is connected to its related controller. Because the controllers are both analog-type, factory wiring is constantly increasing.

When applications from PLC and PAC networking to the control system network become practical and popular, engineers began to look for ways to connect both as networks through the same cable group, it also supplies power to PAC, network devices (such as Ethernet switches), or equipment in the plant.

This power supply method is much simpler when Ethernet and TCP/IP become the standard for all networks (including factory workshops, control rooms, factory offices, and even intranet connection networks. Therefore, the concept of Ethernet power supply (PoE) came into being.

PoE-becoming a reality

PoE is a technology that allows transmission of data and power at the same time through a single 5-class twisted pair as a standard Ethernet connection. PoE can provide conventional 48VDC voltage for traditional category 5 twisted pair wires.

One is a power supply device (PSE), which is mainly a Midspan adapter and an Ethernet switch, it can provide power (and network connection) for the connected PoE device, and is also called a power-receiving device (PD), which can be any device, such as wireless access points, VOIP phones, security cameras, and other digital devices.

The 802.3af Standard specifies two operation modes: In TypeA mode, the power supply and data are transmitted through the same data pair, for example, 10/100 Base-T Ethernet, while in TypeB mode, the project network is copyrighted, the power supply is transmitted through unused data pairs. Whether TypeA mode or TypeB mode is selected depends on the PSE device, not the PD device. In addition, the standard also indicates that the PSE device can use any mode, but not both modes.

As there are many examples of using non-conforming PSE devices, you need to check whether the PSE device complies with the standards before using the PSE device to power the PD device. If the PD device does not comply with the standard, the PSE device automatically stops power supply. Therefore, when Ethernet power is not used, this feature (integrated into each device that complies with the 802.3af standard) can prevent faults of devices that do not comply with the standard.

The PoE device has a 25kOhm resistor and can be used as a PoE "signature ". When a vswitch or another PSE device detects a signed resistor, it automatically detects the device connected to the resistor and powers the device after the device is detected. When the resistor is not connected to any device, the PSE device will turn off the PoE power.

Why is PoE used?

Providing both power and Ethernet data through the same cable means that the number of cables required in the field industrial environment will be greatly reduced. In a typical manufacturing plant environment, wiring costs and maintenance costs are often considerable. The wiring cost is usually 10-USD/foot depending on the location of the factory and industrial environment. Therefore, it is not surprising that engineers and designers are highly concerned about cabling costs.

Network devices and many on-site devices require DC power to work, but using DC power in long-distance industrial environments is not feasible, therefore, the AC to DC power supply must be used within the appropriate range around the device.

Engineers and designers must provide an AC power supply and signal or data cable. Generally in plant environments, such power supplies include a duplex power outlet with a wall-mounted power adapter.

From the perspective of maintenance, it is a great improvement and progress to provide power to Ethernet devices through Category 5 twisted pair wires. The cost of re-wiring is greatly reduced, and the maintenance and troubleshooting become simpler by transmitting power supplies and data through the same cable group. Therefore, while ensuring fast transmission speed, when the cable breaks or is damaged, it becomes easier to maintain the connection.

1. PoE Integrated Wiring
2. A stackable Gigabit PoE smart switch is available for commercial networks.