By Bryson Barney
What if a single wire could carry both energy and information seamlessly—without either getting in the way? The future of connectivity isn’t about more wires—it’s about smarter ways of using your existing cable infrastructure. Serial communication typically relies on three or four conductors: one or two wires for single-ended or differential signaling, plus two more for power and ground. With the help of simple coupling and decoupling networks, it’s possible to merge power and data onto the same pair of wires. How does that work? The answer lies in the frequency domain. Since data signals usually occupy a much higher frequency band than the supply voltage, the two can coexist on the same line—combined during transmission and cleanly separated at the receiver—without interfering with each other.
Figure 1: Frequency Band Overview Chart
Let’s look at the coupling and decoupling filters. Figure 2 outlines a network with two nodes where one node acts as a power source and the other node receives power over the same two wires as the data. RS-485 transceivers are used in this scenario because of the differential signaling capability (using two wires for the data). Typically, with RS-485, power is sent over the cable with a third and fourth wire for the supply line (power+ and power-).
Figure 2: Two Node Network Architecture (Power Source Node and Power Receiving Node)
The inductor and capacitor components connected to the network cable wires form the high-pass filtering and low-pass filtering. The inductors act as a low-pass filter to block transients (data included) from interfering with the supply and load on the nodes. The series capacitors on the lines form the high-pass filtering that prevents the DC supply voltage from entering the transceivers, while allowing the data to pass into the transceiver. Think of filters as frequency gates. A low-pass filter is like a bouncer that only lets the “slow dancers” (low frequencies) into the club, while a high-pass filter admits only the “fast movers” (high frequencies). Together, they’re essential tools in electronics, audio, communications, and even image processing — helping engineers clean, sharpen, and control signals with precision. The method of doing power over data using filters has been understood and used in many systems for years and ultimately reduces the cost of cabling for point-to-point and multi-drop networks. The reason it has not been popular in the past is because it requires special encoding in the protocol stack to utilize it.
Many RS-458-based systems cannot use power over data (PoD) because of their protocols (modBus, bacNet, Profinet, etc). When data is organized into frames for transmission across the cable, the data must be encoded to keep the line DC balanced. This means that the amount of time the line is high and the amount of time the line is low must be equal, and the ones and zeros forming the data packet need to be evenly distributed. The data must not influence the DC supply level. Referring back to Figure 1, unbalanced data can potentially cross the boundary between the low-pass and the high-pass filtering. If the data packet has sections of high or low for too long, the line will drift to common mode on the transceiver side of the series capacitors and data will be lost. To combat this, interface protocols were created that have an encoding layer such as Manchester encoding in their software stack. Ethernet is a high-performance example of this approach. Home Bus, which is designed for lower-speed, low-cost networks, is another example. While adding a software encoding layer allows for power over data with RS-485, it precludes any system that is unable to change their protocol. Many unbalanced protocols (Profibus, Modbus, bacNet, etc.) have been in use for a long time and are difficult to replace. For this reason, ADI has created a technology known as PD-Link, which manages the DC balance of the line at the transceiver level (physical layer) without placing any requirements on the frame organization of the data sent over the line. This opens the door to power over data to all protocols.
Power over data can now be used by any system either by adopting a software stack with an encoding layer, or by using Analog Devices game changing PD-Link technology. Ready to simplify your design? To learn more about PD-Link, contact your local ADI Sales team to discover how PD-Link can transform your serial communications.
See the blogs in the TranscendingConventionalFieldBus series.