2 An Introduction to Power over Data Line

Reducing the costs of components and assembly is an important goal in most hardware designs, regardless of the end market. Power over Ethernet (PoE) enables system designers to achieve this goal by using a single cable to both transfer data and power the device. PoE reduces assembly and material costs, since only one cable must be laid out and connected. It also reduces weight, which is especially important in automotive and other transportation applications.

PoE was initially developed for Ethernet systems using CAT5 or CAT6 Ethernet cables, which contain 4 pairs of wires, and PoE implementations use multiple pairs in these cables. The goal of reducing cost and weight has driven the development of Ethernet standards that require only a single pair of conductors, such as 100BASE‑T1, 10BASE‑T1L and 10BASE‑T1S. Older PoE standards were not designed for only a single pair of wires; Power over Data Line (PoDL) is the solution when using a single pair.

Systems using PoDL are split into two main components: the Power Supply Equipment (PSE) and the Powered Devices (PD). As the name suggests, the PSE is responsible for providing power to the devices connected on the transmission line. The PD is the device being powered by the PSE. PSEs can be described in terms of their voltage, current and quality of provided power. In all cases, PSEs must be able to endure the in-rush current that loads the transmission line and the PD. PDs must have a circuit to separate the power supply from the data.

PoDL standards often include a negotiation protocol to ensure necessary voltage and current are provided. On engineered systems, where the devices on the bus are already known during the design phase, it is possible for the PSE to omit the communication phase. On such systems, the PSE may power the device to the predetermined power class. This is especially useful in systems used in the automotive industry because it saves time during startup.

The IEEE 802.3 working group has defined the standards for PoDL for several point-to-point single pair Ethernet variants, such as in the IEEE 802.3bu and IEEE 802.3cg standards. The IEEE 802.3cg standard defines PoDL for both 10BASE‑T1L and 10-BASE-T1S point-to-point segments. One of the key advantages of 10BASE‑T1S is the ability to have more than two data nodes on one mixing segment. This makes the mixing segment a switch and enables many low bandwidth devices to communicate via standard Ethernet. At the time of creation of this document, there are no PoDL standards for 10BASE‑T1S multidrop mixing segments. For these networks, inspiration from other applications can be found and used instead. This can include applications based on PoE, 10BASE‑T1L systems, systems using point-to-point links, RS-485 based systems, or even USB specifications. Because there are no tight constraints to uphold, every solution to PoDL for multidrop systems is potentially different and unique.

This application note will show examples of PoDL circuits that have been tested with the LAN8670 and the LAN8671, either of which can be used as a starting point for customer design.