Overview:

10BASE-T1L is a full-duplex 10Mbit Ethernet link which requires only a single wire pair and which can operate over distances up to about 2km! This makes it seem a great substitute for CAN and RS485 connections to remote peripherals. Of course it does lack the multi-drop capability, but the seamless back-office integration due to their MAC & TCP/IP based configuration and access can be of great benefit.

Note that a few more single-pair Ethernet variants are in existence; close sibling is 10BASE-T1S, a shared bus for up to 8 devices, also 10Mbit except much shorter distance of only 40 meters. Then there are also versions for 100Mbit (802.3bw) over 15 meters and 1Gbit (802.3bp) over 15 or 40 meters. Unfortunately, none of the SPE variants are interoperable.

In our opinion, 'long haul' SPE only makes sense if the same wire pair can also be used to provide power to the device, particularly for small 'IoT' sensors and the like. But more substantial self-powered devices could benefit from a dual-powered management interface also. Either way, this is where power over data lines or PoDL comes into play.

Power over Data Lines:

PoDL is the single-pair equivalent of Power over Ethernet (POE). The main difference is that with POE, one pair carries either positive or negative power. With SPE, there is only one pair, so the individual conductors of the pair must be biased to facilitate energy transfer.

We mode some proof of concept prototypes for a PoDL injector and a PoDL splitter. The injector takes a normal 10BASE-T1L pair and adds the signature detection, power switch and current monitoring. The splitter receives the powered 10BASE-T1L and presents the signature voltage or switches the output power, depending on the actual input voltage.

For time being, we have not bothered about the serial communication classification protocol (SCCP) feature of PoDL. So identification depends only on the 4.3V signature voltage. And to be honest, it seems a bit of overkill considering all PoDL devices must be able to withstand the highest possible voltage (60V) anyway. It seems the IEEE 802.3cg committee does not have particularly high confidence in this identification protocol either...

The long distance capability of 10BASE-T1L comes with an extra challenge. That's because IEC 61000-4-5 classifies signal cables with a length ≥300m as telecom cables. These require 10/700µs surge testing in addition to the usual 1.2/50µs surge tests. In this test, the energy is released much more slowly, so this must be considered in the design. (Btw our EMC lab was started specifically so we could design and test for this surge.) We also make sure that the interfaces can deal with FCC / TIA-968-B telecom line surges.

Future plans:

In due time, we want to design some real SPE hardware, for example media converter + power injector, SPE hub or switch, bus-powered SPE sensor devices, etc.