Way back when, Ethernet used two wires—sort of. It started with coax but has since moved to multiple twisted pairs and even fiber optics. The surprising thing is the level of compatibility that’s been maintained from the original 10Base2 used by Category 5 coax (Fig. 1).
Ethernet moved to four wires and Cat 5 twisted pair, which has been the mainstay for decades. The ubiquitous RJ-45 jack has remained on the back of servers, laptops, and PCs for a long time (Fig. 2). Cat 5e and Cat 6 are more common these days as transfer rates have climbed from 10 Mb/s to 100 Mb/s to hundreds of Gb/s.
The question, then, is why go back to two wires? The answer is rooted in cost, weight, and simplicity.
Ethernet was driven by performance. Other communication technologies like RS-232 and CAN used only a couple of wires, and their lower speeds were sufficient for many embedded applications. They were simple, low cost, and lightweight.
The demand for high-speed communication in the embedded space has brought the need for single twisted-pair Ethernet to the fore. There are tradeoffs like non-standard connectors or wiring, but these are usually advantages in an embedded application. Connectors like RJ-45 tend to stand out on a printed circuit board that also has a lot of small connectors.
New standards have emerged to address SPE. Some target application areas like automotive that have demanding embedded requirements. The BroadR-Reach, 100Base-T1, and 1000Base-T1 zero in on this space. 1000Base-T1, also known as IEEE Std 802.3bp-2016, is one of many IEEE standards related to the many flavors of Ethernet.
Some of the standards relevant to this discussion include:
- 802.3bp 1000BASE-T1: 1 Gb/s up to 15 m and up to 40 m on one pair of balanced cabling
- 802.3bw 100BASE-T1: 100 Mb/s up to 15 m on one pair of balanced cabling
- 802.3cg 10BASE-T1: 10-Mb/s Ethernet up to 1 km on one pair of balanced cabling
- 802.3bu for power over data lines on one pair of balanced cabling. Yes, this is like Power over Ethernet (PoE)
Products supporting these standards are available now and the number is growing. This includes microprocessors that would typically have Ethernet ports supporting the usual 100Base-T interfaces. For example, NXP Semiconductors’ RDDRONE-FMUK66 flight management unit (Fig. 3) includes single-pair Ethernet support. It can be used to connect sensors and cameras using the standard Ethernet protocol.
