Single Pair Ethernet is Driving IP to the Edge

Single Pair Ethernet

There is a long running show on the History Channel called Mysteries at the Museum. The premise is that in this obscure museum that no one has ever heard of, a mysterious artifact tells an important story of some historical event.  A few years from now, Mysteries at the Museum might find this odd little device (Figure 1) from the earliest days of the Ethernet: one of the very first ways to add nodes to an Ethernet network.

Figure 1

Ethernet in the early 1980s used a passive 9.5mm coaxial cable (Thicknet) in a trunkline/dropline topology. Vampire network taps were used to connect nodes to the network.

We’ve Come a Long Way

Now, instead of that huge, 9.5mm coax, in the near future we’ll have Single Pair Ethernet (SPE), a twisted pair cable carrying both power and data. SPE is smaller, lighter, easier to deploy and supports daisy-chained configurations.

SPE Driven by Three Big Trends

  1. Requirements for vastly more factory floor data. Industry 4.0 is useless unless the Internet of Things (IoT) can deliver the vast amounts of data required to power analytics, machine learning, preventive maintenance and AI applications that are needed.
  2. Desire for one technology. Manufacturers are tired of all the various technologies on the factory floor and the associated spare parts, training and differing procedures that are required to support them. Using IP (Internet Protocol) from the Cloud to the control system to the field sensor is more efficient and less costly than using a varied set of technologies.
  3. Autonomous vehicle requirements. The backup cameras, lane warning systems, adaptive cruise control systems and all the other systems in autonomous vehicles are profligate users of data. More bandwidth, more speed and more sensors are required to get that data. Automotive designers want low weight, inexpensive media and standard connectivity – all fortes of SPE.

It’s been true in the past and continues to be true, that technologies developed for more mainstream and larger-volume applications eventually find their way to the factory floor. Volumes deliveries of SPE in other industries will reduce prices for applications on the factory floor.

Single Pair Ethernet is simply another physical media for moving bits from one node to another in an Ethernet network. Instead of the four pairs of wires we use for 100 Mbit Ethernet or the eight pairs of wires we use for one Gbit Ethernet, SPE uses two pairs. And just like those wired and wireless Ethernet networks, there is a specific Ethernet MAC and PHY for SPE.

The SPE Ethernet MAC is similar to other Ethernet MACs. It manages access to the Ethernet network and the other housekeeping needed to synchronize with other nodes on the network. The Ethernet PHY, like other Ethernet PHYs, converts between the analog signal on the wire and the digital bits used by the MAC.

SPE Chart

Like most of these technologies, it’s easy to get lost in the nomenclature of all the different standards. Here’s a cheat sheet:

IEEE 802.3bp – The 1000Base-T1 (Gigabit Ethernet over single pair balanced copper cabling), in-vehicle standard. The automotive manufacturers, especially the new electric vehicle developers, are enthused about having gigabit SPE Ethernet to quickly move all the sensor data needed for autonomous operation.

IEEE 802.3bw – The 100Base-T1 (100Mbit over single pair balanced copper cabling) standard. This is the one of interest to us in industrial automation. Other than insanely high-speed motion systems, few applications really need more than 100mbit operation.

IEEE 802.3bu – The 100Base-T1 (100Mbit) standard with simultaneous transmission of power and data over single twisted pair. No additional power source is required so this is of interest for IoT sensor applications.

SPE Architecture Issues

SPE presents a number of new architectural challenges. Adding a multitude of SPE sensors not affiliated with a factory floor control system raises a number of questions:

How to address all those SPE devices? Should IoT devices have their own subnet? Do they get included in the often-limited control system subnet?

How to identify all those IP devices? Which device is which? Not difficult with a few devices but a much bigger problem with lots of identical sensors.

How to connect all those devices? Lots of devices contending with control system messages for access is problematic.

SPE Software Issues

Software issues like configuration, messaging and data modeling cloud the future of SPE.

  • Configuration – IO-Link is the technology with the most robust capability to deliver sensor configuration. Can IO-Link configuration be adapted for SPE?
  • Data Modeling – OPC UA has the best technology for data modeling. Powerful data models simplify device integration.
  • Messaging – MQTT is perfect for low bandwidth devices sporadically sending messages. Is MQTT the right software for SPE devices?

The ODVA and PI would like to incorporate EtherNet/IP and PROFINET IO into SPE devices which makes sense for SPE devices added to control systems, but little sense for IoT devices delivering data to Industry 4.0 applications.

It’s not certain how this will play out. One standard? All of them? Some combination? It’s unknown right now.

Standards to Watch

10BaseT1 is the common name for the IEEE 802.3cg standard. There are two parts to that standard that are important to us in manufacturing: 10BaseT1L and 10BaseT1S. 10BaseT1L is the long range version; 1K meters with power. 10BaseT1S is the short range, multidrop version. Multiple drops, no power on wire, 25m max distance. T1S is meant as the replacement for CAN (DeviceNet), PROFIBUS, Hart and possibly IO-Link.

Future Outlook

TI and Analog Devices are releasing parts. Early product announcements from major vendors should come by mid-2021.

SPE appears to be a new, lower cost, media platform to meet high availability, high bandwidth, and fast transport required to support all the new, data-intensive sensors and appliances needed for autonomous vehicles and Industry 4.0. But even though the hardware is nearly ready, it may be a while before we sort out the deployment issues, especially for factory floor applications.