Ethernet in the early 1980s used a passive 9.5mm co-axial cable (Thicknet) in a trunkline/dropline topology. Vampire network taps were used to connect nodes to the network.
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.
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The IEEE specifies a number of standards for Ethernet communications. The 802.3 set of standards specifies the operation of what we know as Ethernet through twisted pair wires. The 802.11 set of standards specifies the operation of what we know as Ethernet through the air. These standards specify the kinds of wire, connectors, electrical signaling, timing, bus control and much more that is required to create a functional Ethernet network.
Single Pair Ethernet is simply another set of 802.3 physical media standard for moving signals over twisted pair cables. Most of us are familiar with the most common 802.3 standard, 10BASE-T where:
The most common current standards for specifying how Ethernet messages move across twisted pair cables are shown in figure below. In the first two, separate pairs of wires are used for Transmit and Receive. In the 1GBASE-T standard, each pair of wires supports both transmit and receive.
Single Pair Ethernet comprises a set of 802.3 standards for moving Ethernet messages across one single pair of twisted cable. Instead of the two four wires used for 100 Mbit Ethernet or the eight wires used for Gbit Ethernet, SPE uses only two wires. And just like those wired and wireless Ethernet networks, there is an Ethernet MAC and PHY for each SPE standard.
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.
Like most of these technologies, it’s easy to get lost in the nomenclature of all the different standards. You can find a cheat sheet of the most important standards in the table below.
|802.3bw||100Base-T1||100 Mbit/s||15m||2015||In Vehicle, Encoders|
|802.3cg||10Base-T1S||10 Mbit/s||25m||2019||Multidrop CAN replacement|
|802.3cg||10Base-T1L||10 Mbit/s||1 Km||2019||Process Transmitters|
|802.3bu||TBD||TBD||TBD||TBD||Power over Data Line|
The 802.3bw standard is the most important one for in-vehicle applications. Automated vehicles require vastly more sensor data, extremely fast communication and short-range operation. The 802.3bw standard is the one to satisfy that application.
802.3bp is the standard for fast Robotic applications. SPE is particularly important to robotics as the bending radius of SPE cabling is vastly improved over today’s options. 802.3bu is the standard that specifies SPE with Power Over Data Lines (PODL) sometimes called “poodle.”
The standard most important to us in automation is the 802.3cg standard. Two 802.3cg standards are important: the short-range standard, 10BASET1S, and the long-range standard, 10BASET1L. The short-range standard provides a method of using IP in discrete applications currently implementing Controller Area networking (CAN), DeviceNet, PROFIBUS DP. The long-range standard provides an IP replacement for process applications using HART and PROFIBUS PA.
The applications for these standards are presented in the below figure.
Single Pair Ethernet is being driven by three big trends.
New 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. Manufacturers are facing a world where they must make machines smarter, processes leaner, factories more efficient and productivity higher. This is nearly impossible to do with much of today’s existing factory floor networking infrastructure.
One media: 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.
Autonomous vehicles: 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.
That last requirement may shed light on the future of industrial automation. What’s been true in the past is that technologies developed for other more mainstream and larger volume applications eventually find their way onto the factory floor. The volumes of SPE delivered to the vehicle market should reduce prices on the factory floor.
SPE is designed to deliver a number of important benefits to the user:
SPE benefits – thinner, lower weight, less costly cabling – are extremely important in transportation applications like rail, automobiles, and trucks. Rail cars, for example, use hundreds of meters of wire and SPE has the potential of vastly decreasing wiring costs.
Building automation is another area where less costly cabling can really add up. Even in an era when most users use wireless Ethernet, the access points must be wired. Using less costly SPE wiring can save hundreds of thousands of dollars in large buildings.
SPE is also touted for process control but the benefits in that industry are less clear. HART® is currently the standard with somewhere near 30 million nodes implemented. HART has many benefits, but speed is not one of them. Operating at 1200 baud, changing even a small percentage of the thousands of HART transmitters in a process plants can take weeks.
SPE in discrete automation is another industry where the benefits are less clear. Some version of SPE for hazardous areas will be popular. It is not expected that SPE can add value to much of what’s already used on the factory floor.
SPE is implemented just like other 802.3 Ethernet applications with a custom MAC and PHY. Several manufacturers, notably Analog Devices and TI, are hard at work on these components and production releases are planned for 2021.
The Texas Instruments DP83TD510E SPE PHY transceiver IC, for example, provides not only basic 802.3cg functionality but extends the 10BASE-T1L specification to provide a maximum distance of 1700 meters. Equivalent devices can be expected from Analog Devices and other manufacturers.
What’s often missed is that SPE is simply a medium for transmission of the IP protocol just like standard wired and wireless Ethernet. Any software application that uses TCP/IP can communicate over SPE. No changes. Nothing different. There are no software issues particular to SPE.
There are many versions of SPE and many applications for it and that makes the future hard to predict. SPE will certainly have an impact on transportation and building automation. The lower cabling costs will drive the deployment of SPE in both industries. Lower power requirements will drive the deployment in hazardous applications. Its impact in process and discrete automation remains to be seen.
Will it drive the replacement of DeviceNet and PROFIBUS? Probably not. Many of those applications are being converted to standard 10BaseT already. DeviceNet and PROFIBUS are heavily used in conveyor applications where cost matters so it could have an impact there. In other factory automation applications, it’s not clear.