What Is 10BASE-T1S?

10BASE-T1S is one of the single pair Ethernet (SPE) physical layers added to IEEE 802.3 by the 802.3cg amendment in 2019. The name spells out what it does: 10 Mb/s, baseband signalling, one twisted pair (T1) and short reach (S). Its sibling, 10BASE-T1L, covers long reach.

What makes it different from almost every other Ethernet physical layer is the multidrop option. Modern Ethernet is point to point, with a switch port for every device. 10BASE-T1S can also run as a shared bus topology, with several nodes tapped onto one pair, much like a CAN or RS-485 network. That lets a designer take Ethernet and IP addressing right down to small sensors and actuators without a switch port for each one.

It was driven by the automotive industry, which wants one network technology from the central computer to the edges of the vehicle, and it is now being taken up for industrial machines, control cabinets and building systems. The OPEN Alliance publishes interoperability and test specifications for it.

On a multidrop segment every node hears every transmission, so the nodes need a rule for who talks when. Classic shared Ethernet detected collisions after they happened. 10BASE-T1S adds Physical Layer Collision Avoidance (PLCA), defined in Clause 148 of IEEE 802.3, which stops them happening.

PLCA works like a turn passed round in a fixed order. One node, the coordinator with ID 0, sends a BEACON to start each cycle. Every node has its own ID and gets a transmit opportunity in turn; if it has nothing to send, the opportunity passes on after a short silence. Because each node is guaranteed its turn, the worst-case wait is predictable and the bus stays efficient under heavy load, which plain collision detection could not promise.

The working figures are modest. The standard specifies a mixing segment of at least 25 m with at least eight nodes, and a point-to-point link segment of up to about 15 m. Many PHYs support more than eight nodes on a bus. A multidrop segment runs half duplex, with the 10 Mb/s shared between the nodes, and the signal uses 4B/5B coding with differential Manchester encoding.

CAN keeps advantages of its own, including priority arbitration and decades of tools, so in vehicles 10BASE-T1S is usually added alongside CAN rather than replacing it at once. The gain is that a sensor on the bus becomes an ordinary Ethernet endpoint, reachable from the vehicle computer without protocol translation.

Most early designs fall into three groups.

The 2019 standard does not define power on a multidrop 10BASE-T1S bus. Point-to-point single pair links can carry power using PoDL, the single pair counterpart of Power over Ethernet, and the IEEE 802.3da project extends 10BASE-T1S multidrop with longer segments and power distribution, so check its current status before relying on it. For now, many multidrop designs run power on separate conductors in the same cable.

Connectors are not set by the physical layer standard. Industrial single pair Ethernet uses the IEC 63171 family, including IP20 and sealed M8 and M12 versions, while vehicles use automotive connector systems. Our white paper on single pair Ethernet in industrial and edge AI computing compares the SPE variants, connectors and power options side by side, and our explainer on twisted pair cable covers why one pair can carry these signals cleanly.

The faster members of the same family already appear in our range. Several of our in-vehicle computers list 1000BASE-T1 automotive Ethernet ports, and our Ethernet and PoE frame grabbers include a 100/1000BASE-T1 PCIe card with PTP.

Where a vehicle or machine uses single pair Ethernet, we build the edge computer with the right T1 interface fitted as standard and install the drivers before it ships.