SPE Mating Faces / 100BASE-T1 bis 1000Base-T1 hybrid pinout

The Actual Breakthrough: Miniaturized Networking

Single Pair Ethernet (SPE) solves a fundamental problem of modern product development: how to bring IP-based communication into devices that were previously too small for it. The decisive advancement lies not in thinner cables or weight savings – these aspects are side effects. The core is the drastically miniaturized electronics enabled by SPE transceivers.

While conventional Ethernet PHYs for 100BASE-TX require significant chip area, complex multilayer layouts, and elaborate magnetics, the 100BASE-T1 electronics shrink to just a few square millimeters. This miniaturization opens entirely new application fields: sensors that previously depended on proprietary bus systems or analog interfaces now communicate directly via TCP/IP – without gateways, protocol converters, or additional infrastructure.

Direct IP Integration: The True Value

The paradigm shift is that end devices become direct parts of the IP network. A temperature sensor becomes a web server with its own IP address. An actuator receives MQTT messages without an intermediary system. A camera streams directly via RTSP without a fieldbus gateway having to convert the data.

This direct integration eliminates system complexity: no more proprietary protocol stacks, no master-slave topologies with central fieldbus controllers, no elaborate configuration tools for bus couplers. Standard IT infrastructure – routers, switches, firewalls, monitoring software – functions without modification. A network administrator can manage SPE devices with the same tools as any other IP endpoint.

Technically, this revolution is made possible by compact SPE electronics. A 100BASE-T1 PHY needs no elaborate hybrid transformers for four wire pairs, no complex echo compensation for bidirectional four-wire transmission. The reduced signal processing shrinks chip size, power consumption, and board space to a level that makes Ethernet practicable for miniaturized devices for the first time.

Hybrid SPE: Why the Separation of Data and Power Is Decisive

In implementing Single Pair Ethernet, a central architectural question arises: Power over Data Line (PoDL) or hybrid solution? The answer has fundamental effects on device size – and thus on the feasibility of the actual SPE advantage.

Hybrid Single Pair Ethernet physically separates the functions: a twisted pair transmits the data, and separate conductors in the same cable carry the power supply. This architecture allows the SPE electronics to remain minimal. The 100BASE-T1 PHY focuses exclusively on signal processing, while conventional power supply circuitry handles energy distribution.

In contrast, PoDL (Power over Data Line) requires significant additional electronics in the device: inductors for coupling supply voltage onto the twisted pair line, filters to suppress interference between data and power signals, and elaborate EMC protection circuits. These components require PCB space, increase component cost, and significantly enlarge the device.

The advantage of SPE – enabling Ethernet where space is tight – is partially nullified by PoDL circuitry. A sensor whose electronics might shrink to 10 × 10 mm thanks to 100BASE-T1 may grow to twice or three times that size due to PoDL circuitry. Hybrid SPE preserves miniaturization instead: the power supply remains conventional and compact and the data electronics minimal.

Daisy Chaining: Hybrid Architecture as a Topology Enabler

Another aspect that makes hybrid SPE a practically superior solution is true daisy chaining. Devices can be connected in series, with each passing both data and power to subsequent participants over a single cable.

This topology dramatically reduces installation and cabling effort: instead of star wiring with individual runs to every sensor, a continuous connection suffices. In a production line with 30 sensors, this means 29 fewer cable runs to the controller. The physical separation of data and power conductors in the hybrid cable prevents mutual influence and allows more flexible power delivery than PoDL systems.

In practical terms, this works as follows: A device receives Ethernet packets via the twisted-pair conductors and is supplied with power via the supply conductors from the shared power circuit. It processes its data and forwards the Ethernet communication to the next device, while all participants are supplied in parallel from the same continuous power circuit originating at the feed-in point. All of this is implemented via a single cable connection. This freedom of topology is particularly valuable in linear installations such as conveyor systems, rows of luminaires, or vehicle wiring harnesses.

Sensor and Adapter: The Complete Package

The miniaturization of SPE electronics also enables retrofitting network functionality. In a hybrid architecture, an M8 or M12 adapter can house a 100BASE-T1 PHY.

The result: intelligent sensors that act as independent network nodes. A sensor with an analog output is turned into an IP device through an SPE-capable adapter – without modifying the sensor itself. The adapter handles ADC conversion, data preparation, and Ethernet transmission.

This integration would be significantly more difficult with PoDL: the additional circuitry would enlarge and increase the cost of the adapter. Hybrid SPE keeps complexity manageable – the adapter concentrates on data communication while power distribution remains conventional.

100BASE-T1: The Economical Reference

Among SPE variants, 100BASE-T1 dominates the market thanks to the automotive sector, which uses this variant hundreds of millions of times. The technology transmits 100 Mbit/s over distances up to 40 m (according to the standard, in reality far beyond 100 m) and thus completely covers industrial standard applications.

Crucially: 100BASE-T1 PHYs are now available, mature, and cost-effective. Manufacturers such as Texas Instruments, Broadcom, or NXP supply transceivers in high volumes. Additionally, less chip area is required than for 10BASE-T1L. As a result, it will remain the most cost-effective T1 standard prospectively.

Compared to 1000BASE-T1 (Gigabit SPE), 100BASE-T1 omits complex multi-level modulation and elaborate echo compensation. The simplified signal processing reduces not only chip cost, but also the required PCB area for external components. For typical sensor, actuator, and control applications, the bandwidth of 100 Mbit/s is fully sufficient.

Practical Pin Assignment: Market Standards for Hybrid SPE

The question of SPE connector specifications or SPE pinouts leads to an important aspect: IEEE standards define electrical parameters, but not mechanical interfaces. This gap is filled by the market through de facto standards that have become established in practice.

The technology is increasingly used in M8 and M12 industrial connectors as well as miniaturized automotive connectors. The clear separation of functions simplifies system design: network components remain slim without PoDL electronics and power supply units operate independently of the PHY layer.

Connector manufacturers, device designers, and system integrators already use this pin assignment productively – not because a norm prescribes it, but because the market recognizes it as economically optimal. The availability of compatible components from different manufacturers confirms the de facto standardization.

Shielding: Practical Reality Instead of Theoretical Requirements

A frequently discussed aspect in SPE implementations is cable shielding — but practice shows a clear reality: the vast majority of sensors and actuators do not have a metal housing with a shield termination. Plastic housings dominate the market for reasons of cost, weight, and manufacturability.

Connecting a shielded SPE cable to a device without a shield termination renders the shielding ineffective — it can only fulfill its EMC protection function if both cable ends are connected to ground.

Therefore, it is economically and technically more sensible to plan SPE systems from the outset with unshielded cables. The twisted-pair structure already provides significant noise immunity through differential signalling. For industrial environments with moderate EMC requirements, this is fully sufficient.

Shielded variants remain reserved for special cases where metal housings with proper shield termination do exist — for example, in control cabinet mounting or high-performance actuators. For the typical SPE application with plastic sensors, unshielded cable is the pragmatic, cost-effective, and technically appropriate choice.

Twisted Pair: The Technical Basis

Data transmission in SPE takes place over a twisted pair cable — a pair of conductors twisted together, whose geometry compensates for electromagnetic interference. Differential signalling uses both conductors for the same signal in opposite polarity. External disturbances that affect both conductors equally are eliminated at the receiver by difference calculation.

This well-established technique makes SPE robust against EMC influences — a critical requirement in industrial environments with motors, welding equipment, or high-frequency systems. The twisted pair structure is not an SPE-specific innovation, but rather a best practice adopted from decades of Ethernet development. The difference in SPE is that only one pair is needed instead of four.

For hybrid implementations, the cable contains, in addition to the twisted pair, separate conductors for power supply. Typical configurations use 4-wire cables, where one twisted pair is allocated for data and two conductors for power.

Standards as a Framework: What IEEE Defines and What It Does Not

A common misconception is that IEEE standards such as 802.3cg or 802.3bw comprehensively dictate what SPE systems must look like. In fact, these standards are limited to electrical parameters (signal levels, impedance, bit error rate), transmission methods (modulation, coding), and protocol layers (Physical Layer, Auto-Negotiation).

Mechanical interfaces, connector pin assignments, cable construction, and power delivery architectures are explicitly outside the scope of the standards. This openness is intentional: it enables application-specific optimization. An SPE system for automotive sensors has different requirements than one for building automation — different connectors, shielding concepts, and power classes are necessary.

The standard only guarantees that 100BASE-T1 transceivers from different manufacturers can communicate with each other. The remaining system details are defined by the market through reference implementations, best practices, and industry consortia such as PROFINET or the OPEN Alliance.