In 1973, Robert Metcalfe presented his ideas for a networking protocol called Ethernet to his superiors for the first time. More than ten years later, in 1985, the IEEE adopted the first Ethernet standard and networking in offices and data centers began. While originally only a LAN standard for transmission rates between 1 and 20 Mbit/s was planned, the 802 working group of the IEEE continued to develop this standard over the years, particularly with a view to higher data rates. The latest status is a transmission rate of 400 Gbit/s, and the IEEE has set up a study group to investigate the possibilities for even higher data rates.

The standard should not only be used in information technology, but also in industrial automation. End-to-end networking from the sensor to the cloud is the declared goal of Industry 4.0 and the Industrial Internet of Things (IIoT). However, there were hurdles to overcome in order to achieve this: The standard Ethernet was not hard real-time capable, the RJ45 connector used was not suitable for the harsh environmental conditions in the field, and for integrating simple sensors with low data rates, the standard Ethernet with its four pairs of conductors was oversized and therefore far too expensive. However, these 'birth defects' have been eliminated and there are now various real-time-capable Ethernet variants - for example Ethernet/IP, Ethercat, Profinet and TSN.

In order to address the harsh environmental conditions in the field - shock, vibration, humidity, extreme temperatures and temperature cycles - various suppliers have worked on the connectors. On the one hand, the RJ45 has been retrofitted to withstand the environmental influences, while on the other, industrial-grade connector standards such as the M12 have been modified to make them Ethernet-compatible.

Connecting simple sensors

However, the connection of simple sensors was not addressed until recently. This is because standard Ethernet is too expensive for a simple presence detector in a washroom in an office building, for example, or a temperature sensor that only needs to transmit a small amount of data over long distances in an extensive process plant. In such cases, a classic fieldbus or a 4-20 mA current loop was previously used. This required an additional gateway in the control cabinet to connect such simple sensors to the cloud end-to-end. This is where Single Pair Ethernet (SPE) comes into play.

Special specifications for SPE

While standard Ethernet always has a fixed maximum transmission length of 100 m, the IEEE has adopted several specifications for SPE in order to address the special requirements in industrial and building automation. IEEE 802.3cg is available for the aforementioned temperature sensor in an extensive process plant, which permits full-duplex data communication at up to 10 Mbit/s with cable lengths of up to 1000 m. With IEEE 802.3bw, up to 100 Mbit/s can already be transmitted with cables of 50 m in length. This standard is aimed at high-speed sensors in industrial and some automotive applications. An even higher data rate of up to 1000 Mbit/s with a cable length of up to 15 m is possible with IEEE 802.3bp.

At this cable length, SPE can be used for some industrial sensor applications, but is particularly suitable for high-definition video as well as small aircraft and commercial automotive applications where high-speed sensor data needs to be transmitted over a relatively short distance.

The IEEE has also considered Power over Ethernet when designing SPE. However, this feature is not called PoE, but PoDL (Power over DataLine). Even if a PoDL injector can supply up to 50 W in accordance with IEEE 802.3bu, significantly less may be available at the supplied sensor or end point due to losses in the cable. In extreme cases, with a cable length of 1000 m using 14AWG wire at 60 V, just 13.53 W is available at the end point.

If more power is required, SPE offers the option of hybrid transmission of power and data. An M8 interface, as proposed by Harting, can transmit up to 60 V/8 A with one SPE contact pair and two additional power contacts. According to the company, a new M12 connector from TE Connectivity can provide up to 11 kW via a single cable.

Standards for the mating faces

While there are no competing standards for the transmission protocol, there is intense competition for the mating face of connectors for Single Pair Ethernet - a total of seven different mating faces are currently vying for the favor of users:

SPE plug according to IEC 63171-2 in protection class IP20 (top) and according to IEC 63171-5 in IP65/67 (bottom).

© Weidmüller

• IEC 63171-1: SPE connector from CommScope based on an LC interlock with IP20 degree of protection for control cabinets and building cabling
• IEC 63171-2: SPE connector from Reichle & De-Massari for use in office environments and IP20 applications
• IEC 63171-3: SPE plug from Siemon based on a pair of Tera plugs with IP20 degree of protection. This has since been withdrawn
• IEC 63171-4: SPE connector from BKS for IP20 applications
• IEC 63171-5: SPE connector from Phoenix Contact and Weidmüller, based on the draft standard IEC 63171-2, but protection class IP65/67
• IEC 63171-6: SPE connectors from Harting, Hirose and TE Connectivity for protection classes IP20 and IP65/67 as well as hybrid M8 interface with one SPE contact pair and two additional power contacts for up to 60 V/8 A
• IEC 63171-7: hybrid M12 interface from TE Connectivity with protection class IP65/67, which offers up to four power contacts plus the functional earth in addition to the SPE wire pair for data.

The IEC 63171-5 and IEC 63171-6 mating faces are of particular interest for industrial and plant automation. Two consortia have been formed to promote these two different mating faces. While the SPE Industrial Partner Network is promoting the mating face according to IEC 63171-6, the Single Pair Ethernet System Alliance is trying to do the same with the mating faces according to IEC 63171-2 and IEC 63171-5.

The TIA (Telecommunications Industry Association) has set up the Single Pair Ethernet Consortium (SPEC) for telecommunications and the IEC 63171-1 sub-standard. The TIA is at the intersection of networking and communications technologies and aims to help the SPEC accelerate the adoption and acceptance of next generation connectivity in networking and building automation. Recently, the TIA SPEC announced a partnership with the Single Pair Ethernet System Alliance.

The mating face in competition

When it comes to the mating face for industrial and plant automation, there is intense competition between the SPE Industrial Partner Network and the Single Pair Ethernet System Alliance.

The SPE Industrial Partner Network likes to point out that the mating face according to IEC 63171-6 is based on a complete and already published standards document with all the necessary specifications and test sequences, which does not yet apply to the competing solutions according to IEC 63171-2/-5. In addition, the IEC 63171-6 solution is also completely symmetrical at its transition to the PCB. As the cable lengths in the socket are exactly the same, its RF properties for differential signaling with SPE are likely to be slightly better than the competing solutions according to IEC 63171-2/-5. However, signal differences of up to 600 MHz are likely to be negligible and the PHY is also unlikely to be exactly behind the connector. In case of doubt, runtime differences in solutions according to IEC 63171-2/-5 can be compensated for by a careful PCB layout.