What are the performance requirements for multidrop SPE connectivity?

Specifications are provided for the insertion loss, return loss, and mode conversion loss in the link and mixing segments of a Single-Pair Ethernet (SPE) network. Connector specifications are also provided for coupling attenuation, impedance, return loss, and dielectric strength.

This article begins by briefly examining where multidrop fits in the range of Ethernet connectivity options. It then reviews the structure of the mixing segments and the related performance requirements and concludes by considering connector specifications, including coupling attenuation, impedance, return loss, transmission properties, and dielectric strength.

There are three basic Ethernet architectures: point-to-point (P2P), daisy chain, and multidrop (Figure 1). Nodes, or devices, are attached to the multidrop bus with fixed wiring or connectors without the need for active components. Both daisy chain and multidrop enables a single transmitter to connect with multiple nodes. A key element in multidrop implementations is the mixing segment that connects several nodes.

802.3cg mixing segment specs

The mixing segment includes edge terminations, a stub for each node, and the node medium-dependent interface (MDI) that connects the node’s physical layer (PHY) to the physical medium. It can be up to 25 meters long and support at least 8 nodes.

Multidrop mixing segments must be terminated using a 100 Ω resistor at both ends and have a maximum stub length of 10 cm (Figure 2). The mixing segment’s insertion loss, return loss, and mode conversion loss are detailed.

Insertion loss quantifies the effect of the cabling link resistance on signal transmission and is measured in dB between any two MDI points using a reference impedance of 100 Ω. For 10BASE-T1S, insertion loss is defined using equations for three different frequency ranges: 0.3 MHz to less than 10 MHz, 10 MHz to less than 33 MHz, and 33 MHz to 40 MHz.

Return loss measures signal reflections caused by impedance mismatches and is specified in dB at any MDI attachment point using a reference impedance of 50 Ω. To limit receiver noise, return loss is specified for two frequency ranges: 0.3 MHz to less than 10 MHz and 10 MHz to 40 MHz.

Mode conversion loss relates to common mode electromagnetic interference and is specified in dB between two MDI attachment points using a reference impedance of 100 Ω. It’s defined for two frequency ranges: 0.3 MHz to less than 20 MHz and 20 MHz to 200 MHz.

Multidrop connector specs

There are also detailed specifications in IEC 63171 for multidrop connectors, including coupling attenuation, impedance, return loss, transmission loss, and dielectric strength:

Coupling attenuation results from the non-uniform structure, inhomogeneity, or the connectors, which give rise to interference on the data pair based on the shielding effectiveness. In addition, the degree of symmetry in the connector influences coupling attenuation.

Impedance is defined as a nominal value of 100 Ω and varies from about 20 to 160 Ω. It’s dependent on the contact diameter, the distance between contacts, and the permittivity of the plastic connector housing.

Return loss is especially important in applications like multidrop SPE with simultaneous transmissions in both directions. It’s quantified in dB and is the logarithmic ratio between the reflected signal and the input signal. In multidrop connectors, return loss is more strictly defined compared to P2P Ethernet requiring that SPE multidrop connectors have a more homogeneous design compared with RJ45 connectors.

Transmission properties for multidrop connectors are divided into three categories:

• Category A from 0.1 to 20 MHz, for 802.3cg 10BASE-1TS and 10BASE-1TL
• Category B from 0.1 to 600 MHz, for 802.3bw and 802.3bp, 100BASE-T1 and 1000BASE-T1
• Category C from 0.1 to 1250 MHz, for 802.3ch 2.5GBASE-T1

Dielectric strength for multidrop connectors must be at least 1000 Vdc between the contacts and at least 1,500 Vac or 2,250 Vdc between the contacts and the shield.

Summary

The three basic Ethernet architectures — P2P, daisy chain, and multidrop — can be recognized by how devices are connected to the network. IEEE 802.3cg includes specifications for the insertion loss, return loss, and mode conversion loss in the link and mixing segments. IEC 63171 includes connector specifications for coupling attenuation, impedance, return loss, and dielectric strength. IEC 63171 also details several types of SPE connectors.

References

802.3 Multidrop Mixing Segment Specifications, IEEE
A guide to single pair Ethernet, Lapp
Automotive STP cable measurements, Rosenberger
Ethernet and PoE Operation Over Single Pair Twisted-pair Copper Cabling (SPE), AEM
SPE Multidrop Enhancements Mixing Segment Considerations Update, Panduit