From the Ground Up: Industrial Sensors and Single Pair Ethernet

Single-Pair Ethernet (SPE) is on its way to becoming the foundation of a fully automated smart factory.

Over a series of blogs, we’ve shared a lot about Single Pair Ethernet and SPE networks:

• Identified the core features of SPE technology
• Explained how adoption will vary across market segments
• Described the advantages it offers to different stakeholders in industrial automation

Now it’s time to explore how SPE can complement Ethernet networks at the sensor/actuator level, along with the obstacles that need to be overcome so manufacturers of active components (switches and routers) and end devices (sensors and actuators) can expand their SPE portfolios.

Calculating the Total Cost of Ownership for SPE

There are four phases to consider throughout the lifecycle of a network:

1. Acquisition
2. Commissioning (a distinction could be made here between hardware installation and software setup)
3. Operation
4. Maintenance
   
   

When comparing implementation costs for an SPE network vs. a conventional network based on a fieldbus or serial interfaces, acquisition costs are often the only expenses considered. Costs and benefits for subsequent lifecycle phases, such as commissioning, operation, and maintenance, are rarely included. When calculating device costs, prices for Ethernet transceivers and additional circuitry (magnetics) are often compared to the prices for simple RS-485 or RS-232 interfaces.

To determine the true total costs of owning an SPE network, however, a more detailed consideration of all four installation phases is critical.

Probability of SPE Adoption at the Sensor/Actuator Level

Conducting cost-benefit analyses across the entire network lifecycle allows sensors to be classified in terms of SPE adoption probability.

Analog Sensors

Analog sensors that have currents (e.g., 4-20 mA) or voltages (e.g., 0-10 V) proportional to measured variables (pressures or temperatures), converted in analog I/O modules and packaged in Ethernet frames, are rarely, if ever, equipped with SPE due to the low benefit-to-cost ratio this scenario offers.

Simple Digital Sensors

Simple digital sensors, where the conversion of the measured variable into a digital signal takes place in the sensor and is coupled to an Ethernet network via digital I/O modules, will integrate SPE to a small extent. This allows the sensors to take advantage of the consistent implementation of Ethernet. Moving the Ethernet transceiver from the I/O module into the sensors is particularly useful for applications with a small number of sensors or that have large distances between them.

Intelligent Digital Sensors

Intelligent digital sensors connected via fieldbuses or serial interfaces benefit from the higher bandwidth that SPE offers. They also benefit from the security features available for Ethernet. In this respect, a significant level of SPE adoption can be expected for intelligent digital sensors.

Intelligent Sensors

Intelligent sensors with high bandwidth requirements are already connected via Ethernet systems today. Consider cameras that require a native bandwidth of between 1.6 Mb/s and 4.3 Mb/s (depending on video quality), with a standard codec (H.264) at a resolution of 2 MP and a frame rate of 20 f/s. This bandwidth requirement increases when additional vital data is transmitted, enabling value-added services, such as maintenance.

This bandwidth requirement increases with the additional transmission of vital data, which enables value-added services. SPE adoption is most likely in this sensor category not only because of this requirement but also (and especially) because of the long ranges of 10BASE-T1L and 100BASE-T1L (the goal of IEEE 802.3dg is 100 Mb/s over 500 m) and the remote power supply via PoDL/SPoE.

Barriers to SPE Adoption from the Perspective of Device Manufacturers

To meet the diverse requirements of different target markets for SPE, device manufacturers have defined standards with different bandwidths, cable lengths, and topologies for the physical transmission layer.

There are a variety of industry protocols used for the higher layers, along with diverse connectors that complement different industrial environments and their requirements for things like:

• Impermeability to dust and moisture
• Resistance to chemical substances
• Robustness against mechanical stress and electromagnetic interference (EMI)
  
  

Because of the number of possible combinations of requirements, manufacturers of active infrastructure components and end devices will likely counter this complexity by offering solutions that apply only to certain applications that offer an economically viable cost/benefit ratio.

While this could slow the overall adoption of SPE networks and technology, early standardization and establishing common industry standards are possible ways to overcome this.

Manufacturers must also contend with the fact that electronic components have already been developed for automotive applications. Their implementation requires additional effort. A prominent example of this are suitable switch and multi-port transceiver (PHY) chips for SPE being offered by semiconductor manufacturers.

For example, it’s currently necessary to connect switch and single-port PHY chips using RGMII, RMII, or MII, which are medium-independent interfaces. With their high number of signals, these interfaces, as well as the use of single-port PHY chips, lead to greater complexity in signal routing on the PCB and increased space requirements.

While modern interfaces, such as SGMII, require four signals per port, 16 signals are required for an MII interface and 8 signals for an RMII interface. In addition, MDIO and MDC management interfaces are required for each single-port PHY chip.

SPE multi-port PHY chips with suitable media-independent interfaces, such as SGMII or QSGMII, are currently not available. Specifically for 10BASE-T1L, these interfaces, which are designed for gigabit operation, are not a focus area for semiconductor manufacturers.

But the development of switch ASICSs is being driven by rapidly increasing bandwidth requirements and, as a result, corresponding MAC-PHY interfaces with multi-gigabit bandwidths are being optimized.

Interface incompatibility must be overcome by additional chips for protocol conversion, making the development of suitable field switches more expensive.

SPE Is Set to Prove Itself

SPE brings innovation potential to the various professionals involved in the four phases of a network installation’s lifecycle. This is also strongly reflected in the various standardization activities in the IEEE and user organizations.

Despite IEEE’s long standardization history, SPE is still a relatively young technology that will prove itself primarily in more complex and intelligent sensors before reaching more simplified sensors. With the increasing availability of components and hardware, SPE will continue to assert itself in the direction of the cheaper sensor segments and ensure harmonization of network components.

Related Links:

• Making Single-Pair Ethernet in Factory Automation a Reality
• Uniting SPE with Norms and Standards: Where Open Systems Start
• Overcoming the Hurdles of Single-Pair Ethernet Proliferation