Jumpstart Ethernet-APL Field-Device Designs with ADI's Reference Platform

Technical Overview: Ethernet-APL brings high-speed Ethernet to process field instruments, but designers must balance tight power budgets, safety, security and system integration. This article reviews Analog Devices’ AD-EthernetAPLDevice-SL reference platform, which integrates communications, sensing, processing, isolation, and power management to simplify the development of connected instruments for hazardous industrial environments. (Overview courtesy of ChatGPT)

Process industry field instruments such as pressure, temperature, flow, and level transmitters generally rely on mature, proven technologies. But while they are deeply embedded in process-automation infrastructure, legacy architectures constrain the amount of information that can be conveyed and often require additional I/O, gateways, or protocol conversion to exchange that information with modern Ethernet-based systems.

Field instruments today typically rely on field-level communication technologies such as legacy 4-20 mA and HART infrastructures, and digital fieldbus protocols like FOUNDATION Fieldbus and PROFIBUS PA. Those protocols are limited by relatively low data rates and specialized architectures, which can make it difficult to accommodate data-rich instruments and to connect field devices directly to Ethernet-based systems.

As customers deploy more advanced field instruments, designers must integrate richer device data into Ethernet-based plant networks while preserving the power efficiency, reliability, and safety expectations of conventional field instrumentation. Modernizing applications hinges on enhancing field-device connectivity without abandoning the two-wire installation model and the safety expectations that process plants depend on.

Ethernet-APL (Advanced Physical Layer) resolves that question by bringing 10 Mbit/s Ethernet to hazardous environments over a 10BASE-T1L single-twisted-pair connection. It creates a pathway for advanced data-driven operations by providing high-speed data communications and power delivery over distances up to 1 km (Figure 1).

Figure 1: Ethernet-APL utilizes 10BASE-T1L to integrate field devices with Ethernet-based plant networks. (Image source: Analog Devices, Inc.)

Ethernet-APL also incorporates 2-WISE (2-Wire Intrinsically Safe Ethernet), a framework for implementing intrinsically safe Ethernet over the two-wire APL connection, including voltage, current, and energy limits. This ensures that Ethernet communication can be deployed in hazardous areas while maintaining safety requirements.

Engineering challenges with Ethernet-APL

Despite the advantages, developing Ethernet-APL field devices can pose engineering challenges. In addition to adhering to intrinsic safety requirements for process applications, designers must address power budgeting, functional safety considerations, software integration, cybersecurity, protocol implementation, and system certification. For example:

• Designers cannot select a MAC/PHY in isolation for an Ethernet-APL instrument. Interdependencies of power, communications, and sensing mean that the entire system can fail its power budget or safety certification if one of those layers shifts.
• In a power Class A implementation, a device receives up to 500 mW of power. However, the Ethernet PHY and protocol-processing requirements can consume a substantial portion of this budget, leaving limited power for the precision ADC, processor, and other supporting circuitry.
• Intrinsic safety must be considered throughout the design of applications intended for hazardous environments. Available electrical and thermal energy must be limited under normal and fault conditions without degrading the signal integrity required for 10BASE-T1L communications. Safety and communication performance can be affected by capacitance, inductance, creepage and clearance, energy storage, and component failure modes, so component selection and PCB layout are crucial considerations.
• Functional safety must be designed at the system level, not simply by using individual safety-rated or safety-related components. The architecture must be able to detect relevant failures, respond appropriately, and demonstrate that the overall safety function meets its required safety integrity level. An application may require monitoring not only the safety-relevant signal or data, but also the health and integrity of the devices, electronics, and communication paths that produce and transmit it.
• Instruments that perform or contribute to safety-related functions may need to meet a specified safety integrity level (SIL), such as SIL 2; others used solely for process monitoring may not have any SIL requirement.
• Although Ethernet connectivity provides additional capabilities, it also introduces cybersecurity risks by expanding the attack surface. Unlike sensors connected through traditional media, Ethernet-APL sensors are exposed to common network-device threats, including malware injection, spoofing, and information disclosure. Security measures must be implemented in the sensor, while meeting the power budget.

Designing an Ethernet-APL field instrument isn't simply a matter of dropping a 10BASE-T1L PHY onto an existing instrument design. The APL interface must be treated from the outset as an integrated power, communications, and safety subsystem, with each element carefully balanced to achieve the requirements of the instrument.

Ethernet-APL reference platform

The task for product designers is not simply how to implement each of these elements, but how to make them work together within the constraints of an Ethernet-APL field device. Analog Devices, Inc. (ADI) offers designers a system-level reference platform that can speed the path from concept to product.

The company's AD-EthernetAPLDevice-SL (Figure 2) utilizes a modular, DIN-B-compatible 40.5 mm form factor to deliver a complete Ethernet-APL reference platform for field device development. It provides a hardware and software foundation that incorporates Ethernet-APL connectivity, intrinsic safety, functional safety readiness, security considerations, and protocol software support. It also enables prototypes that are mechanically compatible with industrial field instrument housings.

Figure 2: The AD-EthernetAPLDevice-SL hardware comprises a complete APL transmitter design in a DIN-B form factor. (Image source: Analog Devices, Inc.)

Designers can take advantage of ADI’s pre-engineered power architecture and system-level integration of interfaces between key functions to ease the path from concept to working prototype. This integration makes it easier to create applications that bridge the gap between traditional field instruments and Industrial IoT operations.

The platform integrates a range of ADI components, each addressing a specific part of the field-device architecture:

• An Ethernet-APL communications interface is provided by an ADIN1110 10BASE-T1L Ethernet MAC/PHY.
• The MAX32690 microcontroller delivers the computing resources for the instrument's application, communications functions and security primitives.
• Converting analog sensor signals into digital data for processing is accomplished with the ADFS7124-4 precision sigma-delta ADC.
• Power and processor health monitoring is provided by the MAX42500 voltage monitor and watchdog.
• Electronics temperature monitoring is achieved with the MAX6613MXK+T temperature sensor.
• Electrical isolation is handled by an ADUM1441 digital isolator.
• An LT8440RS8E#PBF power conditioner regulates the incoming power and limits transient current surges to keep the field device within the constraints of the APL interface and intrinsic-safety requirements.

Together, these components (Figure 3) illustrate the platform's system-level nature. Ethernet connectivity, processing, precision measurement, monitoring, isolation, and power management are designed as an integrated system rather than as independent functions.

Figure 3: Block diagram illustrates the Ethernet-APL reference design's integration of ADI components for a system-level developer platform. (Image source: Analog Devices, Inc.)

ADI’s platform is certified for Ex ia IIC Ga, the intrinsic-safety protection classification for equipment used in potentially explosive gas atmospheres, and meets the 2-WISE requirements for intrinsically safe Ethernet. Achieving that level of protection requires careful attention to the entire power and interface design. That certification does not extend to the final enclosure, so designers incorporating the platform must still address the requirements applicable to a final product.

Getting started with ADI's reference platform

ADI has addressed the most intricate hardware compliance challenges, allowing designers to focus on unique sensing algorithms rather than the complexities of hazardous-area power management and APL physical-layer compliance.

The platform employs a modular board architecture, with power/comms and digital IS boards forming the primary field-device assembly. The MAX32690 microcontroller includes a dual-core Arm® Cortex®-M4 processor with FPU and a RISC-V coprocessor. It embeds cryptographic hardware accelerator, a cryptographic library and supports secure boot and secure firmware updates. For programming and debugging the RISC-V portion during development, a separate digital NON-IS board is used in place of the digital IS board.

Getting started with the platform requires only a few connections. The power/comms board is first connected to the digital IS board. The fully seated board-to-board connectors form a 40.5 mm-high assembly.

A two- or four-wire PT100 RTD sensor can be connected to the platform's dedicated temperature-sense input. The number of sensor wires is independent of the two-wire Ethernet-APL connection; the RTD wiring is used only for the internal measurement, while the APL pair provides the instrument's external power and Ethernet communications.

The Ethernet-APL connection is established by connecting the APL terminals on the power/comms board to the corresponding terminals on a DEMO-ADIN1100D2Z evaluation board using a twisted-pair cable. The DEMO-ADIN1100D2Z functions as the APL power source and media converter, linking the field device's 10BASE-T1L interface and a conventional Ethernet network. Finally, applying 9 V_DC to 15 V_DC to the DEMO-ADIN1100D2Z powers the field-device platform through the APL link.

With these connections in place, the developer has a working Ethernet-APL field-device platform that can be used to evaluate the signal path from the PT100 sensor through signal conversion and processing to Ethernet communication. From that point, development can proceed toward application-specific sensing, processing, and device software.

Conclusion

The Analog Devices AD-EthernetAPLDevice-SL reference platform gives process instrument designers a practical way to extend Ethernet connectivity to field devices while retaining the two-wire power and communications model required in hazardous process environments. Designers can spend less time resolving low-level interface and compliance challenges and more time developing the sensing, diagnostics, and software capabilities that differentiate their final products.