Ethernet-APL is an extended physical layer for single-pair Ethernet (SPE) based on 10BASET1L. It communicates over a cable length of up to 1000 m at 10 Mbit/s full duplex, which is more than 300 times faster than current technologies such as HART or fieldbus. APL is the logical extension of Ethernet and provides the features required for reliable operation in the field of a process plant. Ethernet-APL is a physical layer that supports Ethernet/IP, HART-IP, OPC-UA, Profinet or any other higher-level protocol.

Ethernet-APL thus ensures a logical extension of Ethernet-based communication from company systems to the field level. This 'last' meter of Ethernet connectivity enables the company headquarters to receive data from all regions of its extensive network.

Components and topologies

Ethernet-APL is designed to support various installation topologies with optional redundancy or failover concepts. It explicitly includes only point-to-point connections, with each connection between communication partners representing a 'segment'. Ethernet APL switches therefore isolate the communication between the segments. This eliminates interference and inherently protects the communication from device errors in another segment.

Ethernet APL defines two general types of segments: The 'trunk' provides high power and signal levels for long cable lengths of up to 1000m. The 'Spur' segment provides lower power with optional intrinsic safety for lengths up to 200 m.

Figure 1: Comparison of different communication technologies for the field in process plants.

© PNO

There are two typical types of switches: The power switch feeds electrical energy and communication into one or more trunk ports. It is usually supplied from the outside. The field switch offers at least one port to which a spur line can be connected. It can be supplied with electrical power via the Ethernet APL trunk or externally. Port profiles define the levels for the electrical power supply and the communication signals in order to ensure interoperability. Ports can be classified as P (Powered, energy source), L (Load, energy consumer) and C (Cascade, for daisy chain configurations).

Ethernet-APL specifies fieldbus cable type A, IEC 61158-2 as reference cable of AWG classes 22 - 14 with a wiring cross-section of 0.324 to 2.5 mm2. This enables simple migration strategies for existing fieldbus installations, including support for intrinsic safety. Ethernet-APL prescribes polarity independence for the track, which reduces wiring errors during installation. Figure 1 summarizes the characteristics of Ethernet-APL (10BASET1L) compared to other communication technologies.

Barrier-free transition from OT to IT

Ethernet technology in the field of process engineering systems integrates operational technology (OT) with IT technology and achieves the vision of a standardized network technology.

Field devices often contain a lot of data such as self-diagnostic functionalities. With Ethernet-APL, this data can be accessed in parallel to process control in real time. This makes it possible to develop additional services, business models and unique selling points to differentiate the field devices. Ethernet APL significantly reduces the cost of accessing this useful data compared to conventional technologies by eliminating the need for protocol converters, additional system components or retrofit solutions that would otherwise be required.

There are virtually no limits to the further processing of the data within IIoT applications. Maintenance dashboards or trend monitoring of process values support targeted process optimization.

Figure 2 shows an example of the structure of a process plant with Ethernet APL, whereby the possible digital services described above are summarized under 'Other applications'.

Implementation in field devices

In principle, the implementation of Ethernet APL in field devices requires only limited effort with regard to the PHY and the protocol stack. But beware: there are system engineering challenges, as field devices in hazardous environments are limited in terms of electrical power and space. In particular, the output power of an APL stub line connecting APL field switches to the field devices is limited to 540 mW at 15 V(DC). As the field device is intended to meet the T6 temperature requirement from an Ex perspective, this means that a device may consume significantly less than 540 mW even in the event of a fault. This makes the use of a ULP microcontroller (ultra-low power) absolutely essential.

Figure 2: Example setup of a process plant with Ethernet APL. Access to the field data also enables new digital services according to the business requirements of the process plant.

© PNO

The problem is that ULP MCUs are resource-limited in order to minimize energy requirements: They generally do not have an integrated MAC and rely on lower clock frequencies and low memory requirements. A standard Ethernet PHY chip without MAC, as offered by various chip manufacturers, is therefore not sufficient here. Instead, a combined MAC/PHY chip such as the ADIN1110 from Analog Devices, which integrates both on a single piece of silicon, must be used. Details about the ADIN1110 can be found in Computer&AUTOMATION 2021, issue 6, pages 21 to 23.

The Siemens way

To achieve the power budget and meet the EX specifications, Siemens uses an STM32L4 ultra-low-power microcontroller from ST Microelectronics, paired with the ADIN1110 from Analog Devices. The RTOS embOS and the IP stack emNet from Segger are also used. This results in a generic solution platform consisting of IP stack, driver, MCU and MAC-PHY, which is suitable for many applications in process technology.

The emNet IP stack was chosen because it offers high performance even on the smallest ULP microcontrollers. The dual IPv4/IPv6 TCP/IP stack was developed from the ground up for resource-constrained embedded applications and is characterized by its low memory requirements. The complete stack on a Cortex-M based microcontroller occupies less than 20 KB ROM and only 1.5 KB RAM (without read/write buffer).

One example of the performance-optimizing functions of emNet is the UDP flood protection. This function helps to save execution time for incoming data that is to be discarded anyway. If the device is part of an overloaded network, this optimization frees up CPU time for other tasks.

The flexible PHY driver layer of the IP stack supports virtually any Ethernet PHY transceiver. A generic PHY driver that is compatible with almost all single-port PHYs that follow the IEEE 802.3u standard is included in the stack. Support for other PHY-like devices such as Ethernet switches is also possible.

Due to the independence of the individual layers according to the ISO OSI model, there are no restrictions for the implementation of any Ethernet protocol. Therefore, no adjustments to Ethernet APL are necessary for the implementation of protocols of layers 3-7. Application layer protocols, which have been used for many years, especially in discrete automation, can be easily implemented in accordance with their specifications, guidelines and certifications. As part of the expansion of the specifications on the physical layer of Ethernet-APL, the corresponding test specifications and certifications can also be adapted. This ensures that the implementations comply with the standards.

Implementation in the field devices

As a supporter from the very beginning, Siemens is working flat out on implementing Ethernet APL in devices. The first prototypes were on display at the virtual Achema 2021. The higher data rate in particular now enables convenient adjustment of the devices via the communication line. In addition to the usual parameterization tools, the Ethernet-APL device can be parameterized via an integrated web browser. This means that the device can also be put into operation without special software. This eliminates the need for prior commissioning in the laboratory. Diagnostics are also easier: the devices can be analyzed quickly and in detail via the web server, allowing users to react reliably to problems in the process. The new devices can also be operated more easily than before using standard tools. This is ensured by the standard provision of an FDI package for integration, which guarantees similar operation regardless of the control system provider.

The authors: Dr. Jürgen Ficker (right) is Product Manager in the Process Automation division at Siemens Digital Industries in Karlsruhe. Frank Riemenschneider is Senior Marketing and PR Manager at Segger Microcontroller in Monheim.

© private / Siemens

As the process industry places increased demands on the robustness of a communication line, the Ethernet APL devices are equipped with the terminal block as standard.

In addition to the physical layer, the application layer is crucial for automation. To enable comprehensive applications, Siemens supports the important Profinet and Ethernet/IP protocols.

Initial extensive tests at BASF have confirmed the expected benefits of the technology: flexible installation and simple commissioning have been confirmed. The Ethernet-APL physical layer also proved its stable Ethernet communication over two-wire fieldbus cables. The testing of data transmission via the '2nd channel' parallel to the control system in accordance with the Namur Open Architecture concept also proved successful.