Ethernet-APL and functional safety – can it work?

Ethernet-APL is in the process of becoming the digital backbone for the field level in processing systems. But what's the situation like with the functional safety infrastructure? In safety systems, does Ethernet-APL offer the same availability and reliability as 4 to 20 mA technology? And is Ethernet-APL not prone to cyber attacks? Time to take stock!

For decades now, established 4 to 20 mA technology has been relied on in the process industry and in safety-critical applications. This technology provides basic and reliable analogue signals for measuring and control circuits. However, it is reaching its limits ever more frequently, especially with regard to the requirements of modern Industrial Internet of Things concepts and the increasing networking of digital devices. Although the implementation of digital fieldbus technologies has improved the availability of diagnostics information, the integration of these systems is often complex. What's more, their data rate and data transparency are limited. Ethernet-APL is designed to do away with these drawbacks.

Ethernet-APL (Advanced Physical Layer) offers many advantages: It enables fully digital communication in the field level, is suitable for hazardous areas down to Zone 0 and boasts a transfer rate of 10 Mbps via a two-core cable. The technology makes it easier to connect field devices and means that the same cable can be used for the power supply and data transmission – with the "intrinsic safety i" type of protection for both. This considerably reduces the cabling required and the complexity compared to 4 to 20 mA technology. And that's not all – Ethernet-APL is fully interoperable with industrial standards like PROFINET and PROFIsafe, enabling seamless integration into existing systems.

Despite the obvious benefits of Ethernet-APL, users have some concerns, particularly with regard to reliability, cybersecurity and functional safety. The situation was similar when fieldbuses were first launched: The safety infrastructure was still set up with conventional 4 to 20 mA technology even in systems which used a fieldbus for process control. But because this makes implementation and maintenance more costly and complex, process operators have for some time now been looking for consistent digital communication where control technology devices and safety equipment use the same communication technology. And this is where Ethernet-APL comes in. But the questions asked at the outset still need to be answered.

1. Availability and reliability

Many users ask whether Ethernet-APL offers the same reliability and availability as the established 4–20 mA technology. The answer is a clear yes. Ethernet-APL was specifically developed for industrial applications, including the harsh environments found in the process industry. The technology supports topologies such as star or ring structures, which ensure high availability. In the event of a device failure, the system remains stable because the ring structure provides alternative communication paths.

Field tests, such as those conducted at BASF, have shown that Ethernet-APL operates stably and reliably even under extreme network load and interference. For this purpose, a test setup with 15 Ethernet-APL field switches and 240 APL field devices was built, and disturbances such as an IP storm were introduced—without affecting process communication. Thanks to digital data transmission, process values are also significantly more accurate and reliable than with 4–20 mA signals, which can be distorted by interference without being detected.

2. Cybersecurity

Since Ethernet-APL is an IP-based technology, security concerns regarding potential cyberattacks are justified. However, when comparing Ethernet-APL with the previously used 4–20 mA technology with HART communication or fieldbuses such as PROFIBUS DP or PA, it should not be forgotten that these can relatively easily become targets of internal and, in some cases, even external cyberattacks.

Ethernet-APL, however, uses modern protocols with additional security features, such as PROFINET, which enable secure communication through encryption and authentication. By using public key infrastructures (PKI), it is ensured that only authorized devices have access to the network. In addition, denial-of-service attacks are countered through defense-in-depth strategies that ensure multiple layers of protection take effect. Mature protocol standards such as PROFINET and OPC UA already offer mechanisms to secure the integrity and authenticity of data.

3. Functional safety

Ethernet-APL is capable of meeting the same functional safety requirements as 4–20 mA technology. With Ethernet-APL, the PROFIsafe safety protocol can be used, which is an established and proven safety protocol for industrial networks. PROFIsafe has long been successfully used in factory automation and, via Ethernet-APL, can also be used for the safe transmission of safety-related data between controllers, sensors, and actuators in the process industry.

There are already SIL-3 certified solutions, as demonstrated by a test setup at BASF in Ludwigshafen: it was shown that Ethernet-APL, in combination with PROFIsafe, supports high-availability safety applications. The infrastructure consisting of cables and field switches can also be used in safety-related installations—only field devices and controllers need to support the safety protocol. Regarding the usability of Ethernet-APL field switches, TÜV Rheinland states: “Certification with respect to functional safety is not required when the components (sensor–transmitter) (logic) (receiver–actuator) communicate with one another via a ‘black-channel’ communication. Typically, the fail-safe information is contained in the data packet and can be checked by the receiver for integrity.”

Thus, field switches from manufacturers such as R. STAHL can be used for both safety-related and all other process value transmissions—a major advantage for planning and spare parts management.

Ethernet-APL enables a unified digital communication infrastructure that meets both measurement and control tasks as well as safety requirements. The technology allows direct communication between sensors and controllers without the need for conversions or separate systems. The consistent, end-to-end communication also results in lower maintenance effort: device management is simplified, and devices can be easily replaced in the event of a failure based on application profiles. With PROFIsafe, safety-relevant data can be transmitted over the same infrastructure as process data. This simplifies planning and reduces maintenance effort, as separate systems for process control and functional safety are no longer required.

Early successful implementations of Ethernet-APL in safety-critical applications demonstrate that the technology works in practice and meets high safety requirements. Ethernet-APL is already certified in combination with PROFIsafe for SIL-3 applications, further strengthening confidence in the technology.

The ad-hoc working group “Security and PROFIsafe for PROFINET over APL,” newly established in early 2025 under the leadership of PI and NAMUR, has set itself the task of conducting a large multi-vendor field test. Both the new security features in PROFINET and the safety functions of PROFIsafe will be thoroughly evaluated. By the time of the multi-vendor field test at the end of 2026/beginning of 2027, various plug fests will gradually verify specification-compliant integration. Currently, 40 companies, including R. STAHL, are active in the working group.

Switching from 4–20 mA technology to Ethernet-APL does require an adjustment of the existing infrastructure, but it is easier than expected. Ethernet-APL is compatible with some existing cabling infrastructure, which reduces the cost and effort of the transition. In addition, many of the new devices are backward-compatible, meaning that existing technologies can continue to be used. By reducing cabling, integrating power supply and data transmission on a single line, and enabling the use of digital additional information, significant long-term savings in planning, operation, and maintenance costs can be achieved.

Ethernet-APL is a key enabler for the process plant of the future, as it allows end-to-end digitization at the field level. It supports modern communication protocols such as PROFINET and OPC UA, enabling advanced analysis and diagnostic capabilities. This not only improves efficiency but also enhances the transparency and controllability of processes in real time. Companies that adopt Ethernet-APL benefit from higher data availability, lower operating costs, and simplified maintenance. Ultimately, economic studies, for example by TH Köln, have shown that Ethernet-APL can achieve significant cost savings of over 10% compared to conventional installations.