Field-mounted I/O and junction boxes that terminate Ethernet-APL directly, pushing the marshalling and protocol-conversion layer out of the control room and into the field.
Ethernet-APL (Advanced Physical Layer)
A two-wire, 10 Mbit/s Ethernet physical layer that carries power and data to field instruments, including into hazardous areas. The technology has passed plant-scale testing; for most owners the open question is when the economics of replacing working 4-20 mA wiring make sense.
What changes at the instrument
For forty years the dominant field signal has been the 4-20 mA loop, with HART superimposed at 1,200 bit/s for configuration and diagnostics. Fieldbus (FOUNDATION Fieldbus H1 and PROFIBUS PA) raised that to 31.25 kbit/s and put several devices on one cable, but it needed its own engineering tools, specialist skills and gateways to reach the rest of the plant network. Ethernet-APL replaces the physical layer under all of this with standard Ethernet.
It is built on 10BASE-T1L, the single-pair Ethernet variant defined in IEEE 802.3cg. One twisted pair carries 10 Mbit/s full duplex and power to the device. The typical topology is a powered trunk of up to 1,000 m to a field switch, with spurs of up to 200 m to each instrument; the trunk can run into Zone 1/Division 2 and the spurs into Zone 0/Division 1 1. Intrinsic safety is handled by a port-level concept called 2-WISE (two-wire intrinsically safe Ethernet), defined in IEC TS 60079-47, which lets an engineer verify an intrinsically safe connection by comparing port profiles rather than calculating entity parameters for every segment 1.
Ethernet-APL is a physical layer, not a protocol. PROFINET, EtherNet/IP, HART-IP, OPC UA and Modbus TCP can all run over it. That is the main architectural point: the instrument becomes an addressable node on the same kind of network as the controller, and much of the gateway layer that fieldbus needed disappears.
| 4-20 mA + HART | FOUNDATION Fieldbus H1 / PROFIBUS PA | Ethernet-APL | |
|---|---|---|---|
| Digital data rate | 1.2 kbit/s (HART) | 31.25 kbit/s | 10 Mbit/s full duplex |
| Devices per cable | One | Several on one segment | One per spur; many per field switch |
| Power on the same pair | Yes | Yes | Yes |
| Intrinsic-safety concept | Entity parameters, IS barriers | FISCO or entity | 2-WISE (IEC TS 60079-47) |
| Path to the plant network | I/O card or HART multiplexer | Linking device or gateway | Native Ethernet via field switch |
What the plant-scale tests showed
The question users asked first was not whether a single APL transmitter works, but whether a plant-sized network behaves. BASF made supplier performance tests a condition of awarding contracts. In tests completed in March 2023, ABB ran PROFINET over Ethernet-APL with about 240 devices, 10 field switches and several control systems built on its System 800xA. BASF then ordered ABB control-system equipment for its first greenfield Ethernet-APL project in Europe, and said it plans to equip new European plants with the technology 2.
A second multi-vendor round, reported in June 2025, used close to 240 Endress+Hauser flow, pressure, temperature and level devices plus Samson positioners in a ring topology, with field switches from Pepperl+Fuchs, Phoenix Contact and R. Stahl, connected to an Emerson DeltaV system. Network load and redundancy switchover times met the requirements BASF had set 3. Two features of these tests matter more than the headline: they were defined by an end user rather than a vendor, and they mixed suppliers at the switch and device level, which is where interoperability problems usually hide.
ARC Advisory Group's 2026 assessment describes the market as being in early deployment: several suppliers offer APL field switches and the first waves of APL instruments, most installations are lighthouse projects or controlled expansions, and adoption is led by Europe, especially Germany. ARC lists brownfield conversion cost, hazardous-area documentation, workforce skills, cybersecurity discipline and the breadth of the device portfolio as the main barriers 4.
Where it pays, and where it does not
The business case is strongest on new units with a high instrument count and a real use for device data. Coriolis meters, radar level gauges, analysers and digital valve positioners all produce diagnostics and secondary variables that a 4-20 mA signal discards and HART delivers slowly. On a greenfield project the trunk-and-spur layout can also reduce marshalling and home-run cabling, although the saving depends heavily on the plot plan and should be estimated, not assumed.
Brownfield is different. A working 4-20 mA loop with HART has no failure to fix, and replacing it means new field switches, power, hazardous-area documentation and new device models, usually squeezed into an outage alongside other work. For most existing plants the realistic path is selective: APL for new units, analyser houses and expansions, and for devices whose diagnostics justify the change, while existing loops stay in service. Some owners plan to reuse existing fieldbus cable for APL trunks. That is worth evaluating, but treat it as something to verify against the cable's specification and condition rather than a given.
The device portfolio is the other practical limit. Mainstream pressure, temperature, flow and level transmitters are widely available in APL versions; coverage is thinner for specialised analysers and some valve and actuator types. A project that ends up with a mix of APL, HART and fieldbus devices keeps part of the complexity it was trying to remove.
Engineering it well
Most difficulties reported from early projects are not about the physical layer. They are about planning, ownership and procedures.
- Design the network, not just the instruments. Decide switch locations, trunk redundancy (ring or dual trunk), the power budget per switch and spare ports at the front-end engineering stage.
- Treat it as an IP network from day one. Field switches and instruments become network nodes, so they belong in an IEC 62443 zone-and-conduit design with defined access control, monitoring and patching responsibilities.
- Agree how diagnostics are handled. NAMUR NE 107 status signals should map to defined alarms or maintenance notifications; otherwise richer device data simply becomes more noise.
- Update procedures and training. Loop checks, device replacement and fault-finding change when the tool is a laptop on a network rather than a multimeter on a loop.
Evidence timeline
- 2019IEEE 802.3cg defines 10BASE-T1L single-pair Ethernet, the physical layer Ethernet-APL builds on. 1
- Mar 2023ABB completes BASF's plant-scale test of PROFINET over Ethernet-APL: about 240 devices, 10 field switches. 2
- Feb 2024BASF announces ABB as supplier for its first greenfield Ethernet-APL project in Europe. 2
- Jun 2025Second multi-vendor test (about 240 devices, switches from three suppliers, Emerson DeltaV host) reported to meet load and switchover requirements. 3
- 2026ARC Advisory Group characterises adoption as early deployment, led by Europe and Germany. 4
Why it sits at the Industry Acceptance stage
The core technology has passed end-user-defined, multi-vendor tests at plant scale, and greenfield orders are being placed on the strength of them. That is the Industry Acceptance stage: the question has moved from whether it works to where, how and at what cost to deploy it.
We expect APL to become a normal specification for new process units in Europe within the near-term horizon. Broad brownfield adoption will take longer than that horizon suggests, because the installed base of 4-20 mA and HART is very large and mostly working.
Adoption signals to watch
- Published APL projects outside Europe, particularly greenfield units in North America and the Middle East.
- Broader APL coverage in analysers and valve positioners, not only pressure, temperature, flow and level.
- End-user and EPC standard specifications that make APL the default rather than an alternative.
- Brownfield conversion reference designs with published cost data.
Questions to ask before committing
- Which devices on this project's instrument index are available in APL versions with the hazardous-area approvals we need, and from how many suppliers?
- What is the power budget per field switch, and how many spare ports does the design leave for later additions?
- How is the trunk made redundant, and what switchover time has been demonstrated with our control system?
- Who owns the field switches as network assets: instrumentation, the control-system vendor or OT networking?
- How will NE 107 diagnostics be routed, and to whom?
Terms used on this page
- 10BASE-T1L
- The IEEE 802.3cg single-pair Ethernet physical layer that APL is built on.
- 2-WISE
- Two-Wire Intrinsically Safe Ethernet, the intrinsic-safety concept defined in IEC TS 60079-47.
- Trunk and spur
- The topology of a powered trunk cable with field switches feeding individual device spurs.
Sources
- Advanced Physical Layer (overview of 10BASE-T1L, segment lengths and 2-WISE)
- BASF selects ABB for greenfield projects after a successful Ethernet-APL trial
- Ethernet-APL proves its value: expanded testing shows broad industry adoption
- From spec to site: where Ethernet-APL adoption stands in 2026
Related technologies
See this entry on the full maturity map · All Instrumentation entries · How placements are made. Stage and time-horizon placements are editorial judgements based on the public sources listed above, as of October 2026. They are not engineering, safety or investment advice; see the Terms of Use.