Ethernet-APL (Advanced Physical Layer) is a 10BASE-T1L single-pair Ethernet physical layer that delivers 10 Mbit/s full-duplex plus device power on the same two conductors, with cable reaches up to 1000 m and intrinsic-safety profiles for hazardous areas [S3][S4]. 4-20 mA HART, in contrast, carries one primary process variable as a 4-20 mA current on a 2-wire loop, while HART modulates a 1200 bit/s FSK signal on top of that same loop for diagnostics and configuration [S2]. For a temperature transmitter the engineering choice is no longer 'digital vs analog' in the abstract; it is a structured tradeoff between bandwidth, multivariable support, cable reach, and the cost of coexisting with what is already in the marshalling cabinet.
The relevant population for this decision is process-industry plants: chemical, refining, pharmaceutical, power, and large-scale water or skid OEMs. Skid builders and greenfield EPCs are the early adopters because they can standardise on one cable and one protocol stack; brownfield maintenance and operations teams that already own thousands of HART loops typically extend rather than replace [S1][S2].
Physical layer and bandwidth: the 300x speed gap
Ethernet-APL is specified at 10 Mbit/s full-duplex on a single twisted pair, which VEGA's published material places "around 300 times faster than HART or Fieldbus" in practical data throughput [S3]. 4-20 mA HART uses a 4-20 mA analog current as the primary variable, with HART 7 operating at 1200 bit/s and HART-IP available on higher-speed backbones but not on the analog loop itself [S2][S4]. For a temperature transmitter, the analog channel still carries one engineering value (deg C or deg F) plus a live-zero fault state at 3.6 mA or below 4 mA; HART can additionally deliver device status, ambient temperature from a secondary sensor, calibration drift, and HART Universal / Common Practice commands, but only at the slower 1200 bit/s side-channel rate [S2].
Zero Instrument's 12 September 2026 comparison notes explicitly that Ethernet-APL is "not a communication protocol like PROFINET, EtherNet/IP, or HART-IP" but a physical layer, which is the single most important framing error to avoid in spec writing: APL is the wire and the connector, the application protocol (PROFINET, EtherNet/IP, HART-IP) still rides on top [S4]. On a single temperature loop, that means an Ethernet-APL temperature transmitter can stream PV, device diagnostics, calibration history, and even firmware updates natively, while a HART temperature transmitter streams PV continuously and diagnostics only when the control system polls the device.
Power, cable reach, and hazardous-area integration
Ethernet-APL transmits both data and device power on the same two wires, with trunk segments specified up to 1000 m on IEC 61158-2 type fieldbus cable and intrinsically safe variants for Zone 1 / Class I Div 1 installations [S3]. A 4-20 mA HART loop also powers the device from the loop itself, typically 24 VDC at 20-25 mA budget, with cable runs in the 100-600 m range depending on conductor gauge and shield. For RTD and thermocouple heads in particular, the head-mounted temperature transmitter's loop-powered design has been the dominant field architecture for more than two decades, and Ethernet-APL head-mounts are now entering the same form factor [S1].
The FieldComm Group and the four protocol organisations (FieldComm Group, ODVA, OPC Foundation, PI) jointly developed Ethernet-APL on the IEEE 10BASE-T1L single-pair Ethernet standard, which is what gives APL its 10 Mbit/s ceiling and its 1 km reach on a single pair [S2]. For a greenfield spec, this collapses a separate coaxial run for process video, a Profibus or DeviceNet run for motor control, and the HART run for instrumentation into one shared 2-wire infrastructure, removing the parallel cable layers that have historically been the main cost of marshalling-room buildouts [S2]. The trade-off is that brownfield replacements are incremental, with VEGA's guidance explicitly recommending "gradual changeover via replacement of remote I/Os and field barriers step by step" rather than rip-and-replace [S3].
Data richness: one PV vs full multivariable diagnostics
A 4-20 mA loop "mainly transfers only one primary process variable" per Zero Instrument's 2026 article, and HART on that loop is the way a host retrieves device tag, PV range, SV/TV/QV (secondary / tertiary / quaternary variables), calibration date, and HART command-level diagnostics, but the analog channel itself never carries more than one variable [S4]. A HART 7 temperature transmitter with dual inputs can expose PV, sensor-2 temperature, sensor differential, and cold-junction compensation values via HART commands, but the 4-20 mA current still represents only the configured PV at any moment [S2].
Emerson's sponsored supplement to Control Global framed HART-IP over Ethernet-APL as the route by which multivariable temperature assemblies (for example, a thermowell with multiple RTDs, a differential temperature loop, or a temperature-plus-pressure combo) gain the bandwidth to publish every variable simultaneously at process speeds, instead of having the control system poll and reconstruct them at HART 1200 bit/s [S5]. The same logic applies to a level transmitter or differential pressure transmitter where the user wants the primary measurement on the analog channel and the secondary diagnostics on HART, which is the configuration most plants will retain on existing loops.
Selection criteria: a criterion-by-criterion comparison
Five selection criteria separate the two choices for a temperature transmitter today. (1) Bandwidth: 10 Mbit/s full-duplex on Ethernet-APL versus 1200 bit/s HART over a 4-20 mA loop, a roughly 300x practical throughput advantage to APL [S3]. (2) Cable infrastructure: APL uses one 2-wire IEC 61158-2 type cable carrying data and power up to 1000 m, HART uses one 2-wire shielded pair carrying 24 VDC loop power and a 4-20 mA signal typically up to a few hundred metres [S3][S4]. (3) Variables per device: APL with PROFINET or HART-IP can publish multiple process and diagnostic variables in parallel, HART 4-20 mA carries one PV on the analog channel and exposes others only on poll [S4][S5]. (4) Migration cost: greenfield APL saves parallel cable layers for video, motor control, and instrumentation; brownfield HART reuses existing wiring and I/O cards with no panel rework [S2][S3]. (5) Tooling and training: HART handhelds, DD files, and FDT/DTM workflows are installed-base familiar; Ethernet-APL requires PROFINET, EtherNet/IP, or HART-IP engineering tools plus APL switches and field barriers [S1][S2].
For a pressure transmitter in a hazardous-area service, the same five criteria apply, and the same conclusion follows: APL wins on bandwidth, multivariable, and greenfield cable cost; HART wins on installed base, training, and the absence of any requirement to replace I/O. For an absolute pressure transmitter on a vacuum line, the analog 4-20 mA channel is still the most robust way to ensure a loss-of-signal reads as a true fault, not a network dropout, and this is one of the strongest reasons HART 4-20 mA persists in safety-related loops [S1][S4].
Where Ethernet-APL temperature transmitters fit, and where they do not
Ethernet-APL temperature transmitters fit greenfield process plants, modular skid packages, and any application where multiple variables per device, firmware management over the network, or high-density diagnostic data justify the new cable and switch infrastructure [S1][S3]. They also fit plants that already operate PROFINET controllers and want one protocol from the I/O card to the sensor head. The Emerson 2021 supplement specifically names "multivariable measurements" as "yet another promising use case for HART-IP over Ethernet-APL, making it easier to power and communicate HART" alongside other process variables on the same pair [S5].
4-20 mA HART temperature transmitters remain the right call for brownfield maintenance replacements, for safety-instrumented loops where the live-zero analog fault behaviour is part of the SIL calculation, and for any installation where pulling a single shielded pair is faster than commissioning a new APL switch and field barrier. Control Global's 2024 reality check is explicit: even for greenfield facilities, "that fully realized tomorrow is only a few years away," because users are "necessarily conservative" and will pilot APL before standardising on it [S2]. The practical reading is that 4-20 mA HART and Ethernet-APL will coexist for the rest of this decade, with APL growing at the greenfield and skid-builder edge and HART holding its installed base.
Migration and sourcing signals to track next
Two signals are worth watching through 2026 and into 2027. First, the rate at which head-mounted temperature transmitters ship with APL ports at the same form factor as today's HART head-mounts; the same end-to-end PROFINET or HART-IP over APL stack that capacitance level transmitter suppliers are adopting is the leading indicator for temperature. Second, whether brownfield revamps continue to standardise on remote-I/O-to-APL migration, which is the only field-proven path that avoids the cost of running a parallel APL trunk alongside an existing HART loop plant-wide [S3]. For now, a temperature-transmitter spec is best written as a hybrid: 4-20 mA HART for the analog PV and the live-zero fault state, Ethernet-APL wired and provisioned but left dark until the marshalling-room switch and the control system are both APL-ready.
For comparison with adjacent field-instrument decisions such as cable selection for industrial Ethernet switches with SFP uplinks and HART vs Ethernet-APL pressure transmitter wiring rules in the same marshalling cabinet, the underlying logic is the same: the physical layer decides the cable, the application protocol decides the host integration, and a 1200 bit/s HART side-channel never replaces the 10 Mbit/s APL trunk, it only rides on it [S1][S3][S5].