Two-wire, intrinsically-safe Ethernet that runs all the way to the transmitter — finally bringing native Ethernet, and the bandwidth it carries, into hazardous-area field instrumentation.
Ethernet-APL-native distributed I/O & junction boxes
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.
Short entry. This page summarises our current placement. Full, sourced analyses are published first for the technologies closest to practical use; see the list of full analyses.
How it works
In an Ethernet-APL installation the field switch takes the place of the traditional junction box. It connects to the trunk or to an industrial Ethernet ring, and provides a number of intrinsically safe spur ports, each powering and communicating with one instrument. The switch is housed in an enclosure rated for the hazardous zone it sits in.
Around it sit devices that bring non-APL signals onto the same network. Some field switches include a proxy so that existing PROFIBUS PA instruments can be connected and seen by the host as Ethernet devices. Remote I/O units do the same for conventional 4-20 mA, HART and discrete signals, which will remain the majority of points in most plants for years.
Why it matters
Cuts marshalling cabinet footprint and field cabling scope on new projects, complementing rather than competing with virtualised remote I/O in the control room.
Where it fits
These components are specified together with APL instruments on new units, and are a practical way to migrate an existing fieldbus installation in stages. Moving the conversion into the field removes marshalling cabinets and long multicore runs, at the price of putting active electronics outdoors where they have to be accessed, powered and kept within temperature limits.
Why we place it at the Industry Acceptance stage
Now shipping from multiple I/O vendors as APL adoption accelerates; a natural, low-risk companion purchase to APL instrumentation itself. Estimated time to the Proven in Use stage: 2–5 yrs. What the stages mean.
Key risk
Field-mounted electronics still need environmental and hazardous-area ratings equivalent to the instruments they serve, adding cost versus control-room-housed I/O.
Questions to ask before committing
- How many spurs per switch are planned, and what is lost if one switch or its power supply fails?
- Are enclosures, glands and heat dissipation designed for the worst ambient conditions on the site?
- Is there a defined migration path for existing fieldbus or HART devices, and has it been tested with your host system?
Standards and terms to know
- Field switch
- The APL device that connects the trunk or ring to individual instrument spurs.
- Proxy
- A function that represents a non-Ethernet device to the network as if it were an Ethernet device.
- Marshalling
- The cross-wiring between field cables and I/O channels that distributed I/O is intended to reduce.
Related technologies
See this entry on the full maturity map · All Control Devices entries · How placements are made. Stage and time-horizon placements are editorial judgements based on public information as of October 2026, not engineering, safety or investment advice; see the Terms of Use.