Compares three dominant industrial sensor communication methods: 4-20 mA current loop (NAMUR NE 43), HART digital-on-analogue (Highway Addressable Remote Transducer), and IO-Link (IEC 61131-9 point-to-point digital). Covers wiring topology, cable-length limits, diagnostics visibility, hazardous-area compatibility, and the ROI case for migrating a plant from 4-20 mA to IO-Link.
| Method | Layer | Cable | Max length | Diagnostics |
|---|---|---|---|---|
| 4-20 mA current loop | Analogue only | Twisted pair, shielded | ~1000 m | Loop break + NAMUR NE 43 fault ranges only |
| HART on 4-20 mA | Analogue + superimposed digital (1200 baud FSK) | Twisted pair, shielded | ~1500 m (single-master) | Digital variables, device identity, diagnostics |
| IO-Link (IEC 61131-9) | Digital point-to-point | Unshielded 3-wire, standard M12 | 20 m | Full parameterisation, event codes, process value, cyclic + acyclic data |
The industry workhorse since the 1970s. A 2-wire transmitter draws 4-20 mA proportional to the process variable, sourced by a loop-powered PSU at the DCS side. 4 mA = 0% of range, 20 mA = 100%. NAMUR NE 43 defines the failure signal ranges: 3.6 mA = under-range / sensor fail low, 21 mA = over-range / sensor fail high, 3.8 to 20.5 mA = valid measurement, so the DCS distinguishes a genuine 0% reading from a broken sensor. Simple, well-understood, works with any manufacturer's DCS input card.
HART superimposes a 1200-baud FSK modem signal on the 4-20 mA loop without affecting the analogue reading. A handheld HART communicator or the DCS multiplexer can query the transmitter for extended data: multiple process variables, calibration status, event log, device identity, sensor diagnostics. The core analogue loop still works if the HART modem fails, so the plant does not lose the process variable during HART commissioning issues. HART 7 (2007) added wireless HART (802.15.4-based mesh, IEC 62591) as a wire-free option for hard-to-reach retrofit sensors.
IO-Link is a digital protocol carried on unshielded 3-wire cable with standard M8/M12 connectors — the same connectors that carry legacy 24 VDC discrete sensor signals. Each IO-Link master port supports one IO-Link device (up to 20 m away). The master aggregates IO-Link data from up to 8 ports and uplinks to the PLC via PROFINET, EtherNet/IP, EtherCAT, or CC-Link IE. IO-Link's advantage over 4-20 mA is that the same physical cable and connector now carries process value, device parameters, event codes, and vendor-specific diagnostics — no analogue-to-digital conversion loss, no calibration drift, no separate cable for discrete "sensor OK" signalling.
| Method | Ex protection | Notes |
|---|---|---|
| 4-20 mA loop | Ex ia intrinsic safety (galvanic isolator / zener barrier) | Well-established; every intrinsic-safety isolator OEM has a certified module |
| HART on 4-20 mA | Ex ia; HART signal passes through most zener barriers | Some older galvanic isolators strip HART — check datasheet |
| IO-Link | Not intrinsically safe at cell wire level | Use IO-Link master mounted in safe area, IS-rated sensor + galvanic isolator |