Signal & Protocol Selection

Quick pick (TL;DR)

Single robust loop, simple & familiar → 4-20 mA + HART (the default; HART gives diagnostics over the same pair).
Many devices per cable, less wiring → Foundation Fieldbus H1 or PROFIBUS PA.
Packaged/serial equipment → Modbus.
New high-speed, long-distance digital → PROFINET / Ethernet-APL.
Safety functions → keep them hardwired or on a certified safety bus, not on the basic control bus.

Step 1 — Select by need

Need	Choose	Why
Single, robust	4-20 mA + HART	Proven + diagnostics
Many devices	Foundation Fieldbus / PA	Multidrop
Packaged gear	Modbus RTU/TCP	Universal serial
High-speed digital	PROFINET / Ethernet-APL	Ethernet to field

Step 2 — Why 4-20 mA + NAMUR

Signal detects faults (NE43)

The 4-20 mA scale 4 mA = 0% of range 20 mA = 100% of range Why 4, not 0? a broken wire = 0 mA 0 mA ≠ valid signal → fault is detectable NE43 fault bands ≤ 3.6 mA → under-range / fault 3.8–20.5 mA → valid measurement ≥ 21 mA → over-range / fault So → live-zero gives built-in diagnostics

Live-zero at 4 mA means a broken wire (0 mA) ≠ a real zero — NE43 makes faults auto-detectable.

Step 3 — Wiring & safety

Point-to-point analog; trunk-spur fieldbus.
2-wire loop-powered; FISCO for IS bus.
Safety signals separate from BPCS bus.

Common mistakes

Safety on a basic-control bus.
Overloading a fieldbus segment.
Ignoring NE43 fault bands.

Requisition & datasheet

Function, criticality, device count.
Protocol, topology, power, IS/Ex.

Frequently asked questions

What should I know about Signal & Protocol Selection?

Single robust loop, simple & familiar → 4-20 mA + HART (the default; HART gives diagnostics over the same pair). Many devices per cable, less wiring → Foundation Fieldbus H1 or PROFIBUS PA. Packaged/serial equipment → Modbus. New high-speed, long-distance digital → PROFINET / Ethernet-APL. Safety functions → keep them hardwired or on a certified safety bus, not on the basic control bus.

Which standards or codes apply?

NAMUR NE43 (signal fault levels), NE107 (diagnostics), IEC 61158/61784 (fieldbus), HART/FF/PA specs, IEC 61511 for safety signalling.

Standards — NAMUR NE43 / NE107; IEC 61158/61784; IEC 61511.