Instrumentation Signal Standards

Process control signal types — 4–20 mA HART, FOUNDATION Fieldbus, PROFIBUS, Modbus, OPC UA — with typical applications.

README

Analog Signals

Analog Signal Standards

Signal	Range	Physical	Wiring	Max distance	Typical use	Notes
4-20 mA DC	4=LRV, 20=URV	Two-wire current loop	2-wire shielded TP	~3000 ft	Process transmitters (pressure, flow, level, temp)	De-facto standard; NE43 fail-low <3.6 or fail-high >21 mA diagnostic.
0-10 V DC	LRV-URV	Single-ended voltage	2-wire TP	<50 ft	Analyzer outputs, VFD speed reference	Ground-loop prone; use isolators on long runs.
+/-10 V DC	+/-FS	Bipolar	2-wire TP	<50 ft	Servo drive command, bipolar measurements	Same constraints as 0-10 V.
1-5 V DC	1=LRV, 5=URV	From 250 ohm sense R	Branched from 4-20 mA	Same as 4-20 mA	Legacy DCS AI; still common on Honeywell	Formed across a 250 ohm dropping resistor.
Pt100 / Pt1000 RTD	Resistance	100 or 1000 ohm @ 0C	3- or 4-wire	<200 ft typ.	Temperature	3-wire cancels lead resistance; 4-wire most accurate.
Thermocouple (K, J, T, N)	mV vs Cold Junction	Seebeck EMF	Compensating TC extension cable	<100 ft, more w/ transmitter	High T measurement	Always need cold-junction compensation at DCS/transmitter.
Frequency (pulse)	0-10 kHz typ	Square-wave or sine	Shielded 2-wire	<2000 ft	Turbine/mag flow totalizer	Scale to engineering units in DCS or flow computer.
Namur sensor (IEC 60947-5-6)	<1.2 mA off, >2.1 mA on	2-wire proximity	Shielded 2-wire	<330 ft	IS proximity / limit switches	Loop powered by IS barrier; used in Zone 0/1.
3-15 psi pneumatic	3=LRV, 15=URV	Instrument-air signal	6x1 mm or 1/4 in PE tubing	<300 ft	Legacy controllers & valve positioners	Rarely designed new; mostly retrofit applications.

Digital Fieldbus

Digital / Fieldbus / Industrial Ethernet Comparison

Protocol	Layer	Speed	Topology	Max distance (copper)	Devices/segment	Power on bus	Typical use
HART 7	Superimposed on 4-20 mA	1.2 kbps	Point-to-point / multidrop	Same as 4-20 mA loop	1 (P-to-P) or 15 (MD)	No (host powers loop)	Legacy w/ smart device access; diagnostics, cal data
FOUNDATION Fieldbus H1	IEC 61158-2	31.25 kbps	Bus / tree (trunk+spur)	1900 m trunk	16 (non-IS) / 6-12 (IS)	Yes (9-32 VDC)	Plant-wide control w/ device-resident function blocks
PROFIBUS-PA	IEC 61158-2	31.25 kbps	Bus / tree	1900 m	32 non-IS / 6-12 IS	Yes	Process automation w/ PROFIBUS-DP backbone
PROFIBUS-DP	RS-485	9.6 kbps - 12 Mbps	Bus	100-1200 m (rate dep.)	126 via repeaters	No	Factory automation, drives, I/O
Modbus RTU	RS-485	9.6-115.2 kbps	Bus	4000 ft	247	No	VFDs, power meters, simple devices
Modbus TCP	Ethernet/TCP	10/100/1000 Mbps	Star	100 m/seg	IP-limited	No	PLC to PLC, OPC gateways
EtherNet/IP	ODVA / CIP	10/100/1000 Mbps	Star / ring (DLR)	100 m/seg	IP-limited	PoE option	Rockwell control, drives, motion
PROFINET	PI / IEEE 802	100/1000 Mbps	Star / line	100 m/seg	IP-limited	PoE option	Siemens control, drives; RT/IRT real-time
EtherCAT	ETG	100 Mbps	Line / ring	100 m/seg	65535	No	High-speed motion, servo
OPC UA	IEC 62541	Ethernet-dep	Any IP	Any	Any	No	MES/ERP integration, IT/OT bridge

NAMUR NE43 107

NAMUR NE 43 (4-20 mA Failure) and NE 107 (Diagnostic Categories)

Band (mA)	NE 43 State	Meaning	Typical use
< 3.6	Fault (low)	Transmitter failure, sensor open	Shutdown diagnostic; alarm low
3.6 - 3.8	Saturation low	Under-range	Out-of-range indication
3.8 - 4.0	Live zero margin	Just below 4 mA; valid zero	Normal
4.0 - 20.0	Valid	Normal measurement range	Process signal
20.0 - 20.5	Live full margin	Just above 20 mA	Normal
20.5 - 21.0	Saturation high	Over-range	Out-of-range indication
> 21.0	Fault (high)	Transmitter failure	Shutdown diagnostic; alarm high
NAMUR NE 107 Diagnostic Categories
Failure (F)	Output signal is invalid due to malfunction in the device or its periphery.
Function check (C)	Output signal temporarily invalid (test, local operation, frozen).
Out-of-specification (S)	Deviations from ambient or process conditions outside spec; info only.
Maintenance required (M)	Device operates OK but remaining service buffer is limited; schedule maintenance.

Source: Instrumentation_Signal_Standards_Reference_v1.xlsx

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