SIGNALSOUTPUTS / WIRING / INTERFACES

Industrial Sensor Signals & Output Types

Industrial sensor outputs carry a switching state or measured-value signal from the sensor to a PLC, controller or measuring system. Common electrical outputs include 4–20 mA current loops, 0–10 V voltage outputs, PNP/NPN transistor outputs, and pulse or frequency signals. Their connections are commonly arranged as two-, three- or four-wire circuits.

Match the sensor output to a compatible receiving input and supply arrangement, then verify reference potential, polarity, load, cable characteristics and required response speed.

COVERS4–20 mA0–10 VPNP / NPNPulse / frequency2 / 3 / 4 wirePLC inputs

SIGNAL TYPES

Industrial Sensor Signals & Wiring

Start with the electrical interface used by the sensor, then check the wiring arrangement and the input requirements of the PLC, controller or measuring instrument.

ANALOGUE CURRENT01

4–20 mA Current Loop

Loop current, live zero, loop supply, burden resistance and receiver connection.

OPEN →

ANALOGUE VOLTAGE02

0–10 V Sensor Output

Voltage reference, input impedance, cable effects and common industrial connections.

OPEN →

DISCRETE OUTPUT03

PNP vs NPN Sensor Outputs

Determine the switched current path and PLC common requirements for sourcing and sinking DC outputs.

OPEN →

DYNAMIC SIGNAL04

Pulse & Frequency Outputs

Interfaces that encode motion or quantity as repeated transitions, with limits set by pulse shape, frequency and counter capability.

OPEN →

WIRING05

2-Wire vs 3-Wire vs 4-Wire Sensors

How power and signal conductors are arranged in common switching and analogue sensors.

OPEN →

COMPARISON06

4–20 mA vs 0–10 V

Compare current and voltage transmission by wiring, loading, cable behaviour and fault visibility.

OPEN →

SIGNAL FUNCTION

First decide what the output represents.

A sensor output may report a binary state, transmit a continuously varying measurement, or generate pulses whose count or frequency represents position, speed, flow or another changing quantity.

The same sensing principle can be offered with different output interfaces. A pressure sensor, for example, may be available with 4–20 mA, 0–10 V or a switching output depending on the application.

IO-Link and fieldbus carry structured device data, while HART superimposes digital communication on a conventional 4–20 mA loop.

Sensors may also use relay, solid-state or push-pull switching outputs and analogue ranges other than 4–20 mA or 0–10 V; the exact interface depends on the device.

01 / DISCRETE

Switching outputs

PNP and NPN transistor outputs represent states such as object present, pressure reached or level detected.

02 / CURRENT

Analogue current

4–20 mA represents a measured value as loop current and is widely used for process and field instrumentation.

03 / VOLTAGE

Analogue voltage

0–10 V represents a measured value as voltage relative to a reference conductor or common potential.

04 / DYNAMIC

Pulse or frequency

A pulse train can carry count or rate information; the receiver must meet the required frequency, pulse-width and electrical-level limits.

OUTPUT ↔ INPUT

Match the electrical interface before wiring the sensor.

A sensor signal is usable only when the receiving input is electrically compatible with the output. Signal family, voltage or current range, current direction, input impedance, supply arrangement and maximum switching or counting rate must agree.

Industrial sensor output types and typical receiving-input requirements
Sensor output	Typical receiving input	Critical checks
4–20 mA	Analogue current input	Loop supply, active/passive arrangement, polarity, burden resistance and allowed loop voltage.
0–10 V	Analogue voltage input	Signal reference, input impedance, ground-potential difference, cable length and electrical noise.
PNP	DC digital input compatible with a sourcing sensor	Supply voltage, common conductor, input current and required ON/OFF thresholds.
NPN	DC digital input compatible with a sinking sensor	Supply voltage, current path, input common and whether a pull-up or sourcing input circuit is required.
Pulse / frequency	Counter, high-speed or frequency input	Logic level, maximum frequency, minimum pulse width, edge polarity and input filtering.

WIRING ARRANGEMENT

Wire count describes connection architecture, not one universal signal type.

Two-, three- and four-wire sensors use different ways of distributing power and signal conductors. The exact terminal function must still be taken from the device documentation.

2-wire

Power and signal share the same two conductors. This arrangement is common in current-loop transmitters and also exists in some switching sensors.

POWER + SIGNAL SHARE THE CIRCUIT

3-wire

Two conductors supply the sensor and a separate conductor provides the output. This is common for DC PNP/NPN proximity and switching sensors.

SUPPLY + COMMON + OUTPUT

4-wire

Four conductors may separate power and signal paths or provide complementary outputs. The function depends on the sensor and interface design.

DEVICE-SPECIFIC TERMINAL FUNCTIONS

ANALOGUE TRANSMISSION

4–20 mA and 0–10 V solve the same measurement task differently.

Both interfaces can carry a continuously varying process value, but they differ in how cable resistance, reference potential, receiver loading and fault conditions affect the transmitted signal.

4–20 mA

The measured value is represented by loop current.

Current is the transmitted variable.
4 mA provides a live-zero reference in the standard range.
Loop voltage must cover transmitter and load requirements.
Open-circuit loss of current can be distinguished from a valid 4 mA lower-range value.

0–10 V

The measured value is represented by voltage at the receiver.

Voltage is referenced to a signal common.
Receiver input impedance should be high enough for the source.
Voltage drop and ground-potential differences can become measurement errors.
Cable routing and electrical noise require attention in industrial installations.

ENGINEERING CHECKS

Many apparent sensor-signal faults are caused by the electrical interface.

When a sensor appears to switch incorrectly or an analogue value is unstable, verify the complete electrical path before assuming the sensing element has failed.

Wrong input type

A current output connected to a voltage input, or an incompatible PNP/NPN input, will not be interpreted correctly.

Supply mismatch

Check sensor supply range, loop voltage, polarity and whether the transmitter or receiver provides excitation.

Reference problem

Voltage and discrete interfaces depend on the intended common or reference potential being connected correctly.

Excess loading

Receiver impedance, loop burden, pull-up resistance or multiple loads can move the circuit outside its specified operating range.

Cable and noise

Long cable runs, shielding, routing, capacitance and electromagnetic interference can affect signal integrity and response.