Analog vs Digital Signals in Industrial Automation Explained
Analog vs digital signals explained for automation — discrete vs continuous, the common signal types (24V, 4-20mA, 0-10V), and how each maps to PLC I/O.
In industrial automation, every field device communicates with a PLC through one of two fundamental signal types: digital (discrete) or analog. A digital signal carries a simple on/off state — a pushbutton is pressed or it is not. An analog signal carries a continuous value across a range — a pressure transmitter outputs a current that tracks actual process pressure in real time. Understanding the difference is the first step to wiring field devices correctly, choosing the right I/O cards, and writing PLC programs that scale raw counts into engineering units.
This guide explains both signal types, the common electrical standards used in industry, how each maps to PLC hardware, and when you will encounter one versus the other in the field.
What Digital (Discrete) Signals Are
A digital signal — also called a discrete signal in the controls world — has exactly two states: on or off, 1 or 0, energized or de-energized. There is no in-between value. The PLC sees the signal as a single bit in its input image table.
Common Digital Signal Standards
24 VDC sourcing/sinking is the most common discrete input level in modern automation. A field device provides or draws 24 V to assert a logic-1. Most PLCs accept a threshold around 11–15 V as a definitive high, and anything below about 5 V as a definitive low.
Dry contact / potential-free contacts are simple mechanical or relay contacts with no voltage of their own. The PLC I/O card supplies the voltage — typically 24 VDC — and the contact either connects it to the input circuit (on) or leaves it open (off). Limit switches, pushbuttons, and relay contacts are classic dry-contact sources.
120 VAC discrete inputs appear in older North American installations and in motor control panels that were designed before 24 VDC I/O became universal. The signaling principle is identical — two states — but the input card must handle line voltage and provide appropriate isolation.
What Generates a Digital Signal
Digital signals come from anything that has a clear binary state:
- Pushbuttons and selector switches
- Limit switches and proximity sensors (when used as position detectors)
- Float switches and level switches
- Relay and contactor auxiliary contacts
- Safety light curtains (safe/not-safe output)
- Solenoid valve confirmation switches
On the output side, a digital output card drives devices that are either fully on or fully off: solenoid valves, indicator lights, motor starter coils, alarms, and relay coils.
What Analog Signals Are
An analog signal is a continuously variable electrical quantity — voltage or current — that is proportional to a physical measurement. Instead of two states, an analog signal represents thousands of distinct values across its range. A 4-20 mA pressure transmitter measuring 0–100 bar does not report "high pressure" or "low pressure"; it reports exactly 11.43 mA, which the PLC scales to 45.4 bar.
Analog signals are the language of process instrumentation. Anywhere you need to know how much rather than just whether, you need an analog signal.
Common Analog Signal Standards
4-20 mA current loop is the dominant standard in process plants worldwide. The transmitter regulates DC current in a series loop between 4 mA (0% of range) and 20 mA (100% of range). Current is constant throughout the loop regardless of wire resistance, making 4-20 mA reliable over cable runs of hundreds of metres and highly immune to electromagnetic noise. The 4 mA live zero also allows broken-wire detection — a reading below roughly 3.6 mA indicates a fault rather than a valid zero. See 4-20mA current loop explained for full wiring and scaling detail.
0-10 VDC voltage signal is common in building automation, HVAC, and shorter cable-run applications. A sensor or transducer outputs a voltage proportional to the measured variable. Voltage signals are simpler to source but susceptible to noise over long cable runs and affected by cable resistance — factors that make them less common in heavy industrial environments.
±10 VDC bipolar voltage is used where the measured quantity swings through zero — torque, displacement from centre, or velocity direction. The PLC analog input card must accept a bipolar range and interpret negative voltages correctly.
1-5 VDC is a scaled voltage equivalent of 4-20 mA used with some transmitters and sensors, retaining the live-zero diagnostic benefit.
RTD and thermocouple signals are a special category: very low-level voltage signals from temperature sensors that require dedicated input cards with cold-junction compensation (thermocouples) or precision excitation (RTDs). These are technically analog but handled by specialist I/O modules rather than general-purpose analog cards.
What Generates an Analog Signal
Analog signals come from transmitters and transducers — devices that convert a physical variable to a proportional electrical signal:
- Pressure transmitters (process pressure, differential pressure)
- Temperature transmitters (converting RTD/thermocouple to 4-20 mA)
- Flow transmitters (Coriolis, magnetic, vortex)
- Level transmitters (hydrostatic, ultrasonic, radar)
- pH and conductivity analyzers
- Position transducers and linear potentiometers
- Load cells and force sensors
- Speed transducers (0-10 V tachometer output)
Analog outputs drive devices that require proportional control: variable-speed drives (speed setpoint), proportional control valves (valve position), and current-to-pressure converters.
Digital vs Analog: Side-by-Side Comparison
| Property | Digital (Discrete) | Analog |
|---|---|---|
| Number of states | 2 (on/off) | Continuous (thousands of values) |
| Common standards | 24 VDC, dry contact, 120 VAC | 4-20 mA, 0-10 V, ±10 V |
| PLC data type | Single bit (BOOL) | Integer or real (INT, DINT, REAL) |
| Typical resolution | 1 bit | 12-bit (0-4095) or 15-bit (0-32767) |
| Wiring | 2 wires (signal + common) | 2 or 4 wires depending on type |
| Noise sensitivity | Low (large voltage swing) | Higher — shielded cable needed |
| Typical field devices | Switches, pushbuttons, relays | Transmitters, transducers |
| PLC I/O card | DI / DO card | AI / AO card |
| Scaling required? | No | Yes — counts to engineering units |
| Fault detection | Wire open = 0 (ambiguous) | Wire open = <3.6 mA (4-20 mA only) |
Common Industrial Signal Standards at a Glance
Plants use a defined set of signal levels that have become de-facto standards. Knowing them by sight prevents wiring errors.
24 VDC discrete — by far the most common input/output level in modern PLC systems. IEC standards and most European-designed PLCs default to 24 VDC I/O. Devices are sourcing (PNP) or sinking (NPN), and the I/O card type must match.
4-20 mA — the universal process instrumentation standard. Two-wire loop-powered transmitters require only a 24 VDC supply; the transmitter draws its operating power from the loop current itself. Four-wire transmitters have separate power and signal connections.
0-10 VDC — common for HVAC actuators, variable-speed drive speed references, and building management systems. Requires low-impedance wiring and short cable runs to avoid voltage drop errors.
±10 VDC — bipolar voltage for servo drives, position feedback, and applications requiring signed values.
Dry contact / SPDT relay — used for PLC-to-PLC interlocks, alarm outputs to SCADA systems, and anywhere galvanic isolation is required between signal circuits.
Thermocouple and RTD — not "standard" voltages or currents but dedicated signal types requiring specialist analog input cards. Most modern PLCs handle them with plug-in thermocouple or RTD modules.
Digital vs Analog PLC I/O Cards
PLC chassis accept different I/O cards depending on the signal type. Matching the card to the field device is a fundamental commissioning task.
Digital Input Cards (DI)
A digital input card reads the state of field contacts. Each channel presents a circuit that the card energizes internally; when an external contact closes, the card detects a logic-1 and sets the corresponding input bit. Cards are specified by voltage level (24 VDC, 120 VAC, 240 VAC), current sink/source type (NPN or PNP for DC cards), and the number of channels per card (8, 16, or 32 are typical).
Digital Output Cards (DO)
A digital output card switches field devices. Transistor (solid-state) outputs are fast-switching and suited to solenoids and pilot lights. Relay output cards provide contacts for driving AC loads or for isolation. Output cards are rated by voltage, current capacity per channel, and whether outputs share a common return.
Analog Input Cards (AI)
An analog input card converts a continuous electrical signal to a digital number the CPU can process. The card's analog-to-digital converter (ADC) samples the input at intervals and produces a binary count:
- A 12-bit card produces counts from 0 to 4,095 (4,096 steps over the full range)
- A 15-bit card produces counts from 0 to 32,767 (32,768 steps)
- Some cards express bipolar ranges as signed integers (−32,768 to +32,767)
Cards are specified by signal type (current, voltage, or both), number of channels, and resolution. Many modern cards are software-configurable per channel — the same card can accept 0-10 V on channel 1 and 4-20 mA on channel 2.
Analog Output Cards (AO)
An analog output card takes a digital value written by the CPU and converts it through a digital-to-analog converter (DAC) to a proportional voltage or current output. The PLC program writes a count (e.g., 0-32,767) and the card produces the corresponding 0-10 V or 4-20 mA signal to the field device.
When You Get Each Signal Type: Switch vs Transmitter
The quickest field rule: a switch gives you a digital signal; a transmitter gives you an analog signal.
Discrete Inputs in Practice
A proximity sensor detecting whether a part is present sends a 24 VDC pulse — one bit, wired to a DI card. A pressure switch pre-set at 6 bar trips its contact open or closed — one bit, wired to a DI card. A door interlock switch sends a contact state — one bit.
The PLC program reads these bits directly as BOOL values. No conversion is needed. If the bit is 1, the condition is true.
Analog Inputs in Practice
A pressure transmitter measuring the same 0–10 bar range as that pressure switch outputs a 4-20 mA signal proportional to actual pressure — wired to an AI card. The card converts it to a count; the PLC program scales the count to engineering units (bar, PSI, kPa). You can then implement any setpoint you want in software, trend the value, and detect slow drift — none of which is possible with a switch.
Choosing between a switch and a transmitter is an engineering decision:
- Use a discrete switch when you only need to know if a threshold has been crossed and process simplicity or cost drives the decision.
- Use an analog transmitter when you need the actual measured value, want software-adjustable setpoints, need historical trending, or must detect slow degradation rather than a hard trip. Learn more about field device selection in types of industrial sensors.
Converting Between Analog and Digital: ADC, DAC, and Scaling
Every analog input card contains an ADC (analog-to-digital converter). The ADC samples the electrical signal and produces a binary integer. Every analog output card contains a DAC (digital-to-analog converter) that does the reverse.
Resolution and Accuracy
Resolution describes the smallest increment the converter can distinguish. A 12-bit ADC divides the full input range into 4,096 steps; a 15-bit ADC provides 32,768 steps. For a 4-20 mA card reading a 0-100 bar transmitter:
- 12-bit: each count represents approximately 0.024 bar (100 ÷ 4,096)
- 15-bit: each count represents approximately 0.003 bar (100 ÷ 32,768)
Higher resolution is useful when the process requires precise control, but it does not improve accuracy if the transmitter itself is only ±0.5% accurate. Resolution and accuracy are independent specifications.
Scaling in the PLC Program
The raw ADC count must be scaled to a useful engineering unit in software. Most PLC platforms provide a scale or normalize instruction. The general linear formula is:
Engineering value = ((Raw count − Min count) ÷ (Max count − Min count)) × (EU range)
For a 12-bit card reading 4-20 mA (counts 819–4,095 for a card that maps 0 mA to count 0 and 20 mA to count 4,095):
- 0 bar = 819 counts (4 mA)
- 100 bar = 4,095 counts (20 mA)
Getting this scaling right is one of the most common PLC commissioning tasks. See PLC analog input scaling for step-by-step instruction blocks for the major PLC platforms.
Frequently Asked Questions
What is the difference between analog and digital signals? A digital (discrete) signal has two states — on or off. An analog signal is a continuously variable electrical quantity, such as a 4-20 mA current or a 0-10 V voltage, that is proportional to a physical measurement. Digital signals map to BOOL bits in the PLC; analog signals map to integer counts that the program scales into engineering units.
Is 4-20mA analog or digital? 4-20 mA is an analog signal. The current varies continuously between 4 milliamps (0% of range) and 20 milliamps (100% of range), representing any value within the transmitter's calibrated span. It requires an analog input card and a scaling calculation in the PLC program.
What is a discrete signal? A discrete signal is another name for a digital signal in the controls industry. It carries exactly two states — energized or de-energized — and maps to a single bit in the PLC's input image table. The term "discrete" distinguishes individual on/off signals from analog or network-based (fieldbus) signals.
What signals does a PLC use? A PLC handles both digital and analog signals simultaneously. Discrete input (DI) and discrete output (DO) cards handle on/off signals from switches, relays, and solenoids. Analog input (AI) and analog output (AO) cards handle continuous signals from transmitters and proportional actuators. Modern PLCs also communicate over digital fieldbuses (Profibus, Profinet, EtherNet/IP) that carry large amounts of data over a single cable, but the underlying field measurements still originate as analog or discrete signals at the sensor level.
Understanding the distinction between analog and digital signals is foundational to every task in controls engineering — from reading a P&ID to wiring a panel, selecting I/O cards, and writing PLC programs. Review the PLC programming basics fundamentals guide for a broader introduction to how signals fit into the overall PLC architecture, or start with 4-20mA current loop explained to go deeper on the most common analog signal standard in process automation.
