Industrial automation systems depend on signals. Sensors send information to controllers, controllers evaluate that information, and outputs command equipment throughout the process.
Two of the most common signal categories are digital and analogue signals. Understanding the difference is useful not only for automation engineers, but also for maintenance and purchasing teams trying to identify PLC modules, sensors, transmitters, control equipment and replacement components.
What Is a Digital Signal?
A digital signal represents a discrete state. In a basic industrial control application, that usually means one of two conditions such as on or off, open or closed, healthy or faulted, detected or not detected.
Examples include:
- A push button being pressed
- A limit switch being activated
- A proximity sensor detecting an object
- A motor overload contact indicating a trip
- A float switch indicating high level
- A contactor auxiliary contact indicating status
- An emergency stop circuit providing a permitted status signal to the control system where appropriately designed
The PLC or controller does not necessarily need to know how far a push button is pressed. It generally needs to know whether the required state exists.
What Is an Analogue Signal?
An analogue signal represents a continuously variable process value over a defined range.
Instead of simply indicating high or low, an analogue signal can represent how much pressure, temperature, level, flow or another process variable is present.
Common examples include:
- Tank level
- Pipeline pressure
- Flow rate
- Temperature
- Motor speed reference
- Valve position
- Process weight
The electrical signal is scaled so that a particular signal value corresponds to a particular engineering value.
A Simple Example
Consider a water tank.
A digital float switch might tell the PLC only that the water has reached a specific high level. Its useful information is essentially yes or no.
An analogue level transmitter can provide a continuously changing measurement that allows the PLC or SCADA system to display, for example, the actual level or percentage full across the instrument's configured measurement range.
Both devices can be useful, but they provide different types of information.
Digital Inputs
A digital input module receives discrete field signals.
Common sources include:
- Push buttons
- Selector switches
- Limit switches
- Proximity sensors
- Photoelectric sensors
- Pressure switches
- Level switches
- Relay contacts
- Contactor auxiliary contacts
The exact electrical characteristics depend on the PLC or I/O platform and the field device.
24 V DC Digital Signals
24 V DC is widely used for industrial control signals, but the phrase '24 V DC input' does not by itself provide every detail required for compatibility.
When selecting or replacing equipment, check the module and sensor documentation for voltage range, current requirements, input type, wiring arrangement and switching convention.
Do not assume that every 24 V DC sensor can be connected to every 24 V DC input without checking the relevant specifications.
PNP and NPN Sensors
Three wire DC sensors are commonly described as PNP or NPN according to their output switching arrangement.
In simplified terms, a PNP sensor switches the positive supply toward the load when active, while an NPN sensor switches toward the negative or common side.
The PLC input configuration and field wiring must be compatible with the sensor output type. Terminology such as sourcing and sinking is also used in this context and can cause confusion if the reference point is not clearly stated.
When replacing a sensor, record the exact output type rather than ordering only by sensing technology and physical size.
Digital Outputs
A PLC digital output provides an on or off control command to external equipment.
It can be used, directly or through suitable interface devices, for functions such as:
- Energising a relay
- Operating a contactor coil
- Controlling a solenoid valve
- Operating an indicator lamp
- Sending a run command to a VSD
- Activating an alarm
Relay, Transistor and Triac Outputs
Digital output modules are not all electrically identical.
Relay outputs use electromechanical contacts and can provide useful isolation and switching flexibility, subject to their ratings and expected switching life.
Transistor outputs are solid state DC outputs and can provide fast switching, but polarity, voltage and current ratings must be observed.
Triac outputs are solid state outputs intended for suitable AC loads.
When replacing a PLC output module, the output technology matters. Matching only the number of output points is not sufficient.
Common Analogue Signal Types
Industrial analogue systems use several standard signal ranges. Two of the most familiar are:
- 4 to 20 mA current signals
- 0 to 10 V voltage signals
Other ranges also exist, including 0 to 20 mA, 1 to 5 V, plus or minus 10 V and application specific signals.
The transmitter, PLC input and configuration must all agree on the signal type and range.
Why Is 4 to 20 mA So Common?
The 4 to 20 mA current loop is widely used in industrial instrumentation.
One important feature is the use of a non zero lower measurement value. In a conventionally scaled loop, 4 mA represents the bottom of the configured measurement range and 20 mA represents the top.
This provides a 'live zero' that can help distinguish a valid low process measurement from certain fault conditions such as an open circuit, although actual fault signalling and detection depend on the transmitter, receiving equipment and configuration.
Current loops are also well suited to many industrial installations because loop current is less directly affected by wiring voltage drop than a simple voltage measurement would be, within the permitted loop voltage and load limits.
We cover 4 to 20 mA loops in more detail in a separate Knowledge Centre guide.
How Does Analogue Scaling Work?
The electrical signal has to be translated into an engineering value.
Suppose a pressure transmitter is configured for 0 to 10 bar with a 4 to 20 mA output. Under ideal linear scaling:
- 4 mA corresponds to 0 bar
- 12 mA corresponds to approximately 5 bar
- 20 mA corresponds to 10 bar
The PLC, indicator or control system must be configured with the correct scaling to display and use the process value accurately.
0 to 10 V Signals
Voltage signals such as 0 to 10 V are also common in automation and control equipment.
They can be used for speed references, position commands and measurement signals, depending on the equipment.
Voltage signals require appropriate attention to wiring, reference potential, input impedance, electrical noise and cable length. The manufacturer's wiring recommendations should be followed.
Analogue Inputs
An analogue input module converts an incoming electrical signal into a value the PLC or controller can process.
When specifying an analogue input, important questions include:
- Is the signal current or voltage?
- What is the signal range?
- How many channels are required?
- Are the channels isolated?
- What resolution and accuracy are required?
- How is the transmitter powered?
- What wiring arrangement is used?
Specialised inputs can also be required for devices such as resistance temperature detectors and thermocouples.
Analogue Outputs
Analogue outputs allow a controller to provide a variable command rather than only an on or off state.
Typical applications include:
- Providing a speed reference to a VSD
- Commanding a control valve position
- Providing a setpoint to another controller
- Controlling compatible process equipment
Again, the receiving equipment and output module must support the same electrical signal type.
Two Wire, Three Wire and Four Wire Transmitters
Instrumentation is often described according to its wiring and power arrangement.
A two wire transmitter commonly uses the same pair of conductors for loop power and the 4 to 20 mA measurement signal. Three wire and four wire instruments use different power and signal arrangements.
These descriptions should not be treated as interchangeable. When selecting a replacement transmitter, confirm the original wiring arrangement, supply requirements and output type.
Digital Signals Are Not Necessarily Communications
The word digital can cause confusion.
A simple digital I/O signal is a discrete electrical state. A digital communications network, such as Modbus TCP or PROFINET, transfers structured data between devices.
Both are digital technologies, but they are not the same thing.
A PLC may receive a motor run status through a single digital input, or it may receive extensive operating data from a VSD over an industrial communications network.
Hardwired Signals vs Networked Data
Modern automation systems often combine traditional hardwired I/O with industrial communications.
Hardwired signals remain useful for simple status, control and interface functions. Networked devices can provide much more information through one communications connection.
The appropriate architecture depends on reliability requirements, diagnostics, cost, existing infrastructure, safety considerations and system design.
Signal Isolation
Electrical isolation can be important in instrumentation and control systems.
Depending on the design, isolation can help manage differences in electrical potential, reduce unwanted current paths and protect equipment interfaces.
Some PLC modules provide channel or group isolation, while other applications use external signal isolators or conditioners.
The required isolation should be determined from the system architecture and manufacturer specifications.
Electrical Noise and Signal Quality
Industrial environments contain motors, VSDs, contactors, switching equipment and power cables that can generate electromagnetic interference.
Good signal performance can depend on:
- Correct cable type
- Appropriate screening or shielding
- Correct earthing practices
- Separation of signal and power wiring
- Proper panel layout
- Following equipment manufacturer recommendations
Signal problems should not automatically be blamed on the sensor or PLC module. Wiring and installation conditions can be equally important.
Common Selection Mistakes
Many incorrect automation purchases happen because the description is too broad.
Examples include ordering:
- A '24 V sensor' without specifying PNP or NPN output
- An 'analogue module' without specifying current or voltage input
- A PLC output module without checking whether it is relay, transistor or triac
- A replacement transmitter without confirming its measurement range
- A 4 to 20 mA instrument without confirming its power and wiring arrangement
- An I/O module based only on the number of channels
Exact manufacturer part numbers are extremely valuable when replacing installed equipment.
A Practical Planning Checklist
For sensors, transmitters or PLC I/O equipment, provide as much of the following as possible:
- Manufacturer
- Exact model or part number
- Supply voltage
- Digital or analogue signal type
- Signal range
- PNP or NPN where applicable
- Number of channels where applicable
- Measurement range for instrumentation
- Process connection where applicable
- Electrical connection or connector type
- Environmental or IP rating requirements
- Photographs of existing equipment and labels
If you are replacing a PLC module, include the PLC family, CPU model and neighbouring module part numbers where possible.
If you have an exact manufacturer and part number, include it with your enquiry. If the equipment is already installed and the part number is unclear, send clear photographs of the product label, wiring arrangement and wider installation where safe and practical.
Technical Note
This article provides general automation information. Signal wiring, PLC I/O configuration, instrumentation, electrical isolation and control system design must be verified for the specific equipment and application. Installation and modifications should follow manufacturer instructions, applicable standards and site procedures and should be undertaken by appropriately competent personnel where required.
Related catalogue examples: PLCs & Programmable Controllers. For an equipment enquiry, contact Softcore Group Industrial Solutions at info@softcoregroup.co.za.