Programmable Logic Controllers, usually called PLCs, are at the heart of countless industrial automation systems. They control machinery, monitor processes, respond to sensors and coordinate equipment across factories, mines, water systems, utilities and infrastructure.
You do not need to be a PLC programmer to benefit from understanding what a PLC does. For purchasing, maintenance and operations teams, a basic understanding makes it easier to identify equipment, request replacement hardware and communicate accurately with automation specialists.
What Is a PLC?
A Programmable Logic Controller is an industrial computer designed to perform control functions in machinery and process systems.
Instead of relying entirely on hardwired relay logic, a PLC uses a program to determine how the control system should respond to information received from field devices.
In simple terms, a PLC receives information, processes that information according to programmed logic and then controls outputs.
The Basic PLC Control Cycle
A simplified PLC system can be understood in three stages.
1. Read the Inputs
The PLC receives information from devices such as push buttons, selector switches, limit switches, proximity sensors, pressure switches, level switches, transmitters and other field instrumentation.
2. Execute the Program
The PLC processor evaluates its control program using the current input information and internal data.
3. Update the Outputs
Based on the program logic, the PLC changes outputs that can operate devices such as contactors, relays, solenoid valves, indicator lamps, alarms and motor control equipment.
This sequence is repeated continuously and rapidly while the controller is operating.
A Simple PLC Example
Consider a basic water tank system.
A level sensor tells the PLC whether the tank level is low. The PLC program evaluates that information together with other conditions, such as whether the pump is available and whether any fault is active.
If the programmed conditions are satisfied, the PLC can command the pump to run. When the required level is reached, the PLC can stop the pump.
A real installation may include additional interlocks, alarms, dry run protection, pressure monitoring, duty and standby pumps, telemetry and operator controls, but the basic principle remains the same: inputs are evaluated by programmed logic to determine outputs.
What Are PLC Inputs?
Inputs provide the PLC with information about the machine or process.
They are commonly divided into digital and analogue inputs.
Digital Inputs
A digital input generally represents an on or off state.
Examples include:
- Push button pressed or not pressed
- Limit switch made or open
- Motor overload healthy or tripped
- Proximity sensor detecting an object or not
- Pressure switch active or inactive
Analogue Inputs
An analogue input represents a variable measurement rather than simply on or off.
Examples can include:
- Pressure
- Level
- Flow
- Temperature
- Speed reference
Common industrial analogue signals include 4 to 20 mA and 0 to 10 V, although the correct signal type depends on the instrumentation and PLC hardware.
What Are PLC Outputs?
Outputs allow the PLC to command or influence external equipment.
Digital Outputs
Digital outputs can control devices such as:
- Contactors
- Interposing relays
- Solenoid valves
- Indicator lamps
- Buzzers and alarms
Analogue Outputs
Analogue outputs can provide variable control signals. For example, a PLC might send an analogue speed reference to a variable speed drive or a position command to another control device.
What Is Inside a PLC System?
The exact architecture differs between manufacturers and PLC families, but a system can include several components.
CPU or Processor
The CPU executes the control program and manages the PLC's internal operations and communications.
Power Supply
The PLC system requires a suitable power supply. Depending on the product family, the power supply may be integrated or installed as a separate module.
Input and Output Modules
I/O modules connect field signals to the control system. These can include digital input, digital output, analogue input, analogue output and specialist modules.
Communications Interfaces
Modern PLCs commonly communicate with HMIs, SCADA systems, VSDs, remote I/O, instrumentation and other controllers.
Memory
The controller stores the application program, configuration and operating data. Memory arrangements differ between PLC platforms.
Compact PLCs vs Modular PLCs
Some PLCs are compact devices with the processor and a fixed quantity of I/O built into one unit. Others use a modular architecture where the CPU, power supply, I/O and communications modules are selected separately.
Compact PLCs can suit smaller machines and control applications. Modular systems can provide greater flexibility and expansion for larger or more complex installations.
The correct architecture depends on the required I/O count, communications, redundancy, processing capability, environmental requirements and future expansion.
What Is Remote I/O?
Not every field signal needs to be wired directly back to the main PLC panel.
Remote I/O allows input and output modules to be installed closer to the field equipment and communicate with the controller over an industrial network.
This can reduce long cable runs and simplify distributed installations, although network architecture, environmental conditions and system availability requirements need to be considered.
How Does a PLC Communicate With a VSD?
A PLC can control a variable speed drive in several ways.
A simple system might use digital outputs for start and stop and an analogue output for the speed reference. More integrated systems can use industrial communications.
Depending on the PLC and VSD, protocols may include Modbus RTU, Modbus TCP, PROFINET, EtherNet/IP, PROFIBUS, CANopen or other supported networks.
Network communication can allow the PLC to send commands and speed references while receiving operating values, status information and fault data from the drive.
PLC, HMI and SCADA: What Is the Difference?
These terms are often used together but refer to different parts of an automation system.
PLC: Executes control logic and interfaces with field equipment.
HMI: Human Machine Interface. Provides an operator interface for viewing information and issuing permitted commands.
SCADA: Supervisory Control and Data Acquisition. Typically provides broader supervisory monitoring, control, alarming, data collection and visualisation across a process or multiple systems.
A PLC can operate without an HMI or SCADA system, although many modern installations use them together.
Why Are PLCs Used Instead of Only Relays?
Traditional relay control remains useful for many functions, but complex hardwired logic can require large numbers of relays, timers and wiring changes.
A PLC allows much of the control logic to be implemented in software. This can provide advantages such as:
- More complex control sequences
- Timers and counters
- Mathematical operations
- Process calculations
- Diagnostics
- Data handling
- Communications
- Easier logic changes where appropriately managed
This does not mean every control function should automatically be moved into a PLC. Safety, protection and system design requirements must still be considered appropriately.
Common PLC Applications
PLCs are used across a very broad range of industrial applications, including:
- Conveyor systems
- Pump stations
- Water and wastewater treatment
- Reservoir and borehole control
- Packaging machinery
- Production lines
- Material handling
- Process plants
- Mining equipment
- HVAC and building plant
- Machine automation
- Telemetry and remote infrastructure
What Is a PLC Program?
The PLC program contains the instructions that determine how the controller behaves.
PLC programming environments can support languages defined for industrial control, including ladder diagram, function block diagram and structured text, depending on the controller platform.
The software, programming method and project structure differ between manufacturers and product families.
What Is Ladder Logic?
Ladder logic is a graphical programming approach whose appearance was influenced by electrical relay control diagrams.
It represents control conditions and actions in a format that can be familiar to electrical and automation personnel.
Modern PLC applications can combine ladder logic with other programming methods where supported and appropriate.
What Happens When a PLC Fails?
A PLC related fault does not automatically mean that the CPU itself has failed.
Problems can originate from:
- Power supply failure
- I/O module failure
- Network or communications problems
- Field wiring
- Faulty sensors or actuators
- Program or configuration issues
- Memory or storage problems
- Environmental damage
- The PLC CPU itself
Fault diagnosis should therefore be carried out systematically by appropriately competent personnel rather than replacing the controller without confirming the cause.
Replacing an Existing PLC
Replacing a PLC can be substantially more complex than replacing a simple electrical component because the controller contains software and forms part of a wider automation architecture.
When selecting replacement PLC hardware, provide as much of the following as possible:
- Manufacturer
- Exact CPU model or part number
- Power supply model
- All I/O module part numbers
- Communications module part numbers
- Rack or backplane details where applicable
- HMI and SCADA interfaces
- Connected industrial networks
- Photographs of the complete PLC rack or panel
- Existing electrical drawings
- Program backup availability
- Programming software and version where known
Why the Program Backup Matters
A replacement PLC CPU does not automatically contain the machine or plant program.
If an existing CPU fails, access to a current program backup can be critical to restoring the system. Depending on the platform, hardware replacement can also require firmware compatibility, configuration, communications setup and software conversion.
For important plant and machinery, maintaining controlled backups of PLC, HMI, VSD and other automation configurations should form part of the site's maintenance and asset management practices.
Obsolete PLC Hardware
Industrial control systems can remain in service for decades, which means maintenance teams frequently encounter PLC families that are discontinued or approaching end of life.
An obsolete module may sometimes still be available through specialist supply channels, but availability alone does not necessarily make continued dependence on legacy hardware the best long term strategy.
Depending on the system, options may include finding an exact replacement, using a manufacturer supported successor, migrating individual components or planning a wider control system upgrade.
Migration decisions can affect software, I/O, communications, wiring, HMIs, SCADA, drives and other connected equipment, so they require appropriate technical assessment.
Can You Replace a PLC With Another Brand?
Not as a simple hardware swap in most cases.
Different PLC platforms use different hardware architectures, programming environments, communications configurations and software structures. Moving to another manufacturer can require control program redevelopment, I/O redesign, network changes, HMI changes and recommissioning.
For purchasing purposes, a different brand should therefore not be treated as an equivalent substitute merely because it has a similar number of inputs and outputs.
A Practical Planning Checklist
If you need PLC hardware, the following information provides a useful starting point:
- Manufacturer
- Exact part number for each required component
- Quantity
- Whether the requirement is new or replacement equipment
- Photographs if replacing installed hardware
- Required delivery location
- Required delivery date where known
If you do not know the part numbers, send clear photographs of the labels on the CPU and modules together with an image of the complete PLC assembly.
Products included in our active online catalogue can be purchased or quoted through the Softcore Group Industrial Solutions store. Other brands, models, obsolete components and project specific requirements can be quoted on request, with pricing, availability and lead time confirmed separately.
Where a PLC requirement involves programming, migration, system integration, electrical design or commissioning, the work should be undertaken or verified by appropriately qualified automation and engineering professionals.
Technical Note
This article provides general automation information. PLC hardware selection, software changes, safety functions, system architecture, installation and commissioning must be assessed for the specific machine or process and completed in accordance with manufacturer requirements, applicable standards and site procedures.
Related catalogue examples: PLCs & Programmable Controllers. For an equipment enquiry, contact Softcore Group Industrial Solutions at info@softcoregroup.co.za.