How to Choose the Right PLC for Your Application

Choosing a Programmable Logic Controller is not simply a matter of selecting the PLC with the fastest processor or the lowest purchase price.

The correct controller must match the actual requirements of the machine or process, including I/O count, processing speed, memory, communications, environmental conditions, safety requirements, future expansion, software ecosystem, and lifecycle cost.

This programmable logic controllers selection guide explains the main factors engineers, system integrators, plant teams, and machine builders should evaluate before selecting and sizing a PLC.

A small standalone machine may only require a compact PLC with built-in I/O. A high-speed packaging line may need motion control and fast deterministic processing. A large production system may require distributed I/O, multiple industrial networks, redundancy, safety functions, and integration with SCADA or MES.

The goal is not to buy the biggest PLC available. The goal is to select a controller that fits the application today while providing enough headroom for realistic future requirements.

Why PLC Selection Matters

Selecting an undersized PLC can cause problems such as:

• Insufficient I/O capacity
• Slow program execution
• Limited communication ports
• Memory shortages
• Poor expansion capability
• Difficulty adding new machines
• Limited integration with drives or safety systems

Selecting a controller that is much larger than necessary can also increase:

• Hardware cost
• Engineering complexity
• Spare-parts cost
• Training requirements
• Software licensing cost

A good PLC selection process therefore balances technical capability, scalability, maintainability, and total cost.

Major PLC manufacturers themselves organize controller selection around criteria such as installation environment, performance, application size, I/O requirements, networking, and technology functions.

Start by Defining the Application

Before comparing PLC models, document what the automation system must do.

Define the Machine or Process

Ask questions such as:

• Is it a standalone machine?
• Is it part of a production line?
• Does it control continuous or batch processes?
• Are high-speed operations involved?
• Does it require motion control?
• Will robots or vision systems be connected?
• Does it need remote I/O?
• Will production data be sent to SCADA, MES, or cloud systems?

Classify the Application Size

A practical starting point is to divide projects into three broad groups.

Small Application

Examples:

• Simple conveyor
• Pump control panel
• Small packaging machine
• Automatic gate
• Basic material handling system

Typical requirements may include:

• Limited digital I/O
• Few analog signals
• Basic Ethernet communication
• Simple HMI
• Limited expansion

Medium Application

Examples:

• Assembly machine
• Packaging line
• Multiple conveyor zones
• Food processing skid
• Automated warehouse subsystem

Typical requirements may include:

• More I/O
• Several drives
• Remote I/O
• HMI or SCADA
• Industrial Ethernet
• Data logging
• Multiple machine sequences

Large or Complex Application

Examples:

• Automotive production line
• Process plant
• Large material handling system
• Multi-machine manufacturing cell
• Plant-wide control architecture

Requirements may include:

• Large distributed I/O
• Multiple networks
• High-speed processing
• Advanced motion
• Redundancy
• Safety
• SCADA integration
• MES connectivity

PLC Selection Steps

Step 1: Calculate the Required I/O

I/O capacity is one of the most important PLC sizing parameters.

Create an I/O list before selecting the controller.

Count Digital Inputs

Examples include:

• Push buttons
• Limit switches
• Proximity sensors
• Photoelectric sensors
• Emergency status signals
• Motor feedback
• Door switches

Count Digital Outputs

Examples include:

• Relays
• Contactors
• Solenoid valves
• Indicator lamps
• Buzzers
• Motor start commands

Count Analog Inputs

Typical analog signals include:

• Temperature
• Pressure
• Flow
• Level
• Load
• Position

Count Analog Outputs

Analog outputs may control:

• VFD speed reference
• Control valves
• Actuators
• Process setpoints

Include Special I/O

The application may also require:

• High-speed counters
• Encoder inputs
• Pulse outputs
• Temperature modules
• Load-cell modules
• Safety I/O
• Motion-control modules

Rockwell Automation's PLC hardware guidance, for example, recommends considering not only the amount and type of I/O but also voltage range, isolation, current requirements, noise, distance, and where I/O is physically concentrated.

Add Spare I/O Capacity

Avoid choosing a system that is already at its maximum I/O capacity on day one.

A reasonable engineering approach is to provide spare capacity for likely additions such as:

• Extra sensors
• Additional cylinders
• New conveyor zones
• New quality checks
• New production variants

Future expansion should be planned based on realistic project expectations rather than an arbitrary percentage.

Step 2: Determine CPU Performance Requirements

Not every application requires a high-performance processor.

A basic sequence-controlled machine may have relatively modest CPU requirements, while motion, robotics, high-speed packaging, or large communication loads can demand significantly more processing capability.

Evaluate Scan-Time Requirements

The PLC repeatedly:

1. Reads inputs
2. Executes logic
3. Updates outputs
4. Handles communications and system functions

The time required to complete these operations affects response performance.

Fast-changing applications may require shorter execution times.

Applications That May Need Higher CPU Performance

Consider a more powerful processor when the system involves:

• High-speed packaging
• Servo synchronization
• Motion control
• Large numbers of PID loops
• Extensive mathematical calculations
• High-speed counters
• Large data arrays
• Complex communication
• Multiple robots or intelligent devices

Manufacturer comparison data often includes instruction execution time or processing speed because these values can differ significantly between controller families.

Mitsubishi Electric, for example, publishes PLC selection tables comparing processing speed, program size, memory, I/O capacity, interfaces, and network compatibility across controller series.

Step 3: Check Program and Data Memory

PLC memory requirements have increased as industrial applications become more connected.

Memory may be required for:

• Program logic
• Tags and variables
• Recipes
• Alarm information
• Production counters
• Historical values
• Data logging
• Communication buffers
• Motion data

Do Not Size Memory Only for Today's Program

A machine may initially require a relatively small program but later gain:

• Additional product recipes
• New stations
• Extra diagnostics
• SCADA tags
• Traceability functions
• Production data collection

Choose a controller that provides adequate memory headroom for expected development.

Step 4: Identify Communication Requirements

Communication capability is now a major PLC selection factor.

Before purchasing a controller, list every device and system that must communicate with it.

Examples include:

• HMI
• SCADA
• VFDs
• Servo drives
• Remote I/O
• Robots
• Vision systems
• Barcode readers
• Energy meters
• MES
• Edge gateways

Common Industrial Networks and Protocols

Depending on the application, you may require:

• PROFINET
• EtherNet/IP
• Modbus TCP
• Modbus RTU
• PROFIBUS
• OPC UA
• CAN-based networks
• Vendor-specific industrial networks

Check Built-In Ports

A compact PLC with onboard Ethernet and serial communication may eliminate the need for additional modules.

However, larger systems may require dedicated communication modules or multiple network interfaces.

Check Device Compatibility

Do not assume that every Ethernet-enabled device communicates directly with every PLC.

Verify:

• Supported protocols
• Required communication modules
• Device profiles
• Driver availability
• Maximum connection counts
• Data update rates

Step 5: Evaluate Motion-Control Requirements

If the application includes servo motors or coordinated motion, motion capability should be considered early.

Basic Motion

Applications may include:

• Indexing tables
• Simple positioning
• Cut-to-length machines
• Pick-and-place mechanisms

Advanced Motion

More demanding applications include:

• Electronic gearing
• Electronic cams
• Multi-axis synchronization
• High-speed packaging
• Robotics

The number of motion axes and synchronization requirements can strongly affect controller selection.

Step 6: Determine Safety Requirements

Standard machine control and functional safety are different design requirements.

Applications involving personnel hazards may require:

• Safety PLCs
• Safety I/O
• Emergency-stop circuits
• Guard-door monitoring
• Light curtains
• Safe drive functions

Some controller families offer both standard and fail-safe CPU versions.

For example, Siemens' controller selection information includes fail-safe versions for safety-related applications within its PLC portfolio.

Do Not Select Safety Hardware Only by I/O Count

Safety selection should also consider:

• Required safety functions
• Machine risk assessment
• Applicable standards
• Architecture
• Diagnostics
• Safety-rated field devices

Functional-safety design should be carried out by appropriately qualified engineers.

Step 7: Consider the Installation Environment

A PLC installed inside a clean control cabinet has different requirements from a controller mounted close to a machine in a harsh factory area.

Environmental Factors

Check:

• Ambient temperature
• Humidity
• Dust
• Water exposure
• Vibration
• Shock
• Electrical noise
• Corrosive atmosphere
• Altitude

Installation Location

PLC architectures may support:

• Centralized cabinet installation
• Decentralized cabinet installation
• Field-mounted distributed systems

Siemens' current controller selection guide, for example, differentiates between central IP20 installation, decentralized IP20 installation, and decentralized IP65/67 installation close to the machine.

Step 8: Plan for Future Expansion

Expansion is frequently underestimated.

A machine may later require:

• More sensors
• Additional stations
• New product variants
• Vision inspection
• Traceability
• Additional drives
• SCADA integration
• Remote diagnostics

Check Expansion Limits

Before selecting the PLC, check:

• Maximum local I/O
• Maximum remote I/O
• Number of expansion modules
• Communication module limits
• Maximum network connections
• Memory capacity
• Supported motion axes

Choosing an expandable architecture can prevent expensive redesign later.

Step 9: Evaluate Programming Software and Engineering Ecosystem

Hardware price is only part of the buying decision.

The engineering environment can significantly affect project cost.

Consider:

• Programming software
• License cost
• Programming languages
• Simulation capability
• Diagnostics
• Online monitoring
• Version management
• HMI integration
• Motion configuration
• Safety programming
• Team familiarity

Existing Plant Standardization Matters

If a factory already uses one PLC ecosystem extensively, maintaining compatibility can reduce:

• Training requirements
• Spare-parts inventory
• Engineering time
• Troubleshooting complexity

However, standardization should not override critical application requirements.

Step 10: Evaluate Reliability and Maintainability

Industrial control systems may remain in service for many years.

Before selecting a platform, consider:

• Product lifecycle
• Spare-part availability
• Local technical support
• Replacement strategy
• Backup and restore tools
• Diagnostic capability
• Remote troubleshooting options

Diagnostics Can Reduce Downtime

Useful controller diagnostics may include:

• I/O faults
• Network faults
• Module status
• Device communication status
• Program errors
• System events

Better diagnostics can make troubleshooting faster and reduce machine downtime.

Step 11: Calculate Total Cost of Ownership

The lowest controller price does not always result in the lowest project cost.

Include Hardware Costs

Consider:

• CPU
• Power supply
• I/O
• Communication modules
• Memory
• Safety modules
• Network equipment

Include Engineering Costs

Also consider:

• Programming software
• Licenses
• Engineering hours
• Commissioning
• Training
• Documentation

Include Lifecycle Costs

Long-term costs may include:

• Spare parts
• Maintenance
• Future expansion
• Software upgrades
• Downtime
• Migration requirements

A controller that costs slightly more initially may be more economical if it reduces additional modules, engineering time, or future redesign.

PLC Selection Checklist

Use this checklist before finalizing a PLC.

Example 1: PLC for a Small Conveyor System

Consider a conveyor with:

• 10 proximity sensors
• 6 push buttons
• 4 motor feedback signals
• 5 motor commands
• 3 solenoid valves
• 1 HMI

A compact PLC may be sufficient if it provides:

• Enough built-in or expandable digital I/O
• Ethernet communication
• HMI connectivity
• Modest spare capacity

There may be little benefit in purchasing a high-end modular PLC for this application unless major expansion is planned.

Example 2: PLC for an Automated Packaging Machine

Suppose the machine includes:

• 50+ digital I/O
• Several analog sensors
• Three servo axes
• Two VFDs
• HMI
• Vision system
• Barcode scanner
• Production data collection

Selection should consider:

• CPU execution speed
• Motion capability
• Communication performance
• Memory
• Expansion
• Diagnostics

A basic micro PLC may not be appropriate even if its raw I/O count appears sufficient.

Example 3: PLC for a Large Production Line

A large production system may include:

• Hundreds of distributed I/O points
• Multiple PLCs
• Remote I/O stations
• Servo systems
• Robots
• Safety networks
• SCADA
• MES
• Production databases

In this situation, controller selection depends heavily on:

• Network architecture
• CPU loading
• Connection limits
• Redundancy requirements
• Safety
• Diagnostics
• Plant-wide standardization
• Scalability

The PLC should be selected as part of the complete control architecture rather than as an isolated component.

Common PLC Selection Mistakes

Choosing Only by I/O Count

Two controllers with similar I/O capacity may have very different processing, networking, memory, and motion capabilities.

Ignoring Future Expansion

Using almost all available capacity at commissioning leaves little room for improvements.

Ignoring Communication Compatibility

A PLC may require additional modules to communicate with drives, robots, or legacy equipment.

Oversizing Without a Reason

Buying the most powerful PLC available can increase cost without creating meaningful value.

Ignoring Engineering Software

Programming software, licenses, diagnostics, and team familiarity directly affect implementation and maintenance cost.

Compact PLC vs Modular PLC

Choose a Compact PLC When

A compact PLC may suit:

• Small machines
• Limited I/O
• Simple sequences
• Basic communications
• Cost-sensitive applications

Advantages may include:

• Smaller footprint
• Lower hardware cost
• Integrated I/O
• Simpler configuration

Choose a Modular PLC When

A modular PLC may be more suitable for:

• Larger machines
• Distributed systems
• Large I/O counts
• Advanced networking
• Specialized I/O
• Motion applications
• Future expansion

Its main advantage is flexibility.

PLC Selection Decision Framework

A practical decision sequence is:

Application Requirements

↓

Create I/O List

↓

Determine CPU and Memory Requirements

↓

Identify Communication Protocols

↓

Check Motion and Safety Requirements

↓

Check Environment

↓

Estimate Future Expansion

↓

Evaluate Engineering Software

↓

Compare Lifecycle and Total Cost

↓

Select PLC Family and CPU

This approach reduces the risk of selecting a controller based only on brand familiarity or purchase price.

Conclusion

Choosing the right PLC requires a structured evaluation of the complete industrial application.

A reliable programmable logic controllers selection guide should therefore look beyond basic I/O count and compare:

• Application complexity
• I/O quantity and type
• CPU processing requirements
• Program and data memory
• Communication protocols
• Motion requirements
• Safety requirements
• Installation environment
• Expansion capacity
• Engineering software
• Maintenance
• Lifecycle cost

For a simple standalone machine, a compact PLC may provide everything required at a reasonable cost.

For complex production equipment, advanced motion systems, distributed automation, or plant-wide control, a scalable modular controller may provide better long-term value.

The most effective selection process can be summarized as:

A correctly sized PLC gives the automation system enough capability to perform reliably without paying for unnecessary complexity.