Soft PLC

A Soft PLC is a software-based implementation of a PLC that runs on generic hardware such as an industrial PC, server, Hypervisor or edge platform. Instead of a dedicated hardware Controller, the control logic runs on an operating system such as Windows or Linux, often combined with a Real-time extension or RTOS.

Soft PLCs are increasingly used within Industry 4.0, Edge Computing, Industrial Internet of Things and converged IT OT Convergence architectures. They combine classic industrial control with flexibility from the IT world, including Virtualisation, containerisation, central Lifecycle Management processes and integration with modern data platforms.

In OT environments, Soft PLCs are used for machine control, Process Automation, Motion Control, data acquisition and couplings with SCADA, MES, Historian systems and cloud platforms. The Architecture, however, also brings additional requirements around real-time behaviour, Latency, Cybersecurity and availability.

⚙️ How a Soft PLC works

A Soft PLC consists of software components that perform the same logical functions as a classic hardware-based PLC:

• cyclic task execution
• IO scanning
• process logic
• timers and counters
• communication with field equipment
• protocol stacks
• diagnostics
• alarm handling

The Soft PLC runtime runs on top of:

• a standard operating system
• a real-time kernel
• a hypervisor
• an edge runtime
• a container platform

The application reads input signals from IO, processes logic and writes outputs back to actuators or field equipment.

Typical scan cycle:

1. Input reading
2. Program execution
3. Diagnostics
4. Output update
5. Communication tasks

The Cycle Time varies depending on:

• CPU capacity
• real-time scheduling
• network load
• Virtualisation layer
• protocol use
• motion control requirements

Unlike classic PLCs, Soft PLCs often share resources with other applications, which CAN affect Deterministic Behaviour.

🏗️ Architecture of Soft PLC solutions

A Soft PLC usually consists of multiple layers:

Commonly used platforms:

• Beckhoff TwinCAT
• CODESYS Control
• Siemens WinAC
• Phoenix Contact PLCnext
• B&R Automation Runtime
• Schneider EcoStruxure Control Expert

Soft PLCs often run on:

• industrial PCs
• edge gateways
• virtual machines
• blade servers
• container platforms
• high-availability clusters

🧠 Real-time behaviour and determinism

Within OT environments, deterministic behaviour is essential. A Soft PLC must deliver predictable response times regardless of system load or network activity.

Important parameters:

Soft PLCs therefore often use:

• real-time Linux
• RTX/IntervalZero
• PREEMPT_RT
• dedicated CPU cores
• isolated scheduling
• real-time Ethernet

For motion control and high-speed synchronisation, protocols such as the following are used:

• EtherCAT
• ProfiNET
• Powerlink
• SERCOS III
• TSN

Real-time performance is crucial within:

• Robotics
• packaging machines
• Vision systems
• high-speed conveyors
• turbine control
• energy infrastructures

🔌 Communication protocols

Soft PLCs typically support a wide range of industrial protocols.

North-south communication

Communication towards SCADA, MES and IT systems:

• OPC UA
• MQTT
• HTTP
• HTTPS
• SNMP
• Modbus TCP

East-west communication

Machine-to-machine and controller communication:

• ProfiNET
• Ethernet IP
• EtherCAT
• S7 Comm
• Modbus
• DNP3

Legacy integration

Many Soft PLCs support older interfaces:

• RS-232
• RS-485
• Profibus
• Modbus RTU

This enables hybrid OT environments in which Legacy Systems are coupled to modern IIoT platforms.

🖥️ Virtualisation and containerisation

An important benefit of Soft PLCs is their virtualisability.

Soft PLCs can run within:

• VMware
• Hyper-V
• KVM
• Docker
• Kubernetes edge clusters

Benefits:

• faster deployment
• easy scalability
• Snapshot-based Recovery
• workload consolidation
• central monitoring
• lifecycle management

Applications:

Drawbacks:

• additional latency
• scheduler interference
• hypervisor overhead
• more complex troubleshooting
• larger attack surface

Within critical processes, physical PLC hardware often remains necessary because of certification, SIL requirements or hard real-time behaviour.

🔒 Cybersecurity implications

Soft PLCs enlarge the attack surface compared to traditional PLCs.

Where classic PLCs often use proprietary hardware, Soft PLCs run on standard IT platforms with:

• general operating systems
• network services
• standard drivers
• remote management
• user accounts
• APIs

This creates additional risks.

Attack surface

Important threats:

• Malware
• Ransomware
• privilege escalation
• hypervisor attacks
• container escape
• credential theft
• unpatched OS components
• lateral movement

Hardening measures

Important Security measures:

• Application Whitelisting
• Hardening
• Network Segmentation
• Industrial Firewall
• MFA
• RBAC
• Logging
• Security Monitoring
• Patch Management
• Secure Boot

In addition, standards such as IEC 62443 are important for secure architecture and lifecycle management.

🏭 Soft PLC in industrial environments

Manufacturing

Within discrete manufacturing, Soft PLCs are deployed for:

• assembly lines
• Robot cells
• vision integration
• AGV control
• traceability systems

Integration with MES, ERP and Historian platforms is easier than with traditional PLCs.

Energy supply

Within energy environments, Soft PLCs are used for:

• substation automation
• energy management
• load balancing
• monitoring
• predictive analytics

Often combined with:

• IEC 61850
• SCADA
• IED
• GOOSE

Building Automation

Soft PLCs play a major role within:

• HVAC control
• lighting control
• energy management
• smart buildings

Integrations with:

• BACnet
• KNX
• DALI
• BMS

Water and process industry

Applications:

• pumping stations
• chemical dosing
• filtration
• Batch Control
• remote Telemetry

Here, availability and Redundancy are essential.

⚡ Comparison with traditional PLCs

🧩 Integration with OT architectures

Soft PLCs fit well within modern converged architectures.

Common integrations:

Soft PLCs often act as a bridge between classic OT and modern IT platforms.

📡 Soft PLC and Edge Computing

Within Edge Computing, Soft PLCs are becoming increasingly important.

Benefits:

• local decision-making
• low latency
• protocol conversion
• local AI processing
• filtering of Sensor data
• offline functionality

Edge-based Soft PLCs often combine:

• PLC functionality
• OPC UA server
• MQTT broker
• container runtime
• local database
• analytics engine

This creates multifunctional OT edge nodes.

🧪 Simulation and digital twins

Soft PLCs are highly suitable for simulation environments.

Applications:

• virtual FAT tests
• software-in-the-loop
• hardware-in-the-loop
• process validation
• operator training
• cyber range environments

In combination with Digital Twin concepts, engineers can:

• simulate process behaviour
• analyse performance
• test Safety logic
• reproduce faults

This speeds up commissioning and reduces downtime.

🔄 Lifecycle management

Soft PLCs increasingly follow IT-based lifecycle processes.

Important lifecycle components:

• Version Control
• CI/CD pipelines
• container deployment
• rollback procedures
• central configuration
• Vulnerability Management

This creates overlap with:

• DevOps
• Version Control
• Configuration Management
• Patch Management

At the same time, OT validation remains necessary because of impact on production processes.

⚠️ Operational risks

Soft PLCs introduce new failure modes.

Typical issues

Mitigations

• dedicated real-time cores
• redundant hosts
• isolated VLANs
• real-time monitoring
• failover clustering
• deterministic Ethernet
• OT validation procedures

🛡️ Standards and frameworks

Important standards around Soft PLCs:

For safety-critical applications, additional requirements apply around:

• certification
• real-time guarantees
• Fail-safe behaviour
• redundancy
• validation

📈 Trends and developments

Important developments around Soft PLCs:

• virtual PLCs
• cloud-native OT
• containerised control
• software-defined automation
• TSN networks
• integrated AI
• edge orchestration
• converged compute platforms

More vendors are shifting towards software-based automation platforms in which hardware abstraction is central.

This blurs the boundaries between:

• IPC
• PLC
• SCADA
• edge platforms
• OT servers
• IIoT gateways

🎯 Conclusion

Soft PLCs are an important building block within modern Industrial Automation and IT OT Convergence. They offer flexibility, scalability and strong integration capabilities with modern IT, cloud and edge platforms.

At the same time, they bring new challenges around real-time behaviour, availability, cybersecurity and lifecycle management. Within critical OT processes, design choices around determinism, segmentation, redundancy and validation remain essential.

The shift from dedicated hardware to software-defined automation will accelerate further in the coming years, especially within edge computing, virtualisation and industrial data integration.