Glossary

Soft PLC

A software-based implementation of a Programmable Logic Controller that executes standard IEC 61131-3 control logic on general-purpose computing hardware instead of proprietary physical controllers.

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DEFINITION

What is a Soft PLC?

A Soft PLC is a software-based implementation of a Programmable Logic Controller that executes standard IEC 61131-3 control logic on general-purpose computing hardware instead of proprietary physical controllers.

A Soft PLC is a software application that replicates the deterministic control functions of a traditional hardware Programmable Logic Controller on an industrial PC or edge server. It decouples the control runtime from proprietary silicon, executing IEC 61131-3 languages—such as Structured Text and Ladder Diagram—within a real-time operating system or alongside a real-time hypervisor to guarantee cycle-time determinism.

By virtualizing the control layer, a Soft PLC enables workload consolidation, where logic, HMI, and analytics run on a single platform. This architecture supports infrastructure as code practices, allowing version-controlled, repeatable deployments and seamless integration with Unified Namespace data architectures for enterprise-wide interoperability.

ARCHITECTURAL CAPABILITIES

Key Characteristics of Soft PLCs

Soft PLCs fundamentally decouple control logic from proprietary hardware, enabling a new class of flexible, scalable, and IT-integrable industrial automation systems.

Hardware Independence

A Soft PLC executes IEC 61131-3 control logic on general-purpose industrial PCs (IPCs) or edge servers rather than locked-down proprietary controllers. This abstraction eliminates vendor lock-in, allowing control engineers to select best-of-breed hardware for each application.

Runs on any x86 or ARM64 platform with a real-time operating system
Leverages standard commercial off-the-shelf (COTS) components
Simplifies spare parts management and hardware lifecycle upgrades

Real-Time Determinism

Soft PLCs achieve hard real-time performance through kernel-level scheduling and isolation. The control engine runs as a high-priority task on a PREEMPT_RT patched Linux kernel or a dedicated real-time operating system (RTOS), guaranteeing deterministic scan cycles.

Typical cycle times as low as 50 µs on modern silicon
CPU pinning dedicates physical cores exclusively to the control runtime
Eliminates jitter by bypassing non-deterministic OS scheduling

IT/OT Convergence

By running on standard computing platforms, Soft PLCs natively bridge the gap between operational technology (OT) and information technology (IT). The same hardware can simultaneously host the control engine, a Unified Namespace (UNS) client, and edge analytics.

Native support for MQTT Sparkplug and OPC UA Pub/Sub
Direct database connectivity without protocol translation gateways
Enables Infrastructure as Code (IaC) and Git-based version control for automation projects

Virtualization-Ready Architecture

Soft PLCs are designed to run as virtual machines or containers on Type-1 Hypervisors, enabling true workload consolidation. A single industrial server can host multiple isolated Soft PLC instances, an HMI server, and a predictive maintenance model simultaneously.

Supports live migration for zero-downtime maintenance
Mixed-criticality partitioning ensures safety functions remain isolated
Enables virtual commissioning against a digital twin before deployment

Scalable Fieldbus Integration

Modern Soft PLCs connect to physical I/O and drives through deterministic industrial Ethernet protocols. They leverage Time-Sensitive Networking (TSN) for guaranteed latency and Single Root I/O Virtualization (SR-IOV) to bypass hypervisor overhead for direct hardware access.

Supports EtherCAT, PROFINET IRT, and EtherNet/IP with CIP Motion
Precision Time Protocol (PTP) synchronizes distributed nodes to sub-microsecond accuracy
Software-defined I/O mapping replaces physical wiring changes

High-Availability and Fault Tolerance

Soft PLC architectures enable sophisticated redundancy patterns that are complex and costly with physical controllers. Fault tolerance (FT) is achieved through lockstep execution on redundant hosts, ensuring bumpless failover without a single scan cycle lost.

Active-active and active-standby redundancy modes
Automatic state synchronization between primary and secondary instances
Immutable infrastructure ensures configuration consistency across redundant nodes

SOFT PLC CLARIFIED

Frequently Asked Questions About Soft PLCs

Clear, technically precise answers to the most common questions engineers and CTOs ask when evaluating software-based programmable logic controllers for modern industrial automation architectures.

A Soft PLC (Software Programmable Logic Controller) is a software-based implementation of a traditional hardware PLC that executes standard IEC 61131-3 control logic on general-purpose computing hardware instead of proprietary physical controllers. It operates by running a real-time control engine as an application on an industrial PC, edge server, or virtual machine, leveraging the host's CPU, memory, and I/O subsystems to scan inputs, execute the control program, and update outputs deterministically. The Soft PLC runtime emulates the scan cycle behavior of a physical PLC—reading physical I/O via fieldbus protocols like EtherCAT or PROFINET, processing logic in languages such as Structured Text or Ladder Diagram, and writing outputs—all within a guaranteed cycle time enforced by a real-time operating system or hypervisor. Unlike traditional PLCs with fixed hardware architectures, a Soft PLC decouples the control logic from the underlying silicon, enabling workload consolidation, remote management, and seamless integration with IT systems via OPC UA or MQTT Sparkplug.

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About the author

Prasad Kumkar

CEO & MD, Inference Systems

Prasad Kumkar is the CEO & MD of Inference Systems and writes about AI systems architecture, LLM infrastructure, model serving, evaluation, and production deployment. Over 5+ years, he has worked across computer vision models, L5 autonomous vehicle systems, and LLM research, with a focus on taking complex AI ideas into real-world engineering systems.

His work and writing cover AI systems, large language models, AI agents, multimodal systems, autonomous systems, inference optimization, RAG, evaluation, and production AI engineering.

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Soft PLC

What is a Soft PLC?