IEC 61508 SIL 3 PLC Implementation — PatSnap Eureka
Implementing IEC 61508 SIL 3 in Programmable Logic Controllers
A patent-intelligence synthesis of the hardware architectures, fault-detection mechanisms, software toolchains, and safe parameterization methods used to achieve SIL 3 compliance in PLCs — drawn from 50+ patents across Siemens, ABB, Omron, and more.
Dual-CPU Voting Structures: The Canonical SIL 3 Foundation
The most foundational requirement for SIL 3 in PLCs is hardware architecture. Evidence across the patent dataset consistently points to dual-CPU implementations combined with programmable logic devices (PLDs) as the primary means of reaching SIL 3. As described by Shenyang Zhongke Bowei Automation Technology (2015), the controller adopts two SIL 3-certified CPUs to implement both a 1oo1D (one-out-of-one with diagnostics) and a 1oo2D (one-out-of-two with diagnostics) voting structure. In the 1oo1D configuration, a single CPU with sufficient diagnostic coverage can meet SIL 3, while the second CPU handles inter-controller safety communication; two such 1oo1D controllers can then be combined to realize redundant voting.
Upgrading an existing standard PLC to SIL 3 without replacing the entire platform is addressed by ABB's incremental SIL upgrade method (2010). This approach describes attaching a safety hardware unit — typically a circuit board incorporating a CPU, I/O interfaces using memory chips and FPGAs, local digital output channels, and memory shadowing functionality — to a standard controller via a backplane bus, elevating the composite system to SIL 1 through SIL 4 as defined by IEC 61508.
For safety-critical I&C systems, FPGA-implemented self-diagnosable modules with unified architecture, mutual diagnostics and self-diagnostics at both modular and system levels, and diversity technologies operating units across different clock domains directly serve SIL 3 hardware fault tolerance and diagnostic coverage requirements — as demonstrated by the Bakhmach (2023) data processing procedure patent.
ABB Switzerland's pending 2026 patent on parallel safety controller operation groups safety functions into two classes, each compiled and linked into a separate executable, then dispatched to two independent processors. This architecture directly addresses the IEC 61508 requirement that safety-related systems must operate correctly or fail only in predictable (safe) ways.
Achieving High Diagnostic Coverage for SIL 3 PFD Compliance
Achieving SIL 3 requires not only redundancy but also high diagnostic coverage of dangerous failures. These patent-evidenced mechanisms address the full diagnostic stack — from safety state triggers to analog output verification.
Safety State Trigger — Bypassing the Application Controller
A dedicated safety state trigger monitors all input signals from a supervisory controller, independently identifies fault signals, and bypasses the application controller entirely to send a safety state signal directly to the controlled device. The application controller itself does not need to meet the highest integrity level (SIL 4 or ASIL D), because the safety path is handled by the separate safety state trigger that does satisfy those requirements.
Reduces SIL burden on main CPUEmbedded I/O Safety Function — Reduced Safety Reaction Time
An internal evaluation component embedded directly within the I/O module receives input states, evaluates them, and executes safety functions locally — removing latency from the round-trip through the central controller. This embedded approach is particularly relevant in SIL 3 systems where the safety response time budget is tight.
Tight response time budgetSoft Error Aggregation — Proactive PFD Margin Management
This method aggregates soft error data from multiple safety PLCs, compares actual versus expected soft error rates per I/O module type, predicts future rates, and triggers actions when deviations exceed thresholds — enabling proactive management of latent failures before they accumulate to exceed SIL 3 PFD limits. Soft errors including bit flips in memory must be accounted for in the PFD calculation.
Latent failure preventionFail-Safe Analog Output — Closed-Loop Readback Verification
A three-step verification loop: digital-to-analog conversion, readback conversion using fail-safe criteria into a fault-safe digital value, comparison with the original digital output, and a safety action if deviation or reliability criteria are violated. This closed-loop readback is a direct implementation of the IEC 61508 requirement for output monitoring in safety-related systems.
IEC 61508 output monitoringFail-Safe Counter Module — SIL 3 / CAT 4 / PL e Rated
A standalone SIL 3 / CAT 4 / PL e rated module capable of monitoring speed, direction, and stop-position deviation independently of any external PLC logic. The module executes its Safety Monitoring functions entirely within its own firmware, reporting violations to the customer's safety control program, which then commands the safe state — demonstrating distributed safety functions with independent integrity paths.
SIL 3 / CAT 4 / PL eBypass & Override Coordination — Preventing Systematic Failures
Voting and input function block logic within the safety controller automatically activates bypass or override when a field device enters test mode, and automatically removes it when the device returns to normal — preventing spurious trips and avoiding the human error of forgetting to re-enable safety functions, both of which are systematic failure risks under IEC 61508.
Systematic failure preventionSIL 3 PLC Innovation: Patent Data Visualised
Analysis of 50+ patents across leading assignees and technical approaches reveals where the industry is investing in IEC 61508 SIL 3 compliance.
SIL 3 PLC Patent Filings by Leading Assignee
Siemens leads with the broadest portfolio across hardware modules, analog outputs, and safety circuits; ABB and Omron follow with complementary hardware and software toolchain focus areas.
Dominant Technical Approaches in SIL 3 PLC Patents
Dual/multi-CPU redundant architectures account for the largest share of technical innovation, followed by fault detection mechanisms and software verification toolchains.
Safety Program Design, Verification, and Parameterization
IEC 61508 SIL 3 imposes systematic integrity requirements on software — including tool-supported design, formal verification, and change management. These patents address the full development lifecycle.
Automated Safety Program Evaluation (Omron, 2017)
An evaluation system extracts instructions related to safe operation from the safety program, maps input signals to output signals, allows the engineer to define expected output values for safe operation, and automatically evaluates whether the program's computed outputs match expectations for all defined input transitions. This automated functional safety evaluation supports the systematic V&V process required by SIL 3.
Safety Program Generation Assistance (Omron, 2020)
Generates a safety program by acquiring an input/output setting correlating each input device with the output devices that respond to it, and automatically generating the safety program that transitions output device operational states to safe states whenever the defined input signal changes occur. This reduces manual coding errors — a key source of systematic failures at SIL 3.
Safe Parameterization with Read-Back Verification (KW-Software, 2012)
Parameter values are selected or entered via an operator interface, transmitted to the device, stored in memory, and then read back at least once for verification of integrity — eliminating the systematic risk of data corruption during parameter download. This read-back requirement is explicitly mandated under SIL 3 parameter integrity verification per IEC 61508.
DCS Integration with Physical Key-Lock (State Grid Liaoning, 2016)
A practical SIL 3 implementation guide for integrating a furnace safety system (FSS) and turbine emergency trip system (ETS) with a distributed control system via OPC communication. The procedure includes a physical key-lock mechanism to prevent unauthorized modification of configuration and programs — directly addressing SIL 3 requirements for configuration management and access control.
Key Players and Their SIL 3 Innovation Focus Areas
Based on frequency and depth of relevant filings, these are the leading assignees in IEC 61508 SIL 3 PLC innovation — each with a distinct technical focus area.
| Assignee | Primary Focus Area | Representative Patent | Year | Jurisdiction |
|---|---|---|---|---|
| Siemens AG / Industry | Fail-safe modules, analog output verification, embedded I/O safety, safety circuits with PLDs | Functional Safety High-Speed Fail-Safe Counter Module (SIL 3 / CAT 4 / PL e) | 2023 | CN, US |
| ABB AG / Switzerland | SIL upgrade retrofit, parallel safety controller operation, adaptive safety supervision | Method for Increasing the Safety Integrity Level of a Control System | 2010–2026 | CN, EP, WO |
| Omron Corporation | Safety program generation, simulation support, functional safety evaluation systems | Evaluation System for Safety Programs (automated V&V) | 2017, 2020 | JP, US, EP |
| Fisher-Rosemount / Emerson | Safety & process control integration, field device state management, intrusion prevention | Coordinating Field Device Operations Using Bypass and Override | 2009 | CN |
| Schneider Electric Systems USA | Soft error management for SIS, AI/ML-driven safety logic generation | Soft Error Aggregation Method for Safety Instrumented Systems | 2023, 2025 | JP, CN |
| Infineon Technologies | Safety state trigger architectures that bypass application controller | Safety State Trigger — Dedicated Safety Path Component | 2025 | WO (pending) |
| Shenyang Zhongke Bowei | Dual-CPU 1oo1D/1oo2D SIL 3 PLC architectures for Chinese industrial automation | High-Integrity PLC Controller Based on Functional Safety | 2015 | CN |
| Mitsubishi Electric | Safety program diagnostic devices, device-level safety level assignment | Safety Diagnostic Device and Method for Safety Control Program | 2013 | JP |
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AI, Parallelization, and the Next Generation of SIL 3 Compliance
An emerging trend in the SIL 3 patent landscape is the use of AI-based tools for automated safety logic generation. Schneider Electric's 2025 pending patent trains an artificial neural network on safety instruction sets to automatically generate safety control programs for safety PLCs — promising to reduce the time and resource intensity of SIS configuration while maintaining compliance. This represents a significant shift from manual safety program authoring toward AI-assisted engineering workflows.
At the architecture level, ABB Switzerland's pending 2026 patent on parallel compilation and dual-processor execution of safety functions enables SIL 3 systems to scale to complex safety applications without sacrificing response time or integrity. Safety functions defining logical dependencies between sensor and actuator signals are grouped into two classes, each compiled and linked into a separate executable, then dispatched to two independent processors — representing a leading-edge architecture trend cited explicitly against the IEC 61508 normative framework.
Chinese industrial automation firms are also demonstrating growing domestic capability. Shenyang Zhongke Bowei Automation Technology's architecturally rigorous SIL 3 PLC design and the State Grid Liaoning Electric Power Research Institute's practical DCS integration methods represent a shift in the geographic distribution of SIL 3 innovation. The PatSnap customer base increasingly includes Chinese industrial firms navigating IEC 61508 compliance.
For organizations building or validating SIL 3 systems, PatSnap's open API enables integration of patent intelligence directly into R&D workflows — supporting continuous monitoring of the competitive and regulatory landscape. Authoritative guidance on functional safety standards is also available from IEC and the International Society of Automation (ISA).
IEC 61508 SIL 3 PLC Implementation — key questions answered
The canonical SIL 3 hardware architecture is a dual-CPU implementation combined with programmable logic devices (PLDs). A single SIL 3-certified CPU in a 1oo1D (one-out-of-one with diagnostics) configuration can meet SIL 3; two such controllers combined provide fully redundant 1oo2D (one-out-of-two with diagnostics) voting. A programmable logic device bridges the dual CPUs and manages communication between the controller and external I/O modules.
Under IEC 61508, SIL 3 requires a probability of failure on demand (PFD) in the range of 10⁻³ to 10⁻⁴, with correspondingly stringent requirements on hardware fault tolerance, diagnostic coverage, and systematic integrity.
Yes. ABB's patent describes attaching a safety hardware unit — typically a circuit board incorporating a CPU, I/O interfaces using memory chips and FPGAs, local digital output channels, and memory shadowing functionality — to a standard controller via a backplane bus. The safety hardware unit communicates with the host controller's CPU and elevates the composite system to SIL 1 through SIL 4 as defined by IEC 61508.
A safety state trigger is a dedicated component that monitors all input signals from a supervisory controller, independently identifies fault signals, and bypasses the application controller entirely to send a safety state signal directly to the controlled device. This design means that the application controller itself does not need to meet the highest integrity level (SIL 4 or ASIL D), because the safety path is handled by the separate safety state trigger component that does satisfy those requirements.
Safe parameterization under IEC 61508 SIL 3 requires mandatory read-back verification of all safety parameters after download. Parameter values are selected or entered via an operator interface, transmitted to the device, stored in memory, and then read back at least once for verification of integrity — eliminating the systematic risk of data corruption during parameter download.
Soft error rate monitoring aggregates soft error data from multiple safety PLCs, compares actual versus expected soft error rates per I/O module type, predicts future rates, and triggers actions when deviations exceed thresholds — enabling proactive management of latent failures before they accumulate to exceed SIL 3 PFD limits.
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References
- High-Integrity PLC Controller Based on Functional Safety — Shenyang Zhongke Bowei Automation Technology, 2015
- Safety State Trigger — Infineon Technologies, 2025 (pending)
- Method for Increasing the Safety Integrity Level of a Control System — ABB AG, 2010
- Method for Operating a Safety Controller — Parallelization — ABB Switzerland, 2026 (pending)
- Soft Error Aggregation Method for Detection and Reporting of Risks in Safety Instrumented System — Schneider Electric Systems USA, 2023
- Functional Safety High-Speed Fail-Safe Counter Module — Siemens AG, 2023
- Method and System for Providing Analog Output Values in a Fail-Safe Manner — Siemens AG, 2022
- Module with Embedded Safety Function to Decrease Safety Reaction Time — Siemens Industry, Inc., 2025
- Coordinating Field Device Operations Using Bypass and Override in Process Control and Safety Systems — Fisher-Rosemount Systems, 2009
- Method and Apparatus for Safe Parameter Setting of Electronic Devices — KW-Software GmbH, 2012
- SIL3 Control Method in DCS Systems — State Grid Liaoning Electric Power Research Institute, 2016
- Evaluation System, Non-Transitory Storage Medium Storing Thereon Evaluation Program, and Evaluation Method — OMRON Corporation, 2017
- Information Processing Apparatus, Information Processing Method, and Computer Readable Storage Medium — OMRON Corporation, 2020
- Safe PLC, Sequence Program Creation Support Software, and Sequence Program Judgment Method — Toyoda Koki Kabushiki Kaisha, 2012
- Safety Diagnostic Device and Safety Diagnostic Method for Safety Control Program — Mitsubishi Electric, 2013
- Model-Based Safety Instrumented System Programming Using Artificial Neural Networks — Schneider Electric Systems USA, 2025
- Data Processing Procedure for Safety Instrumentation and Control (I&C) Systems — Bakhmach, Ievgenii, 2023
- Intrusion Prevention for Safety Instrumented Process Control Systems — Fisher-Rosemount Systems, 2009
- Safety Circuit for Outputting Switching Signals — Siemens Aktiengesellschaft, 2011
- IEC — International Electrotechnical Commission (IEC 61508 standard body)
- ISA — International Society of Automation (functional safety standards and guidance)
All data and statistics on this page are sourced from the references above and from PatSnap's proprietary innovation intelligence platform.
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