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Hardware configuration is the better fit when a machine’s physical interfaces, operating environment, existing equipment, lifecycle support, or documented safety design constrain how it can be built. Software-configurable control is often preferable when operating behavior or complex sequences must change. Neither approach is inherently safer, more reliable, or cheaper: choose for the complete system and its requirements.
What “hardware” and “software” configuration mean
Hardware configuration sets behavior through physical components and connections, such as relays, wiring, and selected input/output devices. Software-configurable control uses a programmable controller to implement or adjust behavior through its logic and configuration. Industrial control systems include programmable logic controllers (PLCs), but the choice is not simply between wiring and code: both approaches depend on physical equipment, integration, procedures, and maintenance.
Safety architecture has additional distinctions. A basic hardwired safety relay, a safety controller, and a safety-rated PLC are different options. An ordinary PLC is not automatically suitable for a safety function; identify the required function and use equipment rated and engineered for it. ISA discusses configurable safety logic as more flexible than basic hardwired circuit designs, but flexibility does not establish that a particular design meets a machine’s safety requirements. ISA’s overview of safety relays, controllers, and PLCs
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- Physical interfaces are fixed. Required I/O, deterministic interfaces, or existing equipment may constrain the available architecture.
- The environment limits choices. Installation conditions and equipment constraints can make a particular component arrangement more practical.
- The design must remain supportable. Existing staff skills, compatible spares, and lifecycle support may favor a known, maintainable arrangement.
- A documented safety design constrains the architecture. The selected system must satisfy the machine’s actual hazards, required safety functions, and applicable jurisdiction—not merely resemble a familiar design.
A limited safety function may be straightforward to implement with hardwired relays. The trade-off is that ISA characterizes basic hardwired circuits as inflexible when changes are needed. For larger I/O counts, ISA notes that a safety-controller solution can become comparable to or less expensive than hardwired controls once hardware and installation are considered. That is contextual guidance, not a universal I/O threshold or a current price quote. ISA’s architecture discussion
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When software-configurable control may be the better fit
- Sequences or operating behavior change. Programmable control can make it easier to adapt behavior without redesigning a hardwired circuit for every change.
- Control logic is complex. A programmable controller can manage sequences and equipment behavior in one configured system.
- Embedded automation is needed. A U.S. Department of Energy handbook describes PLCs as flexible and commonly used in embedded OEM automation.
Programmability does not guarantee lower total cost. The DOE handbook says PLCs can offer lower initial costs in its context, but extensive integration and custom programming can erase that advantage compared with a distributed control system (DCS). This is older contextual guidance, not a present-day price comparison or a guarantee for a particular project. U.S. Department of Energy, EMIS Handbook
Compare complete systems, not just components
Use these questions to frame an engineering decision. They are a practical synthesis, not a scoring method prescribed by a standard.
| Decision area | Questions to answer |
|---|---|
| Safety and application | What hazards and safety functions apply? What architecture, verification, and jurisdictional requirements govern this machine? |
| Change frequency | How often will sequences or settings change? Who may authorize, implement, and validate each change? |
| Installed cost | What will controller hardware, wiring, I/O, programming, integration, commissioning, and lifecycle support cost together? |
| Reliability and availability | What failure modes, redundancy, diagnostics, and recovery capabilities does the equipment require? |
| Security | How will access, updates, network connections, and configuration changes be controlled? |
| Lifecycle and maintainability | Can the team maintain the design, obtain compatible spares, preserve configuration records, and support it throughout the equipment’s life? |
Dependability is a lifecycle concern involving more than component selection. IEC 60300-1:2024 provides broad dependability-management guidance and was published on 2024-06-11. IEC 60300-1:2024
Treat safety and security as design requirements
NIST’s SP 800-82 Rev. 2 addresses industrial control system (ICS) security while accounting for these systems’ distinctive performance, reliability, and safety requirements. Security considerations apply to either architecture: define how users gain access, how updates are handled, and how configuration changes are controlled. NIST SP 800-82 Rev. 2
ISA identifies ISA/IEC 62443 as a framework for addressing security vulnerabilities in industrial automation and control systems. ISA/IEC 62443 series
Standards have defined scopes. IEC 60987:2021 covers hardware requirements for conventional hardwired and programmable digital technologies in instrumentation and control systems important to safety at nuclear power plants. It should not be treated as a general rule for every industrial machine or jurisdiction. IEC 60987:2021
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Make the choice against the actual installation
- Define the machine’s requirements. Document its operating behavior, physical I/O, hazards, required safety functions, availability needs, and installation constraints.
- Identify acceptable architectures. Compare hardwired arrangements, safety relays, safety controllers, and programmable control as appropriate to the application. Do not substitute an ordinary PLC for safety-rated equipment.
- Estimate installed and lifecycle effort. Include hardware, wiring, integration, programming, commissioning, staff capability, spares, and ongoing support rather than comparing component prices alone.
- Plan change control and security. Specify who can modify logic or settings, how changes are tested and recorded, and how access and updates are managed.
- Verify the selected design. Check it against applicable regulations, standards, equipment specifications, and the machine’s documented requirements; verify and maintain it over the equipment lifecycle.
The cited material spans a 2015 NIST guide, a 2019 ISA article, IEC standards from 2021 and 2024, and an older DOE handbook excerpt. Confirm current editions, local requirements, product specifications, and lifecycle availability before applying it to a real installation.
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