Understanding the PLC Scan Cycle: Mechanism and Stages for Industrial Control

What is the PLC Scan Cycle?

Key Stages of the Scan Cycle

1. Input Sampling

• Function: Reads all input signals (sensors, switches) and stores them in the input image register.
• Process:
  • Batch Reading: All inputs are sampled simultaneously to avoid timing errors.
  • Filtering: RC circuits eliminate noise, adding 8–10 ms delay (adjustable via software).
• Impact: Input changes occurring after sampling are ignored until the next cycle, creating potential 1-cycle latency.

2. Program Execution

• Function: Processes user logic (ladder diagrams, structured text) using stored input data.
• Process:
  • Sequential Scanning: Executes instructions line-by-line, updating internal registers.
  • Optimization: High-performance CPUs (e.g., Siemens S7-1500 with 10 MIPS) reduce execution time by 40% compared to basic models.
• Impact: Complex programs with nested loops can extend cycle times by 300%.

3. Output Refresh

• Function: Updates physical outputs (relays, motors) based on processed logic.
• Process:
  • Batch Writing: Output image register values are sent to hardware in one go.
  • Module Delays:
    • Relay: 10 ms
    • Transistor: <1 ms
• Impact: Critical for synchronization—e.g., a bottling line’s 0.5 ms transistor outputs prevent overflow.

4. Auxiliary Stages

• Housekeeping: CPU performs self-diagnostics and resets watchdog timers (default 150 ms for S7-300).
• Communication: Exchanges data with HMIs/SCADA systems, adding 2–5 ms via Ethernet/IP.

Real-World Optimization Strategies

1. Cycle Reduction:
   • Use immediate I/O instructions (e.g., Siemens’ L PIW), bypassing image registers for time-sensitive signals.
   • A semiconductor plant achieved 2 ms cycles by prioritizing critical tasks.
2. Interrupt Handling:
   • High-priority events (e.g., emergency stops) override the scan cycle, reducing response time to <1 ms.
3. Hardware Upgrades:
   • Rockwell’s Allen-Bradley ControlLogix cuts cycle times by 22% using dual-core processors.

Future Trends

• AI Integration: Mitsubishi’s AI-PLC predicts scan cycle bottlenecks using neural networks.
• Edge Computing: Siemens’ S7-1500 integrates with cloud platforms for real-time analytics during cycles.