Scan time

Definition

Scan time refers to the duration a programmable logic controller (PLC) takes to execute a complete cycle of input scanning, processing logic, and output updating. This measurement is crucial as it directly affects the responsiveness and efficiency of automated systems, ensuring that the PLC can react promptly to changes in inputs and maintain accurate outputs.

5 Must Know Facts For Your Next Test

1. The scan time is influenced by factors such as the complexity of the program, the number of input and output points, and the processing power of the PLC.
2. Shorter scan times lead to better system performance, allowing for quicker responses to real-time changes in manufacturing processes.
3. Typical scan times for modern PLCs can range from a few milliseconds to several hundred milliseconds depending on their design and application.
4. In systems requiring high-speed operations, minimizing scan time is essential to prevent missed events or system failures.
5. Scan time must be considered in conjunction with other timing parameters like response time and cycle time to ensure optimal PLC performance.

Review Questions

• How does scan time impact the overall efficiency of a PLC-controlled automation system?
  • Scan time significantly impacts the overall efficiency of a PLC-controlled automation system because it determines how quickly the PLC can respond to changes in inputs. A shorter scan time means that the system can react more swiftly to real-time events, reducing delays in processing and output updates. If scan time is too long, it may lead to missed inputs or lagging outputs, which can compromise the effectiveness of automated processes.
• Compare and contrast scan time with cycle time in terms of their relevance to PLC operation.
  • Scan time and cycle time are closely related yet distinct concepts in PLC operation. While scan time refers specifically to the duration taken for the PLC to read inputs, process logic, and update outputs, cycle time encompasses this entire scan plus any additional delays due to processing or communication. Understanding both metrics is crucial for optimizing performance; while minimizing scan time improves responsiveness, managing cycle time ensures that all aspects of control are efficient.
• Evaluate the implications of increasing scan times on an industrial automation system's performance and reliability.
  • Increasing scan times in an industrial automation system can lead to significant performance and reliability issues. Longer scan times can result in delayed responses to critical events, leading to potential failures in control actions or safety mechanisms. As a consequence, production efficiency may drop, and there could be higher risks of equipment damage or operational hazards due to unresponsive controls. Therefore, maintaining optimal scan times is essential for ensuring both reliable performance and safety in automated environments.

Related terms

Input Scan: The phase where the PLC reads and records the status of all input devices connected to it.

Output Update: The phase where the PLC sends signals to output devices based on the processed logic from the input data.

Cycle Time:

The total time taken for one complete operation cycle in a PLC, which includes scan time along with any additional processing delays.