For Siemens S7-300/400 series PLCs, the CPU's cycle time is not directly influenced by the type of instruction used. However, with the S7-1500 series, programs that utilize extended instructions for data transfer may lead to an increase in the cycle time. Understanding what cycle time is essential for optimizing performance and ensuring reliable operation.
**What is cycle time?**
Cycle time refers to the total time taken by the PLC to complete one full scan cycle. It consists of four main components:
1. **Time to complete the main loop program** – This includes the execution of all standard instructions in the main program.
2. **Time to handle interrupts** – When a higher-priority interrupt occurs, the CPU pauses the main program and executes the interrupt routine during the same cycle.
3. **Time required to update the process image area** – This involves updating the input and output images from the physical I/O modules.
4. **Time for operating system communication tasks** – Includes communication with HMI, other PLCs, or network devices.
Asynchronous instructions can introduce additional load on the system, leading to variations in the cycle time. Any asynchronous command that accesses the memory card (such as the SIM Memory Card or SMC) can cause fluctuations in the cycle period. The extent of these fluctuations depends on several factors:
- **Type or version of the memory card** – Different memory cards have different performance characteristics.
- **Amount of data stored** – Larger amounts of data can slow down read/write operations.
- **Number and size of files** – More files or larger files increase the overhead.
- **Number of files per directory** – A large number of files in a single directory can also impact performance.
For example, asynchronous instructions like "READ_DBL" and "WRIT_DBL" access the load store (SMC). If the SMC has reached its storage limit, it increases the management load during power failure scenarios, which affects data consistency and reduces the memory card’s read/write speed. This, in turn, can result in longer cycle times.
To mitigate these issues, consider the following solutions:
1. **Use a high-capacity memory card** – A larger capacity card can reduce the likelihood of reaching storage limits and improve performance.
2. **Adjust the cyclic interrupt OB** – If using a cyclic interrupt OB, increasing its clock period (e.g., from 5 ms to 30 ms) can help stabilize the cycle time.
3. **Avoid configuring the program loop monitoring time limit** – If your program contains many asynchronous instructions like "READ_DBL," it’s better not to set a tight monitoring time limit in STEP 7 (TIA Portal), as this can interfere with normal operation.
By understanding how cycle time is affected and taking appropriate measures, you can ensure smoother and more predictable PLC operation, especially when working with advanced systems like the S7-1500.
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