For Siemens S7-300/400 series PLCs, the CPU's cycle time is generally not affected by the specific instructions used. However, in the case of the S7-1500 series, using extended instructions for data transfer can lead to an increase in the cycle time.
What exactly is cycle time? It refers to the total time it takes for a PLC to complete one full scan of the program. This includes four main components:
1. The time required to fully process the main loop program.
2. The time spent interrupting the main loop to execute a higher-priority program within the same cycle.
3. The time needed to update the process image area.
4. The time taken for the operating system to handle communication tasks.
Asynchronous instructions can introduce additional load on the system, potentially causing fluctuations in the cycle time. Any asynchronous commands that access the memory card—such as the SIM Memory Card (SMC)—can impact the cycle time. The extent of this fluctuation depends on several factors:
1. The version or type of memory card being used.
2. The amount of data stored on the card.
3. The number and size of files present.
4. The number of files in each directory.
For example, asynchronous instructions like "READ_DBL" and "WRIT_DBL" shown in the figure access the load store (SMC). If the SMC has reached its storage limit, it increases the management load during power failure scenarios, which can affect data consistency and reduce read/write performance. This, in turn, can cause the cycle time to increase.
How can you address this issue?
1. Use a larger capacity memory card to reduce the risk of reaching storage limits.
2. If you're using a cyclic interrupt OB, consider increasing the clock period if necessary—such as from 5 ms to 30 ms.
3. If your program contains many asynchronous instructions like "READ_DBL," avoid configuring the STEP 7 (TIA Portal) program loop monitoring time limit, as this can help prevent unnecessary delays.
By understanding how these elements interact, you can optimize your PLC program to maintain stable and predictable cycle times.
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