The cycle time increase the cause and the solution analysis -

by:Coolmay     2020-06-17

for Siemens S7 - 300/400 series of PLC controller, the cycle of the CPU scanning time also has nothing to do with instruction, but for S7 - 1500 series PLC, the procedure of data transmission using extension instruction program may result in the increase of the cycle time.

cycle time is made up of what? — — Four parts:

(1) the main loop program complete processing time

(2) interrupt the main loop program, the implementation of higher priority program processing time ( The current cycle)

3. Update the time needed to process the image areas

(4) the operating system time needed for communication process

asynchronous instruction produces communication load, thus causes a cycle fluctuations. All access card ( 硅镁质记忆卡,SMC) The asynchronous instruction can lead to cycle fluctuation. Such volatility depends on:


memory card version or type (2) the data capacity

(3) the number and size of the file

(4) a number of files in the directory

, for example, in the asynchronous instruction 'READ_DBL' and 'WRIT_DBL' access load storage area ( SMC) 。 Memory card SMC have reached your storage limit, which increases power load management for data consistency to protect, leading to loss of memory card read and write performance, then cause cycles increase.

how to solve? (1) the use of large capacity memory card

2. If you are using cyclic interrupt OB, if necessary, to increase its clock cycles ( For instance from 5 to 30 ms ms) 。

3. If the program includes a lot of asynchronous instruction ( READ_DBL) Don't configuration STEP 7 ( TIA Po road) Program loop monitoring time limit.

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