3.3. Interpretation of the measured variable "PN reaction time" via process image

Determination of the reaction times was deliberately based on configurations as are often used in practical operation. Particular attention was paid to ensure that not only the mere "signal transit times" were measured. This means that the reaction times reflect the complete system, including the PLC program, and not just the individual components.

To be able to classify the measurement results correctly, the next section provides an interpretation of the measured variable "PN reaction time". The table below illustrates the structure of the minimum, typical and maximum PN reaction time:

  Minimum PN reaction time Typical PN reaction time Maximum PN reaction time
Numerical example for 10 ms load program 12 ms 20 ms 26 ms
Position of the input signal in relation to the cycle
Explanation

  1. The input signal occurs shortly before the cycle change.

  2. The inputs are copied to the process input image (within the update time).

  3. The read signals are written to the process output image.
    In the next cycle, the system writes the process image to the outputs.

 

 

Consequence:
The input signal can be directly detected and the mirrored signal can be output at the beginning of the next cycle.

  1. The input signal occurs approx. in the middle of the cycle.

  2. After half a cycle has elapsed, the inputs are copied to the process input image (within the update time).
    These signals are read in the user program.

  3. The read signals are written to the process output image.
    In the next cycle, the system writes the process image to the outputs.


Consequence:
The input signal can only be detected half a cycle later and the mirrored signal is then also output later.

  1. The input signal occurs shortly after the cycle change and after the system has read the inputs.

  2. After a complete cycle has elapsed, the inputs are copied to the process input image (within the update time).
    These signals are read in the user program.

  3. The read signals are written to the process output image.
    In the next cycle, the system writes the process image to the outputs.


Consequence:
The input signal can only be detected after the current cycle has been completed. The mirrored signal can thus only be output after the end of the second cycle or before the beginning of the third cycle.
Structure of the time measured

Time = load program + transmission time of the system (*)

Time = (time_min + time_max)/2

Time = 2 * load program + transmission time of the system (*)
Conclusion This time indicates the best reaction time to be expected (best case). This time indicates the average (typical) reaction time to be expected. This time indicates the worst reaction time to be expected (best case).

(*)