3.4. Interpretation of the measured variable "DP reaction time" via process image

When determining the reaction times, configurations that occur in practical operation were deliberately selected. Particular attention was paid to ensure that not only the mere "signal transit times" were measured. This means that the reaction times mirror the complete system, including 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 "DP reaction time". The table below illustrates the structure of the minimum, typical and maximum DP reaction time:

  Minimum DP reaction time Typical DP reaction time Maximum DP 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 measured time

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).

(*)