Siemens SIMATIC S7-400H Manual page 58

Link-up and Update
In order for the activation of the push button to be recognized by the CPU the
blocking time for priority classes > 15 (see below for definition) must be clearly
below 18 ms.
Since in STEP 7 you are able to set the maximum blocking time for priority
classes > 15 only to 0 ms or a value of between 100 and 60000 ms, you need to
remedy the situation with one of the following measures:
Move the start of link-up and update to a time at which the process state is not
critical. To do this, use SFC 90 "H_CTRL" (see above).
Use a substantially longer cam and / or clearly reduce the velocity of the slide
block before it reaches the push button.
5-14
S7-400H Programmable Controller Fault-Tolerant Systems
A5E00068197-04