Program instructions   
7.6 Counters 
  S7-200 SMART 
268  System Manual, V2.3, 07/2017, A5E03822230-AF 
 
When you use counting modes 6 or 7, and rising edges on both the up clock and down clock 
inputs occur within 0.3 microseconds of each other, the high-speed counter could see these 
events as happening simultaneously. If this happens, the current value is unchanged and no 
change in counting direction is indicated. As long as the separation between rising edges of 
the up and down clock inputs is greater than this time period, the high-speed counter 
captures each event separately. In either case, the program generates no error, and the 
counter maintains the correct count value.