Siemens SIMATIC S7 System Manual page 268

Program instructions
7.6 Counters
HSC modes 6 and 7
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.
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System Manual, V2.3, 07/2017, A5E03822230-AF
S7-200 SMART