What is the maximum permissible frequency a digital input of the MK210 INPUT AND OUTPUT MODULE can handle in case of using this input to read an impulse signal?

It depends on device variant:
-MK210-311 : Pulse frequency, max. is 400 Hz.
-MK210-312 : Pulse frequency, max. is 400 Hz for DI9-DI12 and 100 kHz for DI1-DI8.