Siemens SIMATIC S7 System Manual page 263

● If the device generating the HSC input signals does not drive the input signals both high
and low, then signal distortion can occur at high speeds. This can occur if the output of
the device is an open-collector transistor. When the transistor turns off, there is nothing
driving the signal to a low state. The signal transitions to a low state, but the time to do so
is dependent on the input resistance and capacitance of the circuitry. This condition can
result in missed pulses. You can prevent this condition by wiring a pull-down resistor to
the input signals, as seen in the following figure. Since the input voltage of the CPU is 24
V DC, the resistor has to be rated for a high wattage. A 100 ohm, 5 Watt resistor is a
suitable choice.
Figure 7-1
S7-200 SMART
System Manual, V2.3, 07/2017, A5E03822230-AF
Pull-down resistor wiring for open-collector HSC input drivers
Program instructions
7.6 Counters
263