Plc Operation With And Without Pulse Catch - Siemens SIMATIC S7-200 System Manual

6.3 Pulse Catch
The S7-200 CPUs provide a pulse catch feature for some or all of the local digital
input points. See Appendix A for more information about your particular CPU. The
pulse catch feature allows you to capture high-going pulses or low-going pulses
that are of such a short duration that they would not always be seen when the CPU
reads the digital inputs at the beginning of the scan cycle.
Pulse catch operation can be individually enabled on each of the local digital
inputs. When pulse catch is enabled for an input, a change in state of the input is
latched and held until the next input cycle update. In this way a pulse that lasts for
a short period of time is caught and held until the CPU reads the inputs, assuring
you that the pulse will not be missed. The basic operation of the PLC with and
without pulse catch enabled is shown in Figure 6-4.
Input update
Input
Pulse catch
disabled
Pulse catch
enabled
Figure 6-4
When using the pulse catch function, you must be sure to adjust the input filter
time so that the pulse is not removed by the filter. (The pulse catch function
operates on the input after it passes through the input filter.)
S7-200 Programmable Controller System Manual
A5E00066097-02
CPU scan n

PLC Operation With and Without Pulse Catch

CPU and Input/Output Configuration
CPU scan
Input update
This pulse is missed because it
occurred between the input
updates
n+1
Time
Pulse caught
6-5