Siemens SIMATIC S7-200 System Manual page 60

S7-200 Programmable Controller System Manual
Figure 4-26 shows the basic operation of the S7-200 with and without pulse catch enabled.
Physical Input
Output from
pulse catch
Disabled
Enabled
Figure 4-26 Operation of the S7-200 with the Pulse Catch Feature Enabled and Disabled
Because the pulse catch function operates on the input after it passes through the input filter, you
must adjust the input filter time so that the pulse is not removed by the filter. Figure 4-27 shows a
block diagram of the digital input circuit.
External
Digital Input
Figure 4-27 Digital Input Circuit
Figure 4-28 shows the response of an enabled pulse catch function to various input conditions. If
you have more than one pulse in a given scan, only the first pulse is read. If you have multiple
pulses in a given scan, you should use the rising/falling edge interrupt events. (For a listing of
interrupt events, see Table 6-46.)
Input to pulse catch
Output from pulse catch
Input to pulse catch
Output from pulse catch
Input to pulse catch
Output from pulse catch
Figure 4-28 Responses of the Pulse Catch Function to Various Input Conditions
46
Scan cycle
Input update
Optical Digital Input
Isolation Filter
Scan cycle
Input update
Next scan cycle
Input update
The S7-200 misses this pulse because the input
turned on and off before the S7-200 updated the
process-image input register
The S7-200 catches the pulse on the physical input
Pulse
Catch
Pulse Catch Enable
Next scan cycle
Input update
Input to S7-200