Siemens SIMATIC S7 System Manual page 255

Example CTD count down
LAD
Timing diagram
S7-200 SMART
System Manual, V2.3, 07/2017, A5E03822230-AF
Count down counter C1 current value
counts from 3 to 0
With I0.1 OFF, I0.0 OFF-ON decre-
ments C1 current value
I0.1 ON loads countdown preset value
3
C1 bit is ON when counter C1 current
value = 0
Program instructions
STL
Network 1
LD I0.0
LD I0.1
CTD C1, +3
Network 2
LD C1
= Q0.0
7.6 Counters
255