Now we learn plc programming examples for siemens s7-300 plc,here some ladder diagram program.
FC1 variable 
FC1 ladder diagram program 
OB1 call Ladder diagram program
(1) Control requirements.
using a timer freedom to set the clock pulse generator function (pulse duty factor 1: 1)
(2) FC1 variable
(3) FC1 ladder diagram program
(4) OB1 calls ladder diagram program
http://www.plchelpcenter.com/2016/07/19/siemens-s7-300-plc-using-functional-blocks-to-realize-the-clock-pulse-generator-programming-examples.html