According to ur need draw the logic and differentiate the i/p's and o/p's (helps in programming) .. Siemens and all plc have sepearate software for making ladder program.. Feed ur ideas and logic in the ladder program and send(burn) it to the PLC in programming mode.. Then keep it in run mode and it works fine... We can connect the plc i/p's and o/p's to pc thro SCADA ... Each and every plc have own syntax for programming for addressing , controlling etc.. ( if any mistakes correct me )