Read the instructions for a keypad from the link I included below, and it might help you understand...
http://na.smc.com/wp-content/uploads/2011/07/SMCWK01-Z_Key_Pad_Quick_Start_Guide_Final_031011_A5.pdf
http://na.smc.com/wp-content/uploads/2011/07/SMCWK01-Z_Key_Pad_Quick_Start_Guide_Final_031011_A5.pdf