in the machine, there are sensors. a sensor gives information about different situations in the machine. for example, how fast the motor rotates, what the temperature is or when the current action is completed. all this information is necessary for the program. the sensors give out an electrical signal, each signal goes by wire on an input of the simatic unit. so the program knows what´s going on in the machine. this is very important, because now the program can perform the next correct step in its sequence. for example, if the temperature of the motor is too high, the program has to know it. now the program will switch off the motor.