How do sensors affect PLC inputs ?

I see that PLC's structure has contacts for inputs and if you switch these contacts, the equivalent inputs is on/off, inputs include contacts, sensors but tell me how sensors turn on/off PLC inputs ?
thanks !!!

PLC Inputs for PNP devices respond to an input voltage of whatever its sensitive to; 110VAC, 24VDC, 5VDC are very common inputs voltage levels. PLC inputs that are designed for NPN devices respond to their wire connected to ground/common. When activated, the PLC designates its state as binary 0 or 1 (true or false).

smiles here is a link that will help you understand.
http://www.sea.siemens.com/step/templates/lesson.mason?plcs:2:1:4<br />http://www.sea.siemens.com/step/templates/lesson.mason?plcs:1:1:1

smiles said:

I see that PLC's structure has contacts for inputs and if you switch these contacts, the equivalent inputs is on/off, inputs include contacts, sensors but tell me how sensors turn on/off PLC inputs ?
thanks !!!

Click to expand...

Sensors typically turn on/off PLC inputs via some wire-like device connected between the two.

Sensor outputs can be PNP or NPN.PNP implies output is at logic 1 and vice versa.Based on ur sensor output PLC common can be sourced or sinked.ie)if sensor o/p is PNP your PLC common has to be connected to 0 volts.. and if sensor o/p is NPN your PLC common has to be connected to positive volts.

Hi dear,
Sensors are like contacts but instead of having 0V for open contac and 24V for close contact, they (sensors) have voltage all the time but with different values. This you find for example in proximity switches which is used in (e.g limit switches ). for some manuf. the voltage of the sensor is in the range from 3.5V for OPEN and 7.5V for CLOSE. In some applications the sensor is monitored by a spicial card from the manufacturer and the card output is the normal contact to the PLC.
This type of sensors are used to be sure that the loop is always helthy and no wire break is happened.
Hope this helps you,
Regards