You will prepare and submit a term paper on Programmable Logic Controllers. Your paper should be a minimum of 3000 words in length. The major advantage of a modular PLC is that modules can be added as per the requirement of a particular machine. Modular PLCs are commonly arranged on a rack or railing that allows the easy arrangement of modules. Siemens Step S& PLCs are modular PLCs. Systems can be expanded according to the need.
Rack Mounting PLCs are similar to the modular PLCs. however, rack mounting PLC is standardized as the particular cards that have a slot in a cabinet. Rack mounting PLCs can be expanded according to the requirement.
Typically, PLCs are designed to work with an external 24-volt power supply. However, the input voltages for operation and input relays may range from 10-30 volts DC for input relays (Commonly 12 volt and 24 volts) and 110 to 250 volts AC. Special types of PLCs may have different voltage ranges but mostly PLCs come in the above-mentioned voltage ranges.
Selector Switches and Pushbuttons open or close a circuit that power input relays or transistors of a PLC. The PLC is programmed to accepts the command and proceed with an operation as desired. Selector switches and push buttons are manually pressed by the operator to execute a PLC operation (Ab.rockwellautomation.com, 2015).
Limit switches work on a similar principle as that of the selector switches and push buttons, but limit switches are pressed by the “machine (in a process)” or “arm of a machine”. Limit switches stop or start a process. Like opening and closing of a machine door. A motor opens or closes the door until a limit. As the limit is approached, limit switches either open or close the circuit and send PLC command either to stop the motor or run the motor. Limit switched are either normally close (NC) or normally open (NO). NC terminals complete circuit when the limit switch is in normal position and opens the circuit path when .pressed. NO terminals complete the circuit when the switch is pressed. .Some sensors are fitted with the actuators to enable the total functionality of the actuator.  .