And NC relay is the relay that completes the circuit when it is not energized. In most of the PLC ladder logic programs either “] [“or “| |” symbols is used for the input relays contacts and “( )” is used to denote the output coil.Different PLC ladder logic programs use different sett of instructions but in most of the programs the ladder logic remains the same. The symbols for each of the functions are given at the menu bar, where user can select the physical input relay contacts or other functions according to the requirements. Let’s consider a program to run a motor through PLC with the help of push button. The motor runs till the push button is pressed. In the program used for “Delta PLC”, termed as “WPL soft” writes the program as:When the physical input at Xo is closed by applying 24 volts from the output of the PLC supply output to the physical input Xo through a push button, the out put coil gets energized. It can be seen on the physical PLC device. The indicator LED light of the specific input and out blinks when it is energized. It is better to use the contacts of the output coil and connect it another relay since it internal coil contacts of the PLC are unable to pass high current. The output coil also works as a NO contact. But when the program allows it to operate, it works according to the program. In the similar manner the program can be extended to run more than one motor.The above program can be altered to make an inverse logic meaning that the output coil is initially closed with respect to the input relay contacts, but when the input relays are energized, the output coil gets open.Ladder logic corresponds to the logic of a program to run machine simple ladder logic is based on AND logic, OR logic, NOR logic, NAND logic, Ex OR logic, or other combination for AND-OR logic or other logic combinations. The output is respect to the truth table.In the NAND logic the NC input contacts are used in series instead of NO contact. At the 0 state both the NC relay contact will turn ON the output coil. All other conditions will be zero. A sample program will be such that:In most of the hardware push buttons are used. In most of the programs, when the input is removed or push on the push button is removed, the output coil also opens. On the other hand, with the help of latching, we can have the constant output coil energized with a single push on a push button. The output
Delta.com.tw, Welcome To Delta Electronics, Inc. - Automotive Products. N.p., 2014. Web. 29 Nov. 2014.
Rockwell Automation, Listen, Think, Solve: Basic PLC Programming. N.p., 2014. Web. 29 Nov. 2014.
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