And NC relay is the relay that completes the circuit when it is not energized. I 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. Dfferent PLC ladder logic programs use different sett of instructions but in most of the programs the ladder logic remains the same. Te symbols for each of the functions are given at the menu bar, were user can select the physical input or other functions according to the requirements.
Lt’s consider a program to run a motor through PLC with the help of push button. Te motor runs till the push button is pressed. I the program used for “Delta PLC”, trmed as “WPL soft” writes the program as: Wen 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, te out put coil gets energized. I can be seen on the physical Te 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. Te output coil also works as a NO contact. Bt when the program allows it to operate, i works according to the program. I the similar manner the program can be extended to run more than one motor. Te above program can be to an inverse logic meaning that the output coil is initially closed with respect to the input relay contacts, bt when the input relays are energized, te output coil gets open.
Ldder logic corresponds to the logic of a program to run machine simple ladder logic is based on AND logic, O logic, NR logic, NND logic, E OR logic, o other combination for AND-OR logic or other logic combinations. Te output is respect to the truth table. I the NAND logic the NC input contacts are used series instead NO contact.
A the 0 state both the NC relay contact will turn ON the output coil. Al other conditions will be zero. Asample program will be such that: I most of the hardware push buttons are used. I most of the programs, wen the input is removed or push on the push button is removed, te output coil also opens. O the other hand, wth the help of latching, w can have the constant output coil energized with a single push on a push button. output. ..
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