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Basic PLC Ladder Programming Training Example 12: First in Priority Circuit PLC Ladder Program 

Basic PLC Ladder Programming Training Examples for Beginners. Hi Friends here we are starting a series of Free Training on PLC Ladder Programming. These PLC Ladder Programs are important to get basics of Ladder Programs

PLC Ladder Practice Problem:




  •  There are 3 groups participating in the quiz game: pupils, high school students and professors. If they want to get the chance of answering the question from the host, they must press the answer button on their table first. Other groups’ pressing will be invalid if any group gets the chance successfully
  •  There are 2 answer buttons for the pupil group and professor group and 1 answer button for the high school student group. In order to give preferential treatment to the pupil group, Y0 will be ON if any one of X0 or X1 is pressed. However, in order to limit the professor group, Y2 will be ON when X3 and X4 are pressed at the same time. For the high school student group, Y1 will be ON when X2 is pressed.
  •  If the host presses X5 (Reset button), Y0, Y1 and Y2 will be OFF.

Topics Covered in this example is First in Priority CONTROL PLC Program  


Number of PLC Inputs Required 


X0- Answer button for pupil group
X1- Answer button for pupil group
X2 -Answer button for high school student group
X3 -Answer button for professor group
X4 -Answer button for professor group
X5 -Reset button for host 

Number of PLC Outputs Required 

Y0 - Indicator for pupil group
Y1 -Indicator for high school student group
Y2 -Indicator for professor group

PLC Ladder Programming:


First in Priority PLC Ladder Logic

PLC Ladder Programming Description: 


  • If the host didn’t press the reset button X5, [MC N0] instruction will be executed and the program between MC and MCR will also be executed normally.
  • The answer buttons are connected in parallel connection for the pupil group, and in series connection for the professor group. For the high school student group, there is only one answer button. If one group presses the answer button successfully, its indicator will form a latching circuit, that is, the indicator will be ON even the button is released. 
  • Through the interlock circuit, any other button pressings will be invalid as long as one indicator is ON
  • When the host presses the reset button, X5 = ON. [MC N0] instruction and the program between MC and MCR will not be executed. Y0, Y1 and Y2 will be out of power, and all the indicators for the 3 groups will be OFF. When the host releases the button, X5 = OFF. The program between MC and MCR will be executed normally again, and the new round will begin as well.   
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