RsLogix 500 Training - Program Flow, Which Rung Wins Part 2

In this lesson we are going to find some surprise results from the questions we asked in the last lesson.

This is a continuation of the RsLogix 500 - Program Flow, Which Rung Wins Part 1 lesson.  Make sure you go through Part 1 before doing this lesson.

In the questionnaire in Part 1, you were asked the following questions:

Which rung will control O:0/0 and why? The most common answer given for the question and the one that is shockingly found in many books is that the last rung wins, rung 1 will overwrite anything that rung 0 does.  So the Green Button will turn on the Green Light and the Red Button will turn off the Green Light.  

Create a program with the following two rungs, download it to your PLC, and go online.  Continue through the rest of the answers to the questions below and then we will circle back around to this question at the end of this lesson.



What will happen when Switch 1 is turned to the right(on)? According to the last rung wins rule, Switch 1 will not have any affect on the Green Light.  Turn Switch 1 to the right.  Were you surprised to find the Green Light on?

Study the image below very carefully.  In rung 1 the Green Light has itself sealed in so the only thing that can turn the Green Light off is the Red Button right?  Proceed to the next image to find out.




What will happen when Switch 1 is turned back to the left (off)? Are you surprised?  In Part 3 of this lesson series you will go through a detailed explanation but for now move on to the next image.



What will happen when the Green Button is pressed? Pressing the Green Button turns on the Green Light.



What will happen when the Red Button is pressed while Switch 1 is to the right (on)? Pressing the Red Button with Switch 1 on turns off the Green Light.





Circling back to the original question of Which rung will control O:0/0 and why?  What are your thoughts now?  The clear answer which you were not given as a choice is rungs don't control outputs. Data boxes associated with outputs control outputs in part 3 of the scan process.  Instructions can read and write to data boxes, and rungs are the structure that determines which instruction is executed when.  


Were you surprised by the results?  Proceed to the RsLogix 500 - Program Flow, Which Rung Wins Part 3 lesson where we will break the scan cycle down to each step and explain each of these results in detail

Next Steps

Go to the Allen Bradley RsLogix 500 PLC Training PLC Training Getting Started Lesson series to select your next lesson.  There are also many other Lesson Series on PLC Programming and Industrial Automation.