RsLogix 500 - Sequencing Machine Steps Part 3 - Cascading Timers

In this lesson you will dive deep into how the LAD 4 TIMER routine of the previous RsLogix 500 - Sequencing Machine Steps uses cascading timers to sequence through steps.

Cascading Timers are timers that are locked together in such a way that when one timer completes then the next timer begin.  This can be repeated for as many timers as you like.  In our example the first timer is no lights, then when it completes it starts the inside lights timer, then when it completes then it starts the middle lights timer, then when it completes it starts the outside lights timer, then when it completes it restarts with the no lights timer and repeats.  

1.  Download the CougarTurnSignalTimer.RSS file.  This is a simplified version of the RSS file you used in the Sequencing Machine Steps lesson, only including the timer sequence programming.

2.  Open the RSS file in RsLogix 500, download it to your PLC, switch the PLC to Run Mode, and Go Online.

3.  Open up the LAD 4 TIMER routine.

4.  Turn on I:0/0.  It should still be wired to the Green button from the previous lesson.

5.  You screen should be sequencing through rungs 0-5 similarly to the video below.  You may have to right click the image below and open and new tab depending on your browser.

timerturnsignal4.gif

 

Below is the explanation of how each timer is triggered by the previous one creating the cascading timer sequence.  We are ignoring the XIC I:0/0 left turn signal in the explanation below for simplicity.

Rung 1 - As long as the B3:1/0 "No left light on" bit isn't set, this timer will be enabled.  But once the T4:0 "No lights on timer" in the right of rung 1 is done, it will latch the B3:1/0 "No left light on" bit which is used to start the timer in rung 2.

Rung 2 - As long as the B3:1/0 "No left light on" bit is set and the B3:1/1 "Inside left light on" bit is set, this timer will be enabled.  But once the T4:1 "Inside left light timer" in the right of rung 2 is done, it will latch the B3:1/1 "Inside left light on" bit which is used to start the timer in rung 3.

Rung 3 - As long as the B3:1/1 "Inside left light on" bit is set and the B3:1/2 "Middle left light on" bit is set, this timer will be enabled.  But once the T4:2 "Middle left light timer" in the right of rung 3 is done, it will latch the B3:1/2 "Middle left light on" bit which is used to start the timer in rung 4.

Rung 4 - As long as the B3:1/2 "Middle left light on" bit is set and the B3:1/3 "Outside left light on" bit is set, this timer will be enabled.  But once the T4:3 "Outside left light timer" in the right of rung 4 is done, it will latch the B3:1/3 "Outside left light on" bit which is used in rung 5.

Rung 5 - Once the B3:1/3 "Outside left light on" bit is set, it will move a "0" to B3:1, which sets all of B3:1 bits to 0 including B3:0/0, B3:0/1, B3:0/2, and B3:0/3.  This takes you back to rung 1, repeating the cascading timer.

 

Continue on to the next lesson, RsLogix 500 - Sequencing Machines Part 4 - Counters.

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.

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