RsLogix 500 - Sequencing Machine Steps Part 2 - Controlling Outputs

In this lesson you are going to learn about structuring outputs when sequencing machine steps.

We find many beginners can get the hang of programming a step by step process as long as there are never any variations in the process.  However, this is rarely the case in real world machines.  There is probably an automatic process that the machine normally steps through, but most machines also have manual jog functions, a setup process, and maybe even several other machine sequence variations.

This is a continuation of the RsLogix 500 - Sequencing Machine Steps lesson.  In it you received a brief overview of several methods of sequencing steps in machines.  One major part that was left out was the outputs that were contained in the LAD 3 OUTPUTS routine.


In our Cougar Turn Signal program, we have modes of:

  • Left Turn Signal
  • Right Turn Signal
  • Brake
  • Emergency Flasher

What makes this example even more interesting is that you can't simply program for turn signal mode, brake mode, or emergency flasher mode.  You can have your turn signal on at the same time that you have the brake press and the emergency flashers are on.

While we stress throughout the lessons to avoid creating "rules" to avoid programming errors, by not having the turn signal sequence code actually control the outputs and just have it set internal bits instead, it allowed us to make this simplified rung seen here.  Always remember, when someone is troubleshooting your program later, they won't start by asking why the machine won't get through step 3.  They will be asking why the output isn't turning on.  

One other feature that was introduced in this rung is the free running clock of the PLC.  S:4/6 is on for 1.28 seconds then off for 1.28 seconds.  You will learn more about the S status file in later lessons but here are the values of the S:4 free running clock.

Address Milliseconds 
S:4/0 20 ms
S:4/1 40 ms
S:4/2 80 ms
S:4/3 160 ms
S:4/4 320 ms
S:4/5 640 ms
S:4/6 1280 ms
S:4/7 2560 ms
S:4/8 5120 ms
S:4/9 10240 ms
S:4/10 20480 ms
S:4/11 40960 ms
S:4/12 81920 ms
S:4/13 163840 ms
S:4/14 327680 ms
S:4/15 655360 ms


Next you will go through the details of how each machine sequencing method works starting with the RsLogix 500 - Sequencing Machine Steps Part 3 - Cascading Timers.

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.