Ladder Logic 208: System Routine 2

Today’s post is a continuation of the series on PLC Ladder Logic I started several years ago; for more of these post click on the PLC tab at the top of the site.

In particular this continues my discussion of the System Routine. In Ladder Logic 205 I showed a method of starting an Auto Cycle state or mode, but left much to the imagination as far as how the system or machine might actually get to that state (notice the reference to the Auto Start Sequence). Today I am going to go into a bit more depth on machine starting and safety.

This post also shows a method of generating a pulse train with two timers.

Often it is important not to start machinery instantly, but rather to warn personnel around the machine that it is about to move. This is required for most machinery, with the exception of simpler machine such as test stations. The logic above shows a method of pulsing a horn to warn people that the system is starting. It requires the person starting the machine to hold the button to create the Cycle Start Request signal; the next logic shows how to use the signal to actually start the system.

This is by no means the only way to do this, but it gets across the general idea. A few things to note:
1. The operator must hold down the cycle start button for the entire time for the system to start, if he lets go the timer starts again.
2. The system won’t start unless in Auto Mode with no faults.
3. The system will not stop if in the middle of an auto sequence; this should be modified to taste. It should allow sequences to come to rest in a natural position; a specific sequence step or position can replace the sequence active bit.
4. A fault stops the Auto Cycle immediately. Again, this may not apply in every case.

The Auto Cycle bit or status is not really a mode in itself, it is more of a state within Auto Mode. Generally it is used to allow auto sequences to start or proceed, but not to disable output energization.

The techniques I have listed in these numbered series of Ladder Logic articles are things I teach in my PLC training classes. While most courses do a good job of explaining the instructions themselves, they don’t cover programming technique. If you are interested in learning more, come take a class!


Electrical Engineer and business owner from the Nashville, Tennessee area. I also play music, Chess and Go.