PLC - Learn Structured Ladder Diagram Programming

If you do a quick search for "plc sequencer" in posts on LinkedIn you will see the many ways that sequencers or finite state machines are implemented in PLCs.

This will be an example of the technique I have settled on after teaching Electrical Automation at Idaho State University for many semesters. My students are given a Grafcet/SFC of a simulated piece of machinery and are required to implement that scenario using Ladder Diagram.

Let's start with an simple example:

This is a simple Grafcet that covers continuous actions, stored actions (latches Steps 1,10), Or branches, And branches, and timers (transition after Step 12)

I call this technique Step Ladder and it is based on the STL instruction you will find in Asian based PLCs (Mitsubishi, Delta). Note that STL here stands "Step Ladder" not Statement List for you Siemens folks.

There is one USA manufacturer that implements something similar. That would be Automation Direct's Domore PLC implementing Stage Programming.

Domore Intro to Stage Programming

Here is an snippet of Delta PLC implementing the above Grafcet:

In the above Delta PLC example, when we are in Step 1 and transition i0/X0 is true we set Step 2 and Step 1 is automatically reset.

Now using this thought process, we can now implement this in any PLC.

Here is an example using Codesys:

Now I know a lot of you are big fans of the EQU/MOVE technique when doing PLC sequencers. But I think outside of simple sequences it gets complicated. For example looking at the And Branch in the above Grafcet now we have to deal with multiple pointers in our EQU and MOVE instructions. See below:

Now if you had to implement the below Grafcet it would get really complicated.

Using Step Ladder Technique on IntroToSFC Grafcet Setting And Branches

Using Step Ladder Technique on IntroToSFC Grafcet Resetting And Branches

This was a quick explanation of my preferred technique for implementing "PLC Sequencers". If you would like more detailed videos of this technique and others,

see my Udemy course with the same name as this title.

Regards,

Gregory Brooks