Ladder Logic for Dummies

No Room For Dummies in Ladder Logicbook-background-003


I have written a book that will teach you how to program and troubleshoot ladder logic. It  is an excellent way to learn about PLC,s and starting 2/9/2015 it is free for a very limited time, so go ahead and grab a copy for yourself while it is free!

The free period is over, however the book is still available to buy on Amazon. It’s worth it!

Ladder Logic is a graphical based industrial programming language used to program and configure Programmable Logic Controllers, or PLC’s. Ladder programs consist of rungs that house instructions. Once compiled and downloaded to a PLC the ladder program is scanned like a book; from top-to-bottom, left-to-right.

The following illustration is a common “latching circuit” used to turn devices such as motors on and off. If you are familiar with electrical schematics you will notice the relay and coil symbols.

Ladder latch circuit

 What Makes Up a Ladder Logic Program?

Coils, relays, timers as well as other instructions can be placed on rungs. Variables store data that is checked and manipulated by instructions. How and where these instructions are placed on rungs determine the output logic of a rung. The software has a programming instruction set that covers bit instructions, timers, counters, string instructions, and file instructions just to name a few.

Ladder instructions are unique. They have built in watch windows. For example let’s take a look at the Timer On or TON instruction. Notice the three variable fields:Timer On Instruction

  1. Timer                    Name of the timer
  2. Preset                   Set time like a stop watch
  3. Accum                  How much time has accumulated?

Now let’s look at the –(EN)- and –(DN)- symbols on the right side of the instruction. If the timer is timing the –(EN)- is highlighted green. If the timer is done timing the –(DN)- symbol is highlighted green. This is atypical of programming languages. Most languages require a programmer to configure a watch-window to monitor variables at run time. Having feedback directly on the instructions makes for some quick and easy troubleshooting.

Making PLC Programs is Easy.

Most programs use a combination of just a few of the basic instructions. Master the following list of instructions and associated variables and you will have the basics mastered.

  1. XIC                        Examine If Open
  2. XIO                        Examine If Closed
  3. OTE                        Output Energize
  4. OTL & OUT          Output Latch & Unlatch
  5. MOV                      Move data into a variable
  6. COP                        Copy data into an array of variables

Online PLC Training For Dummies

This site is dedicated to teaching the novice, the dummy,  how to program. How do we accomplish that? We offer online training. Put your credit card away most of the material you will find here is completely free. Free doesn’t mean you won’t learn, quite the contrary. Take a look around the site and I think you will agree there is a bunch of useful material. Check out the main page and be sure to watch the tutorial you will learn more from this than a month of lectures.

Be sure to check out our online training videos. Here you will find a series of videos that teach you how to program. Make sure to watch them all, and if you get hung up on instructions you can get definitions and examples here.

If you’re hungry for more, check out this, and this, and this.