PLC Programming: Ladder Logic vs Structured Text Explained from AutomationDirect

For many years PLCs have been a fantastic solution for reliable and easy to use control in industrial automation. One of the things that makes them so easy to use is the programming language that most PLC projects are coded in. It is called ladder diagram, and it’s designed with a plant electrician in mind rather than a pure programmer. In an electrical circuit, positive voltage is applied to one side of the circuit. The current then flows through a switch or multiple switches, that all must pass that current. If all the switches are on, the circuit is then able to power a load, or multiple loads such as a light bulb. Coding in ladder looks a lot like that same circuit diagram. Each rung of code represents a logical operation with conditions that must all be true, like switches being closed, prior to the logic being able to perform a function, like turning on a lightbulb. This makes it very easy to understand, work with, and troubleshoot, however the simplicity and visual nature of the language can limit it. It is hard to view a HUGE amount of information, such as the status of 60 different digital inputs or outputs simultaneously, or 3 or 4 function blocks that are part of the same rung of logic. It is also hard to code more advanced functions in a way that makes sense. Often you must nest multiple different layers of ladder together to get the desired result using add-on instructions, or user defined data types. Sometimes it is challenging to perform the desired function with ladder diagram alone. It can be tough to track programmer comments and program change notes, and often those comments aren’t stored in the PLC program but only in the offline project. Further, it can be a little annoying trying to copy/paste the same function many times and often requires exporting, manipulating, and then re-importing the logic to the ladder editor. Finally, if a user is familiar with .net and other high level programming languages, it can be cumbersome and frustrating to be “slowed down” by ladder programming. While there are work arounds for most of these limitations in ladder diagram, one simple option to use is a different programming language altogether. Structured Text programming helps with all these struggles. It is text based, so it is easy to see massive amounts of information on the screen simultaneously. Whether this is 20 functions or 100 variables, performing complex analysis on small programming monitors is easier. Structured Text also allows nested programming for more advanced functions without the need for multiple ladder views. Normal programming conventions such as parentheses can be used as logical separators within a single line of code. Comments can be embedded in the code with special characters to tell the controller that the information is for comment only. They travel with the program and are easy to trace and read for a troubleshooter trying to understand the programming intent in the future. Copying one function many times is also very simple as you can copy-paste a few or many lines, and then make minor alterations to each as needed, all within the programming environment. No need for time-consuming exports to other software environments to accomplish fast replication. All of these ideas are well known in high level programming languages, which makes structured text much easier to pick up, understand, and troubleshoot for someone with a non-PLC programming background. While structured text is not the end-all be-all of programming languages, it does have some advantages over ladder, and may be advantageous in certain circumstances. Either way, both ladder diagram and structured text have strengths, and weaknesses, and they are both IEC languages well worth learning and using as needed for each application. If you would like to know more about these two programming methods, check the link in the description to head on over to the LS PLC section of our free training portal where you can find helpful instruction on using both Ladder and structured text in a PLC application… If you want to see more videos on our LS Electric PLC lineup including structured text capable PLCs, click here…. If you want to see more helpful how-to videos