Ladder Logic 102: Benefits, Drawbacks, and When to Use It

Ladder Logic 102: The Pros and Cons

As we’ve seen in the previous article, Ladder Logic 101, choosing the right tool for your programming job is super important. We took a look at the conception and fundamentals of one of those tools, called ladder logic, and now we will finish the introduction to ladder with an overview of its pros and cons. Understanding what ladder does and doesn’t do best is key to getting the most use out of it, and should it be the programming method you choose to use for your project, you’ll want to know where it really shines.

The Pros

Ladder logic has a lot of unique benefits inherent to its diagram-like form, and here are the top ones for today’s industry:

• Self-Documentation: A ladder diagram is an excellent graphical representation of circuit design concepts, and as you code up a circuit, you are writing its design documentation as well.
• Intuitive: Along the same lines, ladder logic’s direct representation of circuits makes for an exceptionally intuitive coding environment for anyone who has ever worked on an industrial control circuit.
• Easily Debugged: Modern ladder logic debugging tools can provide an animation showing live “power flow” through your diagrams. This makes it very easy to understand exactly what your logic is doing and to debug faults.
• Widely Stylistic: Ladder logic code and diagrams come in many shapes and sizes, and programming teams are free to write their code any way they prefer, they could choose a method such as rung by rung (a drawn out and simple approach), or rungs with multiple branches (more sophisticated), each type of coding has its own pros and cons, and being able to choose which one you need is an important benefit.

The Cons

No system is perfect, and although ladder logic has a lot of great benefits, it also has some cons like the following:

• Logic Encapsulation: Ladder programs without function blocks are unable to encapsulate code for reuse, and are unmanageably long. This can make coding, debugging, and editing difficult.
• Data Structure: Ladder diagrams address memory in single memory bits or registers and although it can read and write to variables anywhere in the program, this makes it difficult to protect data or to group data as a structure. Unprotected data is at risk for accidental modification by faulty code elsewhere in the program.
• Execution Control: Rung execution timing is defined by the speed at which the PLC can scan and execute, and multi-rate applications will require a “jump” command to move to a different part of the diagram. This can cause problems while designing systems that need to respond with particular timing, such as PID.
• Arithmetic Operations: Ladder logic requires the use of function blocks to perform mathematical operations, but because the function block inputs and outputs are referenced to memory locations and are not wired together, complex algorithms involving large numbers of variables and intermediate results can be cumbersome to program, document, debug, and edit.

Ladder Logic 102

As one of the many different types of programming methods in the industry today, it is important to understand ladder logic in order to know when to use it instead of something else. Ladder logic is possibly the most beneficial with its direct representation of the circuit logic it is implementing, but it has more pros and cons than first meet the eye.

Our developers here at PanelShop.com are experts in their areas, and they know exactly when a tool like ladder logic will work best. If you have any questions about the use of ladder logic, or some other form of controller programming language, give us a call today!

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