![]() ![]() Ladder logic is based on principles from digital logic gates. There are three basic types of digital logic gates: the AND Gate, the OR Gate, and the NOT Gate. Combine symbols to create a logical flow. Just know that these names are interchangeable and synonymous.ģ. I learned on Allen Bradley PLCs, so I’ll refer to the XIC / XIO / OTE combination. Common symbols are called “Normally Open”, “Normally Closed” and “Output.” Check out the chart below. You’ll notice a few basic symbols in the ladder program, above. Ladder rungs are read starting from the left to the right and then sequentially, top to bottom.Ģ. This builds out a program that looks like a ladder. There are two rails (left and right) and they are connected by rungs. Rungs are Read Left to Right and Top to Bottom Ladder Logic is very easy to understand once you get the hang of a few basic principles. When I was first learning ladder logic, I had a hard time wrapping my brain around the symbology. Ladder Logic is a rule-based language, meaning the conditions must be met for the output to be energized. Programs in Ladder Logic are written differently than embedded or flow-chart programming. ![]() Ladder Logic Fundamentals (How to read and write Ladder Diagrams) Now that we have a general overview of how the PLC executes the program scan cycle, we can talk about ladder logic fundamentals. It’s just something to be aware of if you’re working with larger-scale systems. Therefore, we don’t have to worry so much about scan times. I also want to point out that the ladder programs we’ll be looking at are very simplistic. Scan time is influenced by a few factors including the processor module speed, length of the ladder program, type of instructions executed, and the actual true/false conditions. This is important because if the controller has to react to an input event faster than the scan time, it may never detect the change. Scan times can vary from 1 millisecond to 20 milliseconds. The diagram below shows a model of the PLC Program Scan Cycle. Because inputs can change at any time, this process repeats over and over again while the PLC is in RUN mode. During the Program Scan Cycle, the PLC reads the inputs, solves the Ladder Logic, and energizes (or de-energizes) outputs. When a PLC executes a program, it must know, in real-time, when external devices controlling a process are changing. Learn PLCs by Working with Real Hardwareīefore we can dive into the programming aspect of PLCs, it’s important to understand how the PLC works.Ladder Logic Fundamentals (How to read and write Ladder Diagrams). ![]()
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |