the method used by the plc to write a ladder logic program is called

Pretty simple, right? And in its very basic form, that’s what Ladder Logic is. The option you choose is dependent on your application and on how the system is expected to perform. In this case, the door opening or the motion detection will trigger the alarm. With the OR gate, the inputs are wired in parallel and only one of the conditions is needed. The door will have to be opened and the motion detector tripped before the alarm is triggered. When wired in series, as with the AND gate, both conditions will have to be met before the alarm is activated. To equate this to a control circuit, let’s say Input A and Input B (often referred to as permissive bits) are the Door Open and Motion Detected inputs of a home alarm system. These truth tables can be duplicated with relay contact logic by connecting normally open contacts in series (AND gate) or parallel (OR gate) as seen below.

As the truth tables below show, the AND gate needs all inputs (A and B) to be true before the output becomes true, whereas the OR gate needs just one input (A or B) to be true to make the output true. The two logic gates that you will see represented most often in Ladder Logic are the AND and OR gates. Instead, we’ll just look at a few logic gates and how they work. Now, this blog is not intended to be a Digital Systems class so put away your Karnaugh maps, you won’t need them. And to do that, we need to first understand Boolean math and logic gates. So now that we have a understanding of what Ladder Logic is, we can dig a little deeper into how ladder instructions work. In Understanding Ladder Logic we touched on the origins of Ladder Logic, its structure and execution.

The method used by the plc to write a ladder logic program is called