PLC Programming Interview Questions - Building a 2 Button Motor Starter System Ladder Logic RSLogix

PLC Programming Interview Questions - Building a 2 Button Motor Starter System Ladder Logic RSLogix

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There is a number of scenarios one can create to test PLC Programmers. One of the most simple, yet challenging for a beginner, questions is the famous motor starter. A simple system which utilizes two buttons and an output stumbles those who've little to no experience with PLCs. Furthermore, this question will reveal if the candidate asks the right questions, makes the correct assumptions and is able to reason through a simple assignment presented to him.

Before you start implementing a solution to any problem, it's important to stop and think about the requirements. This is true in every field and especially controls. Spending a moment to reason your implementation will save you debugging time down the road.

The problem is most commonly presented as follows. Two buttons are used to control a motor starter. One should start the motor and the other should stop the motor. In certain cases, the interviewer will specify the polarity of the buttons (Normally Closed or Normally Open) as well as the type (Momentary or Latched), but I personally wouldn't expect them to. This is where critical thinking comes in.

Based on the specs mentioned above, you should start by asking basic questions to clarify the problem. Does the motor need to stay running when the "Start" button is released? Common sense would dictate so. If it's a maintained push button, the hardware will take care of it; if it isn't you'll need to consider it in code. Are the buttons normally open? In certain cases, the "Stop" buttons are always Normally Closed. This is due to the fact that you can easily use them in that state to build a hardware-based circuit. In other words, you don't need any programming at all and the current flows through the button terminals.

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