PLC alarm logic

Now that we have set up an alarm, we need to get into the guts of the alarm, which is what I like to call the alarm logic. There is nothing fancy or complex about triggering an alarm. As we have seen, all we have to do is set a variable to true or false. However, understanding when to set the variable is the trick. For most things in automation, we use bounds or operating ranges to determine whether the part is in a healthy state or not. In other words, many things, such as heaters, motors, and so on, have an optimal operating range that they should always be in. Straying from the optimal range can easily affect the performance of the machine. Some of the most common situations that need immediate alarms are situations that can result in the injury of a person or the surrounding environment. In these cases, you will not want to set up a tolerance range; you will simply want to throw an alarm.

Typically, when you’re working with a range, you will use the HMI...