Planning for “Abnormal Exits”

Planning for “AbNormal Exits” Introduction The purpose of this paper is to discuss the “abnormal exit” and how it plays into your planning for mechanical design and into your programming. What is it? An abnormal exit is something that happens that causes the system program to trip or interrupt or come to a halt and that may or may not happen in a graceful manner. The system response will also be dependent on the “fail safe” strategy that was incorporated into the mechanical and/or process design. Example ─ Power Failure One of the more obvious causes of abnormal exits is the proverbial power failure.  With a power failure, the programming logic has no vote in how the system handles itself.  When a power failure happens, the system behavior is dependent on the failsafe strategy of the mechanical designer.  The failsafe strategy is not usually as critical in machine control as it is in process control. When recovering from a power failure it’s important to know whether you had a graceful shutdown which can be accomplished by setting flags within the program to keep track of whether …