That was necessary for the LogixPro Elevator because a new floor request could come in at any time, unlike your stripped-down bare-bones 2-floor crippled version that only goes up then back down automatically in response to ONE 2nd-floor request.ĮDIT: Further testing indicates that the alarm output in the 26062013 version only needs to be in the Main, Sub 5 and Sub 6. In the 19062013 version, and also in my LogixPro Elevator Student Exercise 4, I did return to the Main every scan where possible, so that the main ladder logic always called EACH subroutine.

Because you have to lock in the Open Door and Close Door subroutines to prevent exiting until they are finished, that means that if something goes wrong and someone presses the alarm button while the PLC is inside one of those subroutines, the alarm will not go off unless there is an output inside each subroutine (or you use an alarm flag in each subroutine and manage to set up your logic so it always returns to the Main on each scan, where you have only 1 version of the alarm output). Where you have stacked subroutines as in the last version 27062013, it is difficult to avoid reusing an output, for example your elevator alarm output. Otherwise it could get left ON, wrecking havoc in your program and causing it not to work correctly. That worked okay IF you put in interlocks to insure that each of those outputs had done its job then was turned OFF again before exiting that subroutine. For example, in your case when you only had 2 subroutines, Going Up and Going Down, the Open Door and Close Door outputs had to be used 2 times each. I remember as well the double-coil syndrome.įrom reading your responses, possibly you do not realize that (unlike double coils in the Main ladder) it is okay to reuse the same coil or output in a subroutine IF you set up your logic to prevent unwanted consequences.

Logixpro elevator simulation download