Pop Machine problems

Hi all,

Working on another project in class, a Pop Machine this time.

I currently have everything working, except for the change return.

Using the compare commands (GEQ, LEQ, EQU) to return the proper change. I'd like to know if there's away to make a loop in this subroutine. i.e. if the subtract command leaves an integer in the regsister, it will loop back up and subtract again until the register reads "0".

Thanks, I appriciate all the help I received on my last question.

Rickk

You **can** probably use a loop, but I'd venture a guess to say that that would be the worst possible approach, as in a real-world situation, you would be stuck in that loop for an awful lot of scans, which could time out the processor watchdog.

Since the logic should be evaluated every scan, as long as a 'Change_Due' flag is set, you should be evaluating your coin return logic. Also, you should probably have an overall timeout alarm to determine if it takes more than say 10 seconds to return all change.

Do Not Re-invent the Wheel

Rick K,

Take a look at what another student did on the vending machine.

http://www.plctalk.net/qanda/showthread.php?p=177036&mode=threaded&highlight=Vending+Machine#post177036

Take his ideas and improve on them. Member Atlascycle's final program is available in the next-to-last post in that thread. He did not "loop" in the Change subroutine, but used subtraction commands to remove the coins deposited from the amount due, until the balance was 0.

For an application like this, it is unlikely that you need to loop back within the program. You can accomplish the same result by doing the calculation in successive scans.

Anyone who has ever programmed a loop knows how easy it is for the program execution to get stuck in one. This is just as true for a PLC executing ladder logic code as for a PC executing a Visual Basic or C application. Most PLCs have a built-in watchdog timer. If the program gets stuck in a loop for longer than the watchdog timer, the PLC shuts itself down and turns all of its outputs off. Because of this, it is generally better to avoid the use of loops when there are alternative methods of getting the job done.