Which is the best procedure to reset a SFC finite state machine

[paulovedder 0]

Hello, One thing that I constantly find hard to do when I program in SFC is to properly reset a FSM. If something goes wrong during the process I like to be able to reset the program. In Ladder this is quite simple to accomplish. Usually, I have a variable (non volatile) which is responsible for the step transition. If I want to reset the FSM I just use the function MOV to disable every step variable, which turn off the entire FSM. If I want to restart the program I use the MOV function again to enable only the first step. So my question is: how can we accomplish that in a SFC program? Lets not consider the option of having a reset transition in each step... Best regards, Paulo Edited 6 Feb 2015 by paulovedder

[OriolFM 0]

I suggest you to use integers to track the current and next states. I would also do the variables volatile.

If there is an error, you can just send the machine to whatever standby state you have.

If there is a power cut, the machine can initialize the state machine to whatever integer you want on the first scan (even with ladder, you can achieve that with a function block). Personally, I have a special initialization state that is triggered after the machine powers up and that loads the hardware configuration. In this way, I do not rely on persistent variables that could be lost if there is a power cut and the battery in the PLC is depleted.