writing to SFC_STEP.X in RSLogix - accidentally enter any step in a SFC???

I am used to using SFC in Schneider unity and we use the step.X bit in the logic to see if the step is active. We don't use parallel branches so only one step can be active at a time.



I have found in RSLogix I can simply toggle the STEP.X bit for any step. It seems dangerous if the PLC operates some outputs when a step is active because we are depending on other things to have happened in the previous steps, but if I accidentally CTRL+T in a ladder routine with the XIC STEP.X accidentally selected then the step becomes active (as well as the other step that was actually active) and the PLC operates the outputs accidentally.


Is there some setting to make it impossible to write to the STEP.X bits?

Anybody who uses SFC in Logix5000 how do you avoid this?

Thanks for any insights

I’ve been using the sfc in rslogix for quite some time. Until now, I never knew that you could write a step true with ladder or toggle. I guess the question is why would you do that? That kind of completely bypass the reason for using SFC.

If you go under controller properties there are some sfc settings that can be manipulated. I don’t know them off the top of my head as I haven’t modified the defaults in a long time.

I do read the step.x bits to write to outputs in ladder that would be to cumbersome to code in sfc. However I have never felt the need to make a step bit true outside of the sfc that contains it.

Edit: I re read your post and I see know you are trying to prevent an accidental toggle. I read that wrong. I’ll poke around on Monday and see if I find anything.

Hi, thanks for the reply and edit.



being able to accidentally set a step true seems like a real land mine. If something can happen by accident and then it results in 'unexpected operation' of the plant, environmental consequences etc ... it seems like SFC can't safely be used and can't be relied upon.



There are always some things in a plant that are "don't do that" but I've already done this twice in the last two years (programming not on actual plant), once by an accidental CTRL+T and once by OTE when i wanted XIC.


I don't see anything in the step properties, controller properties, that would prevent this. I already have the external access on the tag set to read-only.


All I can think of is to do an XOR with all the steps to make sure only one is active at a time and if it is false then trigger a fail-safe emergency shutdown of the plant since I will know the PLC is confused about what it should be doing and can't be relied upon.