Step7 - problem translating from STL to FB

The following code snippet is part of a longer STL Network monitoring the press-fitting of a bearing. The error condition gets set if the timer times out.


In order to better understand what exactly is going on, I wanted to convert it to FB, which I'm more comfortable with for this sort of thing. However, try as I might, I can't get it to come up with an identical listing when I convert my FB version back to STL.

This was pretty well my nearest approach so far (I replaced the Timer with M1000.0 for test purposes):



When I convert it back to STL I got the following:


which is not quite right. Had the final "NOT" been after the second closing bracket, I might have accepted it as being equivalent, but not where it is.

If I try moving the negation to immediately before the assign command (=) in the FB instead of on the preceding &-input (which is where I reckon it ought to be), the the outer bracket pair disappears completely, which is also not quite right!

Would anybody like to have a go at producing an FB (or Ladder) version of the commands which translates exactly to the first version.

Here you go Roy:


... and here it is in FBD format:

Thanks Simon,
I had a feeling that I was mising an AND-gate, but I wasn't happy with any of the solutions that I came up with. I must admit, I'm still struggling to convice myself that the above code is identical to the original. I guess I'll finish up building a truth table of all possible states to convince myself!

Cheers

Roy

Edit: Just noticed that my original scribbles on the proverbial back of the envelope (actually the back of some old listings) finished up identical to your solution above. Looks like the main problem I'm having is conviincing myself that the two slightly different STL renderings are logically identical.

I ran both implementations in the simulator and compared the output results for all possible input combinations to prove they were logically equivilant.

Yes, I got round to drawing up a truth table yesterday and finally convinced myself - with five variables that's still doable. Don't think I'd like to try it with a few more though!