Step7 ladder logic branch selection

[Peter Quiring]

In Siemens Step7 Ladder Logic if there is a branch, which direction does the PLC go first? I'm assuming down. Is that correct?

 

[Tharon]

It should be left to right, top to bottom, same as the rungs (or networks, I think Step 7 calls them) themselves.

 

[mk42]

In general left to right, top to bottom is the rule, but things can get tricky when there is complex branching. I've seen cases where the bottom rung in a branch went before the top.

The easiest way to see what will happen for sure (if using a 300/400) is to look at the LAD in STL. That requires a bit more complex knowledge to parse, but STL is (ignoring jump commands) always top to bottom and linear.