By 'multiple ladders', do you mean subroutines? If so, then they would be scanned in what you refer to as series. When a subroutine is called, it jumps to that subroutine, then returns to the point where it was called, and continues on.
By 'multiple ladders', do you mean subroutines? If so, then they would be scanned in what you refer to as series. When a subroutine is called, it jumps to that subroutine, then returns to the point where it was called, and continues on.
The scan of multiple ladder rungs (or just rungs) is sequential. This is the primary difference between a PLC and had wired relay control where the effect of all contacts and outputs is simultaneous. Sometimes failing to remember this can lead to problems. At other times knowledge of this fixed sequence can be used to great advantage.