First pass - OSR

[FiestaEng]

I am trying to understand and will be cleaning up a AB ladder logic program. The first rung has;

|----- NOT S:1/15 ------- OSR B3:0/0 -------------------------- ( B3:0/1) --------|

B3:0/1 then turns B3:0/2 on and holds itself on with a branch around B3:0/1

Most, if not all, rungs are then only true if B3:0/2 is on.

1) What is S:1/15?

2) What advantage is it to set this bit on during the "First Pass" as it's labeled?

Thank you in advance for your help

 

[xyzz]

S:1/15 is "First Pass", that is first scan pulse.
This is a system read-only bit active only in the first scan after power up or switching from programming to run.
The B3:0/1 in the shown code is "not first scan pulse", that is "the second scan".
If the B3:0/2 is being set by "the second scan" and self-retain, it will mean "not the first scan", which is the same as NOT S:1/15. The goal of such unnatural definition of "not the first scan" cannot be clear without seeing the program.

 

[FiestaEng]

Ok, so I looked into it further and found one other use of the B3:0/1. Way down in the program, B3:0/1 is used as one way another bit is unlatched.

So it looks like nothing happens the first scan;
second scan gets the bit unlatched;
and the third scan the whole program is looked at it.

Does that make sense? Am I thinking correctly?