Why use the "B:"?

Good day, I have attached the example of the paper drilling machine example from SLC500 ref manual. My question is, why would they assign the "RUN" output a B:3/0? Why wouldn't they use say an O:3/1? Is it because it's not connected to a discrete output? Thank you

Sorry, here's the attachment

Greetings ceilingwalker ...

read the following post about using internal (B-type) bits throughout the rungs of the program - and then "mapping" these bits to the real world outputs at the very end ...

http://www.plctalk.net/qanda/showthread.php?p=356564&postcount=70

note that some people LOVE this approach - and some people HATE it ...

Thank you

One advantage of this method is, if a physical output has multiple OR conditions which could turn it on (condition1 or condition2 or condition3) then each of condition1, condition2 etc can be evaluated in its own rung and the output assigned to a bit. Make sure to assign a name to the bit which describes it.

Then, near the end of the scan, have one rung for each real output. Bring together all the previously defined bits in parallel (branching) and then place the output. That way you could read the final rung (I do it out loud, people stare) as "If condition1 OR condition2 OR condition3 then turn on the output." It makes the logic much simpler to follow.

Note that a certain condition may turn on more than one output. It's perfectly fine.

Make sure to assign a name to the bit which describes it.

Your advice above is well taken! Most of them are not described in a ladder written by someone else. Makes it hard to know what can and can't be removed from a project. Thank you

They were probably originally documented in that ladder that was written by someone else, but poor practices have caused that documentation to be lost.

You are so very correct: Properly documented programs are essential!! Learn that lesson now, grasshopper, and you'll go far!