Rockwell Automation 1761-HHP-B30 MicroLogix 1000 with Hand-Held Programmer (HHP) - Manual (Page 109)

Chapter 6
Programming Overview
6–12
In a ladder diagram, each of the input devices are represented in series or
parallel combinations across the rung of the ladder. The last element on the
rung is the output that receives the action as a result of the conditional state
of the inputs on the rung.
Each output instruction is executed by the controller when the rung is
scanned and the conditions on the rung are true. When the rung is not
scanned or the logic conditions on the rung do not create a true logic path,
the output is not executed. The maximum number of instructions per rung
is 128.
Series Connections
One form of logical continuity is series (AND) logic. This means that when
all input conditions in the path are true, energize the output.
a b c
In the above example, if A and B are true, energize C.
Parallel Connections
Another form of logical continuity is parallel (OR) logic. This means that
when one or another path of logic is true, energize the output.
a c
b
In the above example, if A or B is true, energize C.
Parallel logic is formed using branches in your program. Branches can be
established at both input and output portions of a rung. The upper limit on
the number of levels which can be programmed in a branch structure is 75.