Chapter 5
Ladder Program Basics
5–8
The following 4-rung ladder program uses the same 3 bit addresses as our
simple 1-rung diagram. It also uses an external input bit address and an
external output bit address. Note that individual bits are addressed
repeatedly. For example, B3/11 is addressed with an XIC instruction in
rungs 1 and 4, and it is addressed with both an XIC and an OTE instruction
in rung 2.
During normal controller operation, the processor checks the state of the
input data file bits then executes the program instructions individually, rung
by rung, from the beginning to the end of the program; as it does, it updates
the data file bits and the appropriate output data file bits accordingly.
When XIC instruction I:0/1 goes true (because an external momentary push
button closes):
•
Rung 1 is evaluated as false, because XIC instruction B3/11 is false at this
time.
•
Rung 2 is evaluated as true. XIC B3/11 in the branch of this rung goes
true to maintain continuity in the rung.
•
Rung 3 is evaluated as true.
•
Rung 4 is evaluated as true because XIC B3/11 has gone true. The
external device represented by OTE O:0/2 is energized.
A 4-Rung Ladder Program