Rockwell Automation 9323-PA1E USER MANUAL APS 6.0 - Manual (Page 56)

Preface
Advanced Programming Software User Manual
4–2
Understanding Ladder Logic Programs
Ladder logic is a graphical programming language based on electrical relay
diagrams. Instead of having electrical rung continuity, ladder logic is looking for
logical rung continuity. A ladder logic diagram identifies each of the elements in an
electromechanical circuit and represents them graphically. This allows you to see
how your control circuit operates before you actually start the physical operation of
your system.
1
]/[
I
] [ ( )
O
1
I
input instructions output instruction
0
The ladder logic program you enter into the controller’s memory contains bit (relay
logic) instructions representing external input and output devices. It also contains
other instructions, as described in the Instruction Set Reference Manual.
As your program is scanned during controller operation, the changing on/off state of
the external inputs is applied to your program, energizing and de-energizing external
outputs according to the ladder logic you have programmed.
To illustrate how ladder logic programming works, we chose to use bit (relay logic)
instructions, since they are probably the easiest to understand. The three
instructions discussed in this section are:
] [
Examine if Closed (XIC)
Analogous to the normally open relay contact. For this instruction, we ask the
processor to Examine if (the contact is) Closed."
]/[
Examine if Open (XIO)
Analogous to the normally closed relay contact. For this instruction, we ask the
processor to Examine if (the contact is) Open."
( )
Output Energize (OTE)
Analogous to the relay coil. The processor makes this instruction true (analogous
to energizing a coil) when there is a path of true XIC and XIO instructions in the
rung.
Keep in mind that operation of these instructions is similar but not equivalent to that
of relay contacts and coils. In fact, a knowledge of relay control techniques is not a
prerequisite for programming with ladder logic.