Rockwell Automation 1772-L8_LW_LWP_LX_LXP,D17726.5.8 User Manual - Manual (Page 20)

Fundamentals of a
Programmable Controller
Chapter 2
2-6
Program Storage
Program storage takes up the largest portion of memory. This is where the
user’s program is stored. Each program is made up of a set of statements.
Each statement does two things:
It describes an action to be taken. For instance, it might say, “Energize
motor starter number one.”
It describes the conditions that must exist in order for the action to
take place.
Statement
Statement
Statement
Statement
Statement
Statement
Program Storage Area
of Memory
Action Conditions
Program
Statement
Program
For example, you may want this action to take place: “Whenever a certain
limit switch closes.” So your condition could be: “If limit switch number
two is closed,...” The action would be: “energize motor starter number
one.” Therefore, when limit switch number two at the machine closes, the
programmable controller energizes the motor starter. If limit switch
number two does not close, the programmable controller does not energize
the motor starter. Thus, when limit switch number two opens, the
programmable controller de-energizes the motor starter because that action
is implied in the statement.
A program is made up of a number of similar statements. Typically, there
is one statement for each output device on the machine. Each statement
lists the conditions that must be met and then, states the action to be taken.
Each condition is represented by a specific instruction; therefore, each
action is represented by a specific instruction. These instructions tell the
processor to do something with the information stored in the data
table.Some instructions tell the processor to read what’s written in the
image table. When the processor is instructed to read from an image table,
it examines a specific bit to see if a certain I/O device is on or off.
Other instructions tell the processor to write information into the image
table. When the processor is instructed to write into the output image
table, it writes a one or a zero into a specific bit. The corresponding output
device will turn on or off as a result.