Rockwell Automation 1772-LS_LSP,D17726.8.6 PROG/OPER MANUAL-MINI PLC-2/05 - Manual (Page 20)

An Introduction to
Programmable Controllers
Chapter 2
2Ć6
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.” The entire
statement is then: “If limit switch number two is closed, then 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.
Instructions
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.
Input
The second section is the input, which serves four very important functions:
termination
indication
conditioning
isolation
Termination
The input provides terminals for the field wiring coming from the sensing
devices on the machine.
Indication
The input of most modules also provides a visual indication of the state of each
input terminal with indicators. The indicator is on when there is a voltage
applied to it terminal. It is off when there is no voltage applied to its terminal.
Since the indicator reveals the status of its terminal, it’s usually called an input
status indicator.