Rockwell Automation 1772-LV Mini-PLC - 2/15 Programmable Controller (Series B) Programming and Operations - Manual (Page 15)

An Introduction to Programmable
Controllers
Chapter 2
2Ć8
For example, you may want the 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 would then read, “If limit switch number two is closed, then
energize motor starter number one.” Therefore, when limit switch number two
at the machine is closed, the programmable controller would energize the motor
starter. When the condition is not met, however, the action, “energize the motor
starter” is not taken. thus, when limit switch number two opens, the
programmable controller responds by de-energizing 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 first lists the
conditions that must be met and second, 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 CPU to do
something with the information stored in the data table.
Some instructions tell the CPU to read what’s written in the image table. When
the CPU 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 CPU to write information into the image table. When
the CPU 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.
Let’s look at a simple example to see the sequence of events that take place in
controlling a machine with a programmable controller (Figure 2.4). Suppose
you are making a unit. This unit would be carried to the work area by the motor
driven conveyor. The limit switch will detect when the part has arrived at the
work area. When that happens, we want the conveyor to de-energize so work
can be done on the part.
Notice how the limit switch and motor are wired to the programmable
controller. The limit switch, wired to terminal 02, is normally-closed. The
arriving part will open the switch. Therefore, the program statement controlling
the conveyor motor must read, “If there is voltage at input terminal 02 (limit
switch), then energize output terminal 02 (conveyer motor).” The conveyor
motor is wired to output terminal 02.
NOTE: Figure 2.4 is for demonstration purposes only. We do not label
associated wiring, a motor starter, or an emergency stop button.
PC Control Sequence