What is a
PLC?
In this module you will be introduced to a controller called a PLC. The
initials PLC stand for programmable logic controller. A PLC is often
used as the "brain" in a wide range of modern controlled systems.
A PLC is essentially a low level computer capable of interfacing with field
instrumentation to read input data and control output data. PLCs can generally
be programmed from a PC, typically using ladder logic, sequential
function charts, and/or structured text.
In some cases, PLCs are connected to a central control computer. Each PLC works
independently to control its assignment while feeding information back to the
central computer.
PLC's form the basis of what is known as "distributed control". Just
as computer systems have migrated away from large mainframe computers, control
systems have distributed the "decision making" to PLCs (smart control
nodes).
How Does It
Work?
Programmable logic controllers take real world inputs from sensors and convert
these inputs into electrical signals. These signals update an input table in
the "computer".
An I/O board provides the communucation link between the processor and the
sensor. The input signals are used to determine which outputs are to be turned
on and off.
The PLC is
commonplace in today's industrial world. For example, because of their memory
input control capabilities, many elements of an assembly plant utilize PLC's.
In fact, just about every existing automation line is controlled by a PLC.
These small units are microprocessor-based and contain non-volatile memory
locations (memory that maintains data storage when power is removed) that
contain specific instructions programmed by a user for a given application. A
PLC can control a wastewater pumping facility for the utility department in a
city, while another PLC in a nearby factory controls a smelting furnace.
PLCs are used to control processes that need feedback in order to create
consistency. For example, in the Janesville WI, General Motors Truck Facility,
programmable logic controllers are used in the body shop to interface with
robots, sealing equipment, and automated hard tooling that welds the
sheet-metal body of a Suburban Utility Vehicle together.

Some
applications for PLC's in the Janesville plant are the following:
- Hard
Welding Automation

- Flexible
Manufacturing Systems

- Sealing
Applications

- Fluid
Fill Systems

- Facility
Monitoring Systems

The following tasks require the use
of hardware.
If you have access to the PLC Trainer System with the Allen-Bradley SLC 500 PLC
controller, click Start A/B SLC 500 Hardware Tasks below.
If you have
access to the PLC Trainer System with the MicroLogix PLC controller,
click Start MicroLogix Hardware Tasks below.
If you do not have access to hardware, click Skip Hardware Tasks below.
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What are
Inputs and Outputs?
In the following activity, you will activate and use a PLC to control a number
of devices. These devices can be divided into two main categories: input
and output devices.
- Input
devices transmit data to the PLC. The status of the input devices reflects the
status of the controlled system or an operating requirement. The PLC constantly
monitors the status of these devices and checks it with a pre-defined set of
instructions.

- Output
devices receive activation instructions from the PLC. The status of the
output devices is determined by the status of the input devices and the control
instructions.

Input and Output
Devices
As you have already learned, programmable logic controllers control real world
inputs and outputs in order to automate a process. Move your mouse over the
image to see some input and output devices. Input and output devices include:
- Proximity
Switches
- Flow
Meters
- Limit
Switches
- Solenoid
Valve

- Electromagnetic
Relay

- Motor
Starter

- Pressure
Gauges

- Horn
fitted in a car

Real-World
Example
Observe the example of a PLC-controlled level control system. A float is fitted
on a tank at the level marked as 2. The float is an input device that is used
as a level sensor. By controlling an on/off valve that serves as an output
device, the user can control the water level.
The control instructions are stored in the PLC's memory. The PLC constantly
receives an input signal (in this case, the water level). The PLC controls the
output device (valve) according to the input (water level) and the instructions
programmed in its memory.

Discrete and
Analog
I/O devices refer to the data read and written by the programmable
controller.
There are two types of input and output:
Discrete
I/O consists of data represented by 1s and 0s. Discrete I/O systems are
used to control systems that can be in only one of two states, such as an ON or OFF state for a motor or a lamp.
Analog
I/O uses a number of bits to represent an integer value. For example, the pressure in a pneumatic system could be any
value between 0 and the highest integer value available on a particular
pressure gauge's scale. 
Discrete input signals are usually used to determine if a certain event has
happened (for example, the piston is fully extended or the pressure is above 3
bars). Discrete output devices are used to control elements that can be only in
one out of two possible states.
You will learn more about this concept in the next module.




Introduction
to Ladder Logic
In this module, you will use a virtual PLC and input devices to control output
devices. You will program a virtual PLC using ladder logic programming
language, a programming language typically used in PLC programming.
Throughout this module, you will learn basic commands in the ladder logic
programming language. You will then program ladder diagrams using those
commands to control various industrial-type control systems. This graphical
language closely resembles electrical relay logic diagrams.
In this
activity you will learn about programmable logic controllers (PLC) and their
use in today's modern industrial world. You will experiment with a Virtual
Input/Output panel to learn about the difference between input and output
components.
This activity includes the following topics:
- Programable Logic Controllers
- Inputs and Outputs

OBJECTIVES
In this activity you will accomplish the following:
- Define
the term programmable logic controller.

- Define
the terms input and output.

- Learn
about typical uses of PLCs in industry and daily life.

- Distinguish
between discrete and analog inputs and outputs.

- Determine
whether devices are input or output.