Lesson 1
PLC BASICS
PLC Definition

Programmable Logic Controllers are industrial computers
that control machine and other applications.

PLC have replaced hardwired devices such as relays &
contacts

Automation and Control: Real time I/O

Field device: switches, output devices and drives
Advantages of PLC
1.
Less Space: PLC requires less space than a computer or a relay
panel to do the same job.
2.
Lower Cost: Can cost as little as 4 or 5 relays, but can replace
hundred of relays
3.
Resistant to Environment – Industrially hardened to withstand harsh
environment
4.
Direct Interface – Standard computers require a complex and
limited system to interface to real world components. PLC
interface is directly wired to components
5.
Easy Programming – PLCs most often use ladder logic, language
based on industrial standard symbols used in hardwired relays.
6.
Flexible: programming can be changed quickly
PLC Applications
PLC can control any number of applications:

Assembly Machines

Automatic Transfer Lines

Packaging Lines

Robots
Components of a PLC
1.
Input modules
2.
Output modules
3.
Controller (Processor Module)
4.
PLC power supply
5.
I/O chassis
6.
Programming device
All industrial PLCs use similar configurations.
890-AB5500 PLC
Controller of a PLC
PLC Processor Module, also called a controller, is the actual computer
that controls the PLC. It consist of the CPU and memory. The program
is stored in the processor memory. ControlLogix processors usually
contain at least 2MB of user memory.
Prominent processor components include
alphanumeric display, LED status indicators,
the energy storage module (ESM),
SD card cover, and the USB communications port.
PLC PROGRAMMING

All PLC must have user develop project to operate. Projects can be
entered into the PLC memory:
1.
A personal computer
2.
SD card
3.
Handheld terminal
Personal Computers
RSLogix 5000 software is used to develop ControlLogix projects,
configure inputs and outputs, develop ladder routines, and transfer
projects to the PLC and monitor its operation.
Secure Data (SD) Card

Used to store backup programs on nonvolatile memory cards to be
restored to the controller if a memory failure occurs.

Program is developed in RSLogix 5000 software, downloaded to the
PLC memory, and transferred to SD card.

The user configures the card for the conditions that cause the
transfer of the program to the controller, such as a power and
memory failure.
Handheld Terminal

Used to enter, monitor, or change PLC user programs.

Easy to carry

Not used with ControlLogix controller
Basic Operation of PLC

The basic operation of a PLC is to scan instructions, monitor the status of input
devices, and produce outputs based on the status of the inputs.

The status of field input devices, such as limit switches, represent conditions in the
machine or process.

Outputs provide control by turning on and off output devices, such as motor starters
and solenoid valves.

PLC is a specialized computer that works with binary information.

There are 4 basic data types connected to the PLC: discrete input, discrete output,
analog input, analog output. Hardware modules in the PLC chassis or rack use
electronics and firmware to process the input signals and generate the output signals.
Discrete Signal

Discrete input represented by two-state, electricity flowing (T) or not
flowing (F). The binary system relates this to 1 or 0 values of software.

Examples of discrete input signal are: switch, photocell, pushbutton,
contact or proximity sensor.

The discrete input module would represent this presence or absence
of electrical signal as a 1 or 0.

The discrete output is also also a 2 state signal, represented by
electricity flowing or not. Examples are lamps, motor contacts and
solenoid valves.
Analog Signal

Analog signals are represented by a range of electrical signals, such
as 1-5 VDC or 4-20 mA. This electrical signal is generated by a
transducer, which converts the field value to a proportional
electrical signal. The analog input module samples the input signal
and converts it to a 16 digit binary number in the range of 32,787.
The low end would be represented as 0, while the high end of the
signal 5V or 20 mA will be represented by 32787. The current value
of the voltage or current is converted to a binary number which is
proportional to the electrical signal between the two limits.

Analog outputs begin as internal 16 bit values, and are converted
into an electrical signal of 1-5VDC or 4-20mA. Examples are valve
positioners, turn drive speed controllers and any other variable
control device.
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