PLC Programming

Introduction

Welcome to another course in the STEP series, Siemens

Technical Education Program, designed to prepare our

distributors to sell Siemens Industry, Inc. products more

effectively. This course covers Basics of PLCs and related

products.

Upon completion of Basics of PLCs you should be able to:

• Identify the major components of a PLC and describe

their functions

• Convert numbers from decimal to binary, BCD, and

hexadecimal

• Identify typical discrete and analog inputs and outputs

• Identify key differences of the various S7-200 models

• Identify the types of expansion modules available for

S7‑200 PLCs

• Describe the types or programming available for S7-200

PLCs

• Describe the operation of commonly used program

functions such as timers and counters

• Identify the proper manual to refer to for programming or

installation of an S7-200 PLC

This knowledge will help you better understand customer

applications. In addition, you will be better able to describe

products to customers and determine important differences

between products. You should complete Basics of Electricity

before attempting Basics of PLCs. An understanding of many

of the concepts covered in Basics of Electricity is required for

this course.

After you have completed this course, if you wish to determine

how well you have retained the information covered, you can

complete a final exam online as described later in this course. If

you pass the exam, you will be given the opportunity to print a

certificate of completion.

PLCs

A programmable logic controller (PLC), also referred to as

a programmable controller, is the name given to a type of

computer commonly used in commercial and industrial control

applications. PLCs differ from office computers in the types of

tasks that they perform and the hardware and software they

require to perform these tasks. While the specific applications

vary widely, all PLCs monitor inputs and other variable values,

make decisions based on a stored program, and control

outputs to automate a process or machine. This course is

meant to supply you with basic information on the functions

and configurations of PLCs with emphasis on the S7-200 PLC

family. Basic PLC Operation The basic elements of a PLC include input modules or points,

a central processing unit (CPU), output modules or points,

and a programming device. The type of input modules or

points used by a PLC depends upon the types of input devices

used. Some input modules or points respond to digital inputs,

also called discrete inputs, which are either on or off. Other

modules or inputs respond to analog signals. These analog

signals represent machine or process conditions as a range of

voltage or current values. The primary function of a PLC’s input

circuitry is to convert the signals provided by these various

switches and sensors into logic signals that can be used by the

CPU.

The CPU evaluates the status of inputs, outputs, and other

variables as it executes a stored program. The CPU then sends

signals to update the status of outputs.

Output modules convert control signals from the CPU into

digital or analog values that can be used to control various

output devices.

The programming device is used to enter or change the PLC’s

program or to monitor or change stored values. Once entered,

the program and associated variables are stored in the CPU.

In addition to these basic elements, a PLC system may also

incorporate an operator interface device to simplify monitoring

of the machine or process.

example shown below, pushbuttons (sensors)

connected to PLC inputs are used to start and stop a motor

connected to a PLC output through a motor starter (actuator).

No programming device or operator interface are shown in this

simple example.

Hard-Wired Control Prior to PLCs, many control tasks were performed by

contactors, control relays, and other electromechanical devices.

This is often referred to as hard-wired control. Circuit

diagrams had to be designed, electrical components specified

and installed, and wiring lists created. Electricians would then

wire the components necessary to perform a specific task. If

an error was made, the wires had to be reconnected correctly.

A change in function or system expansion required extensive

component changes and rewiring.