Programmable Logic Controllers 4th Edition

Book Preface

Programmable logic controllers (PLCs) continue to evolve as new technologies are added to their capabilities. The PLC started out as a replacement for hardwired relay control systems. Gradually, various math and logic manipulation functions were added. Today PLCs are the controller of choice for the vast majority of automated processes. PLCs now incorporate smaller cases, faster CPUs, networking, and various Internet technologies.

This Fourth Edition of Programmable Logic Controllers continues to provide an up-to-date introduction to all aspects of PLC programming, installation, and maintenance procedures. No previous knowledge of PLC systems or programming is assumed. As one reviewer of this edition put it: “I honestly believe that someone with little or no background to PLC systems could take this book and teach themselves PLCs.”

The primary source of information for a particular PLC is always the accompanying user manuals provided by the manufacturer. This textbook is not intended to replace the vendor’s reference material but rather to complement, clarify, and expand on this information. With the current number of different types of PLCs on the market it is not practical to cover the specifics of all manufacturers and models in a single text. With this in mind, the text discusses PLCs in a generic sense. Although the content is of a nature to allow the information to be applied to a variety of PLCs from different manufacturers, this book, for the most part, uses the Allen-Bradley SLC 500 and ControlLogix controller instruction sets for the programming examples. The underlying PLC principles and concepts covered in the text are common to most manufacturers and serve to maximize the knowledge gained through attending PLC training programs offered by different vendors.

The text is written at a level and format understandable to students being introduced to PLCs for the first time. Feedback from instructors indicates that the information is well organized, to the point, and easy to understand. The content of this new Fourth Edition has been updated and reflects the changes in technology since the publication of the previous edition.

Each chapter begins with a brief introduction outlining chapter coverage and learning objectives. When applicable, the relay equivalent of the virtual programmed instruction is explained first, followed by the appropriate PLC instruction. Chapters conclude with a set of review questions and problems. The review questions are closely related to the chapter objectives and require students to recall and apply information covered in the chapter. The problems range from easy to difficult, thus challenging students at various levels of competence.

This new Fourth Edition has been revised to include a number of new features:

How Programs Operate When the operation of a program is called for, a bulleted list is used to summarize its execution. The list is used in place of lengthy paragraphs and is especially helpful when explaining the different steps in the execution of a program. Representation of I/O Field Devices Recognition of the input and output fi eld devices associated with the program helps in understanding the overall operation of the program. With this in mind, in addition to their symbols, we provide drawings and photos of field input and output devices.

New ControlLogix Chapter Some instructors have felt that students tend to get confused when switching back and forth from SLC 500 Logic to Logix 5000–based programming within the same chapter. For this reason, a new Chapter 15 has been added that is devoted entirely to the Allen-Bradley ControlLogix family of controllers and the RSLogix 5000 software. Each part of the new Chapter 15 is treated as a separate unit of study and includes ControlLogix:

• Memory and Project Organization
• Bit-Level Programming
• Programming Timers
• Programming Counters
• Math, Comparison, and Move Instructions
• Function Block Programming