Siemens PLC Basics

  

Basic Plc's

Basic Plc's covers the following topics :

[ Plc overview , Plc Hardware , Numbers System ,Codes and Data Types, Inputs and Outputs,Plc programming concepts, Basic ladder diagram programming, and Basic function block diagram programming. ]

What is a PLC?

• Most factories contain dozens or even hundreds of machines, from the very simple to the complex. Some are still operated and controlled only by skilled workers.

• However, most complex machines Rely on programmable logic controllers, PLC'S for short A PLC is an industrial computer that is used to control machines and processes.

• PLC'S differ from office computers in the types of tasks that they perform and the hardware and software they require.
  

• While the specific applications vary widely, all PLC'S get inputs from switches and sensors, make decisions based on a stored program, and control outputs to automate machines and processes.

• Before PLC'S were invented, machines were often controlled by Electromechanical devices with complex wiring. This approach is called hard-wired control.
  

• Hard-wired control is still used for simple tasks, but for larger applications it is difficult to design, install, and maintain.

• Because tasks performed by PLC'S are controlled by software and internally stored values, use of PLC'S offers exceptional flexibility.

• As a result, PLC'S are now used to run almost any industrial application, and almost every new machine that comes into a factory has a PLC controlling it. Consequently, many industrial jobs require knowledge of PLC'S.

History of PLC'S

• The very first PLC'S were designed for use in the automotive industry in the late 1960s.
  

• Prior to that time, control of an auto assembly line relied heavily on Electromechanical Relays, Contactors, Timers, and Related devices.
  

• Every year, as design changes were made to different car models, these circuits had to be rewired.
  

• Rewiring circuits was a complicated and time- consuming task, and even when the relays performed well initially, the wear on all those physical contacts resulted in costly maintenance.
  

• Additionally, as manufacturing processes became more and more complicated, more relay circuits were needed. This resulted in even greater cost and complexity.

• Because the hard-wired circuits replaced by PLC'S used control circuit diagrams referred to as ladder diagrams, early PLC'S used ladder diagram software programs, sometimes referred to as ladder logic, to make it easier for someone familiar with control circuits to program a PLC.
  

• Unfortunately, every PLC manufacturer had its own version of ladder diagram programing.
  

• This variation in programming grew as PLC'S were developed to handle a wider range of tasks.

• Today, most PLC'S can still use ladder diagram programing, but the IEC 61131 international standard now defines this PLC programming language.

• This same standard also defines other types of programming languages available for PLC'S, so that they can be used for the most complex applications.
  

• In addition even a small PLC can easily outperform hard-wired  Electromechanical circuits.

  
  

  
  

NEXT

Chapter 1 we will discuss about Plc Components, How PLC works,

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