PLC (Programmable Logic Controller)
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Last download on: Tue May 21, 2013 12:04:37 PM
This chapter brings together the digital elements introduced in Chapter 4 and Chapter 5 to show how a microprocessor based computer system can be created. The chapter begins with a more detailed examination of the state machine concept introduced in Chapter 4 and moves on to the microprocessor and its interaction with other elements via memory mapping techniques. The elements are then brought to show how a computer system is formed. This chapter also covers interrupt programming and polling techniques, multi-tasking and paging.
In order to maintain an accurate perspective on the capabilities of the computer, a microprocessor, or Central Processing Unit (CPU), should be viewed as general- purpose, software-programmable state machine. This device can be coupled to a few basic elements, such as memory chips and other interfacing devices, to create simple control systems. However, we can also combine a microprocessor with other devices such as memory chips, disk drives, graphics controller cards, screens, keyboards, etc. to form another standard package that can be used for both analysis and control. We call that device a computer. The term microprocessor can be somewhat misleading. Although it is intended to convey the size of the silicon onto which it is embedded, many still equate the term "micro" with low performance and this is certainly no longer the case. In general, we refer to a microprocessor as being a single chip that is used as the CPU of a computer or computer-based system. Many computers have CPUs that are composed of a number of chips. Those chips, working together, have the same overall functionality of a single microprocessor. Historically, larger computer systems used multiple-chip CPUs and smaller computers used single-chip CPUs (microprocessors). However, there is currently no simple way of telling whether a multiple-chip CPU is more or less powerful than a single-chip CPU (microprocessor). There are many factors that influence performance and the historical demarcation is no longer relevant. In this text, therefore, when we talk of microprocessors, we are really talking about CPUs, since the basic principles apply to both single and multiple chip systems. In discussing microprocessors, one inevitably comes across a "sister" product known as a Digital Signal Processor or DSP. There are some key differences between the architecture of a microprocessor and that of a DSP. Microprocessors have traditionally been developed on the so-called "Von Neumann" architecture, whereas DSPs have been developed along the so-called "Harvard" architecture. The difference is that microprocessor based program execution depends upon program instructions and data being stored together, whereas DSP program execution depends upon isolation of program instructions and data. We shall see the ramifications of this a little later when we examine microprocessor program execution. However, in simple terms, the Harvard architecture enables a DSP to carry out common control functions (digital filtering, Fast Fourier Transforms, etc.) for a lower cost per given speed than the Von Neumann architecture. This performance advantage is offset by the relatively low volumes in which DSPs are produced. DSPs are therefore used for specialised control functions, involving significant amounts of digital processing, whereas microprocessors are used for both control and general computing.
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"State Machines and Microprocessor Systems "
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