PLC英文参考文献和翻译 第2页

language that provides coordination of program sequences, supporting alternative sequence selections and parallel sequences.  FBD uses a broad function library to build complex procedures in a graphical format. Standard math and logic functions may be coordinated with customizable communication and interface functions.  LD is a graphic language for discrete control and interlocking logic. It is completely compatible with FBD for discrete function control.  ST is a text language used for complex mathematical procedures and calculations less well suited to graphical languages.  IL is a low-level language similar to assembly
code. It is used in relatively simple logic instructions.  Relay Ladder Logic (RLL), or ladder diagrams, is the primary programming language for programmable logic controllers (PLCs). Ladder logic programming is a graphical representation of the program designed to look like relay logic.  Flow Chart is a graphical language that describes sequential operations in a controller sequence or application. It is used to build modular, reusable function libraries.  C is a high level programming language suited to handle the most complex computation, sequential, and data logging tasks. It is typically developed and debugged on a PC.  BASIC is a high level language used to handle mathematical, sequential, data capturing and interface functions.

    Programmable logic controllers can also be specified with a number of computer interface options, network specifications and features.  PLC power options, mounting options and environmental operating conditions are all also important to consider.

2 。INTRODUCTION

For simple programming the relay model of the PLC is sufficient. As more complex functions are used the more complex VonNeuman model of the PLC must be used. A VonNeuman computer processes one instruction at a time. Most computers operate this way, although they appear to be doing many things at once. Consider the computer components shown in 本文来自优,文'论#文^网，毕业论文 www.youerw.com 加7位QQ324,9114找源文 for both input and output for storage. (Note: the directions of these arrows are very important to engineers, always pay attention to indicate where information is flowing.) This figure can be redrawn as in Figure 2 to clarify the role of inputs and outputs.

Figure 2      An Input-Output Oriented Architecture
In this figure the data enters the left side through the inputs. (Note: most engineering diagrams have inputs on the left and outputs on the right.) It travels through buffering circuits before it enters the CPU. The CPU outputs data through other circuits. Memory and disks are used for storage of data that is not destined for output. If we look at a personal computer as a controller, it is controlling the user by outputting stimuli on the screen, and inputting responses from the mouse and the keyboard.

A PLC is also a computer controlling a process. When fully integrated into an application the analogies become;

inputs - the keyboard is analogous to a proximity switch
input -circuits - the serial input chip is like a 24Vdc input card
computer - the 686 CPU is like a PLC CPU unit
output - circuits - a graphics card is like a triac output card
outputs - a monitor is like a light
storage - memory in PLCs is similar to memories in personal computers

It is also possible to implement a PLC using a normal Personal Computer, although this is not advisable. In the case of a PLC the inputs and outputs are designed to be more reliable and rugged for harsh production environments.

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