【正文】 中的發(fā)送和接受中斷、定時和計數(shù)的時鐘中斷、還有 WDT 復位中斷等，它們豐富了 CPU 在處理各種事務時響應種類。經過再次編程的梯形圖程序說明我們在執(zhí)行程序時高級計數(shù)器會自動的執(zhí)行對應的工作，從而將高級計數(shù)器的級別升至高一級別。 可編程邏輯控制器（ PLC ） 14 Programmable Logic Controllers (PLC) 1. About Programmable Logic Controllers (PLC) Everyone knows there’s only one constant in the technology world, and that’s change. This is especially evident in the evolution of Programmable Logic Controllers (PLC) and their varied applications. From their introduction more than 30 years ago, PLC has bee the er stone of hundreds of thousands of control systems in a wide range of industries. At heart, the PLC is an industrialized puter programmed with highly specialized languages, and it continues to benefit from technological advances in the puter and information technology worlds. The most prominent of which is miniaturization and munications. Programmable logic controllers I/O channel specifications include total number of points, number of inputs and outputs, ability to expand, and maximum number of channels. Number of points is the sum of the inputs and the outputs. PLC may be specified by any possible bination of these values. Expandable units may be stacked or linked together to increase total control capacity. Maximum number of channels refers to the maximum total number of input and output channels in an expanded system. PLC system specifications to consider include scan time, number of instructions, data memory, and program memory. Scan time is the time required by the PLC to check the states of its inputs and outputs. Instructions are standard operations (such as math functions) available to PLC software. Data memory is the capacity for data storage. Program memory is the capacity for control software. Available inputs for programmable logic controllers include DC, AC, analog, thermocouple, RTD, frequency or pulse, transistor, and interrupt inputs. Outputs for PLC include DC, AC, relay, analog, frequency or pulse, transistor, triac. Programming options for PLC include front panel, hand held, and puter. Programmable logic controllers use a variety of software programming languages for control. These include IEC 611313, sequential function chart (SFC), function block diagram (FBD), ladder diagram (LD), structured text (ST), instruction list (IL), relay ladder logic (RLL), flow chart, C, and Basic. The IEC 611313 programming environment provides support for five languages specified by the global standard: Sequential Function Chart, Function Block Diagram, Ladder Diagram, Structured Text, and Instruction List. 可編程邏輯控制器（ PLC ） 15 This allows for multivendor patibility and multilanguage programming. SFC is a graphical language that provides coordination of program sequences, supporting alternative sequence selections and parallel sequences. FBD uses a broad function library to build plex procedures in a graphical format. Standard math and logic functions may be coordinated with customizable munication and interface functions. LD is a graphic language for discrete control and interlocking logic. It is pletely patible with FBD for discrete function control. ST is a text language used for plex mathematical procedures and calculations less well suited to graphical languages. IL is a lowlevel 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 (PLC). 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 plex putation, 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 puter interface options, work 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 plex functions are used the more plex VonNeuman model of the PLC must be used. A VonNeuman puter processes one instruction at a time. Most puters operate this way, although they appear to be doing many things at once. Consider the puter ponents shown in Figure 1. 可編程邏輯控制器（ PLC ） 16 Figure 1 Simplified Personal Computer Architecture Input is obtained from the keyboard and mouse, output is sent to the screen, and the disk and memory are used 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 InputOutput 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 puter as a controller, it is controlling the user by outputting 可編程邏輯控制器（ PLC ） 17 stimuli on the screen, and inputting responses from the mouse and the keyboard. A PLC is also a puter controlling a process. When fully integrated into an application the analogies bee。s machine bine, for resolve the very plicated problem to lo