【正文】 the logic. This prevents potential problems that might occur if an input that is used in multiple places in the ladder logic program changed while half way through a ladder scan. Thus changing the behaviors of half of the ladder logic program. This problem could have severe effects on plex programs that are developed later in the book. One side effect of the input scan is that if a change in input is too short in duration, it might fall between input scans and be missed. When the PLC is initially turned on the normal outputs will be turned off. This does not affect the values of the inputs. 3 。 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. 畢業(yè)設(shè)計（論文）外文翻譯 ３ Figure 1 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 畢業(yè)設(shè)計（論文）外文翻譯 ４ 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 puter controlling a process. When fully integrated into an application the analogies bee。畢業(yè)設(shè)計（論文）外文翻譯 １ Programmable Logic Controllers (PLCs) 1 。 About Programmable Logic Controllers (PLCs) PLCs (programmable logic controllers) are the control hubs for a wide variety of automated systems and processes. They contain multiple inputs and outputs that use transistors and other circuitry to simulate switches and relays to control equipment. They are programmable via software interfaced via standard puter interfaces and proprietary languages and work options. 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. PLCs 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 PLCs include DC, AC, relay, analog, frequency or pulse, transistor, and triac. Programming options for PLCs 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, 畢業(yè)設(shè)計（論文）外文翻譯 ２ Ladder Diagram, Structured Text, and Instruction List. 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 g