【正文】 there are actually multiple binations of inputs that will result in the output Y turning on. In the left most part of the rung, power could flow through the top if C is off and D is on. Power could also (and simultaneously) flow through the bottom if both E and F are true. This would get power half way across the rung, 遼寧科技大學畢業(yè)設計（論文） 第 34 頁Figure A5 A Simple Ladder Logic Diagramand then if G or H is true the power will be delivered to output Y. In later chapters we will examine how to interpret and construct these diagrams. There are other methods for programming PLCs. One of the earliest techniques involved mnemonic instructions. These instructions can be derived directly from the ladder logic diagrams and entered into the PLC through a simple programming terminal. An example of mnemonics is shown in Figure . In this example the instructions are read one line at a time from top to bottom. The first line 00000 has the instruction LDN (input load and not) forinput00001. This will examine the input to the PLC and if it is off it will remember a 1 (or true), if it is on it will remember a 0 (or false). The next line uses an LD (input load) statement to look at the input. If the input is off it remembers a 0, if the input is on it remembers a 1 (note: this is the reverse of the LD). The AND statement recalls the last two numbers remembered and if the are both true the 。在我即將離開學校，踏上社會的時刻，讓我真心的說一聲謝謝曾經幫助過我的老師和同學們，祝老師今后工作順利，身體健康，家庭幸福、美滿！ 遼寧科技大學畢業(yè)設計（論文） 第 28 頁參考文獻[1] 周奇才，自動化倉庫主要運行參數(shù)的優(yōu)化確定[M].《上海鐵道大學學報》,1999[2] 李友善，自動控制原理與應用[M]. 北京：國防工業(yè)出版社，1998[3] 蔡尚峰，自動控制理論應用技術和設計實例[M]. 北京：機械工業(yè)出版社，2022[4] 王兆義，西門子公司的 S7300 系列 PLC 開發(fā)導航 [M]. 北京：機械工業(yè)出版社，2022[5] 胡學林、宋宏等編，電氣控制與 PLC 應用[M]. 北京：冶金工業(yè)出版社，2022 [6] Analog Devices． Programmable Logic Controllers[J]．Version 2022. 遼寧科技大學畢業(yè)設計（論文） 第 29 頁附錄 A（外文文獻 )INTRODUCTION OF PROGRAMMABLE CONTROLLERSProgrammable controllers systems have evolved to not only replace electromechanical devices, but to solve an everincreasing array of control problems in both process and nonprocess industries. By all indications, these microprocessor powered giants will continue to break new ground in the automated factory into the 1990s.1 HistoryIn the 1960s, electromechanical devices were the order of the day as far as control was concerned. These devices monly known as relays, were being used by the thousands to control many sequentialtype manufacturing processes and standalone machines. Many of these relays were in use in the transportation industry, more specifically, the automotive industry. These relays used hundreds of wires and their interconnections to effect a control solution. The performance of a relay was basically reliableat least as a single device. But the mon applications for relay panels called for 300 to 500 or more relays, and the reliability and maintenance issues associated with supporting these panels became a very great challenge. Cost became another issue, for in spite of the low cost of the relay itself, the installed cost of the panel could be quite high. The total cost including purchased parts, wiring, and installation labor, could range from$30~$50 per relay. To make matters worse, the constantly changing needs of a process called for recurring modifications of a control panel. With relays, this was a costly prospect, as it was acplished by a major rewiring effort on the panel. In addition, these changes were sometimes poorly documented, causing a secondshift maintenance nightmare months later. In light of this, it was not unmon to discard an entire control panel in favor of a new one with the appropriate ponents wired in a manner suited for the new process. Add to this the unpredictable, and potentially high, cost of maintaining these systems as on highvolume motor vehicle production lines, and it became clear that something was needed to improve the control processto make it more reliable, easier to troubleshoot, and more adaptable to changing control needs. 遼寧科技大學畢業(yè)設計（論文） 第 30 頁Control engineering has evolved over time. In the past humans were the main method for controlling a system. More recently electricity has been used for control and early electrical control was based on relays. These relays allow power to be switched on and off without a mechanical switch. It is mon to use relays to make simple logical control decisions. The development of low cost puter has brought the most recent revolution, the Programmable Logic Controller (PLC). The advent of the PLC began in the 1970s, and has bee the most mon choice for manufacturing controls.PLCs have been gaining popularity on the factory floor and will probably remain predominant for some time to e. Most of this is because of the advantages they offer Cost effective for controlling plex systems. Flexible and can be reapplied to control other systems quickly and easily. Computational abilities allow more sophisticated control. Trouble shooting aids make programming easier and reduce downtime. Reliable ponents make these likely to operate for years before failure.2Ladder LogicLadder logic is the main programming method used for PLCs. As mentioned before, ladder logic has been developed to mimic relay logic. The decision to use the relay logic diagrams was a strategic one. By selecting ladder logic as the main programming method, the amount of retraining needed for engineers and trades people was greatly reduced. Modern control systems still include relays, but these are rarely used for logic. A relay is a simple device that uses a magic field to control a switch, as pictured in FigureA1. When a voltage is applied to the input coil, the resulting current creates a magic f