1、 电子与信息工程学院 本科毕业论文 (设计 ) 外 文 文 献 翻 译 译文题目： Programmable logic controllers Hardware, software architecture 学生姓名： 吴凡 专 业： 电气工程及其自动化 指导教师： 李东京 2012 年 12 月 Programmable logic controllers: Hardware, software architecture Many respects, the architecture of the programmable logic controller (PLC) resembles
2、a general-purpose computer with specialized input/output (I/O) modules. However, some important characteristics distinguish a PLC from a general-purpose computer. Most important, a PLC is much more reliable, designed for a mean time between failures measured in years. Second, a PLC can be placed in
3、an industrial environment with its substantial amount of electrical noise, vibration, extreme temperatures, and humidity. Third, PLCs are easily maintained by plant technicians. Basic PLC hardware architecture The basic architecture of a PLC consists of main components-the processor module, the powe
4、r supply, and the I/O modules. The processor module consists of the central processing unit (CPU) and memory. In addition to a microprocessor, the CPU also contains at least an interface to a programming device and may contain interfaces to remote I/O and other communication networks. The power supp
5、ly is usually a separate module, and the I/O modules are separate from the processor. The types of I/O modules include discrete (on/off), analog (continuous variable), and special modules like motion control or high-speed counters. The field devices are connected to the I/O modules. Depending on the
6、 amount of I/O and the particular PLC processor, the I/O modules may be in the same chassis as the processor and/or in one or more other chassis. Up until the late 1980s, the I/O modules in a typical PLC system were in chassis separate from the PLC processor. In the more typical present-day PLC, som
7、e of the I/O modules are present in the chassis that contains the processor. Some PLC systems allow more than one processor in the same chassis. Smaller PLCs are often mounted on a DIN rail. The smallest PLCs (often called micro-PLCs or naon-PLCs) include the power supply, processor, and all of the
8、I/Os in one pack-age. Some micro-PLCs contain a built-in operator interface panel. For many micro-PLCs, the amount of I/O is limited and not expandable. Basic software, memory architecture (IEC 61131-3) The IEC 61131-3 standard defines a memory and program model that follows modern software engineer
9、ing concepts. This model incorporates such features as top-down design, structured programming, hierarchical organization, formal software interfaces, and program encapsulation. Fortunately, extensive training in software engineering techniques is not necessary to become a proficient programmer. If
10、fully implemented, the model is reasonably complicated. The main disadvantages of the model are its complexity and its contrast to the simplicity of the early PLCs. The overall IEC 61131-3 memory pro-gram and memory model are described. (For various implementations of the standard, visit www.isa.org
11、/link/Erickson_PLC.) The IEC 61131-3 memory model (what the standard calls the software model) is layered-each layer hides many of the features of the layers beneath. Each of the main elements is now described. The configuration is the entire body of software (program and data) that corresponds to a
12、 PLC system. Generally, a configuration equates with the program and data for one PLC. In large complex systems that require multiple cooperating PLCs, each PLC has a separate configuration. A configuration communicates with other IEC configuration s within the control system through defined inter-faces, called access paths. The choice of the term configuration conflict with the historic use of this
