【正文】 s like the absolute being boat is to adopt this kind of way to make airship go up the sky. The development of the higher layer needs our continuous effort to obtain. The PLC emergence has already affected a few persons fully, we also obtained more knowledge and precepts from the top one experience of the generation, ing to the continuous development PLC technique, push it toward higher wave tide. Knowing the available PLC work options and their best applications will ensure an efficient and flexible control system design. The programmable logic controller39。s) ability to support a range of munication methods makes it an ideal control and data acquisition device for a wide variety of industrial automation and facility control applications. However, there is some confusion because so many possibilities exist. To help eliminate this confusion, let39。s munications versatility, let39。s automatically coded as 65 by the sending equipment. The receiving equipment translates the 65 back to the letter A. Thus, different devices can municate with each other as long as both use ASCII code. ASCII module: This intelligent PLC module is used for connecting PLCs to other devices also capable of municating using ASCII code as a vehicle. Bus topology: This is a linear local area work (LAN) arrangement, as shown in Fig. 2A, in which individual nodes are tapped into a main munications cable at a single point and broadcast messages. These messages travel in both directions on the bus from the point of connection until they are dissipated by terminators at each end of the bus. CPU: This stands for central processing unit, which actually is that part of a puter, PLC, or other intelligent device where arithmetic and logical operations are performed and instructions are decoded and executed. Daisy chain: This is a description of the connection of individual devices in a PLC work, where, as shown in Fig. 3, each device is connected to the next and munications signals pass from one unit to the next in a sequential fashion. Distributed control: This is an automation concept in which portions of an automated system are controlled by separate controllers, which are located in close proximity to their area of direct control (control is decentralized and spread out over the system). Host puter: This is a puter that39。re familiar with these terms, let39。 what39。s not always necessary, or practical for that matter, to have a controller at each site. Nor is it practical to individually hard wire each I/O point over long distances back to the CPU. For example, remote I/O systems can be used in acquiring data from remote plant or facility locations. Information such as cycle times, counts, duration or events, etc. then can be sent back to the PLC for maintenance and management reporting. In a remote I/O configuration, the master controller polls the slaved I/O for its current I/O status. The remote I/O system responds, and the master PLC then signals the remote I/O to change the state of outputs as dictated by the control program in the PLC39。 because all munications occur transparently to the user, munications programming is reduced to simple readandwrite statements. In a peertopeer system, there39。s possible to designate one of the PLCs as a master for use as a type of group controller. This PLC then can be used to accept input information from an operator input terminal, for example, sending all the necessary parameters to other PLCs and coordinating the sequencing of various events. Host puter links PLCs also can be connected with puters or other intelligent devices. In fact, most PLCs, from the small to the very large, can be directly connected to a puter or part of a multi drop host puter work via RS232C or RS422 ports. This bination of puter and controller maximizes the capabilities of the PLC, for control and data acquisition, as well as the puter, for data processing, documentation, and operator interface. In a PLC/puter work, as shown in Fig. 4C, all munications are initiated by the host puter, which is connected to all the PLCs in a daisychain fashion. This puter individually addresses each of its worked PLCs and asks for specific information. The addressed PLC then sends this information to the puter for storage and further analysis. This cycle occurs hundreds of times per second. Host puters also can aid in programming PLCs。t have the software ability to interface with individual PLC models. Instead, they typically send and receive data in fixed formats. It39。s responsibility to provide the necessary software interface. The easiest way to provide such an interface to fixedformat intelligent devices is to use an ASCII/BASIC module on the PLC. This module is essentially a small puter that plugs into the bus of the PLC. Equipped with RS232 ports and programmed in BASIC, the module easily can handle ASCII munications with peripheral devices, data acquisition functions, programming sequences, number crunching, report and display generation, and other requirements. Access, protocol, and modulation functions of LANs By using standard interfaces and protocols, LANs allow a mix of devices (PLCs, PCs, mainframe puters, operator interface terminals, etc.) from many different vendors to municate with others on the work. Access: A LAN39。s in possession of a special electronic message called a token. The token is passed from node to node, allowing each an opportunity to transmit without interference. Tokens usually have a time limit to prevent a single node from tying up the token for a long period of time. Protocol: Network protocols define the way messages are arranged and coded for transmission on the LAN. The following are two mon types. Proprietary protocols are unique message arrangements and coding developed by a specific vendor for use with that vendor39