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【正文】 nt process or the procedure is busy, movement time of the procedure exceeds WDT constitution time, the CPU turn but the WDT reset the appearance. The procedure restarts the movement, but will not carry on the breakage to the interruption. The PLC development has already entered for work ages of correspondence from the mode of the one, and together other works control the plank and I/ O card planks to carry on the share easily. A state software can pass all se hardwires link, more animation picture of keep the view to carries on the control, and cans pass the Inter to carry on the control in the foreign land, the blastoff that is 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 (PLC39。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 list what munications are available and when they would be best applied. To understand the PLC39。s munications versatility, let39。s first define the terms used in describing the various systems. ASCII: This stands for American Standard Code for Information Interchange. As shown in Fig. 1, when the letter A is transmitted, for instance, it39。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。s used to transfer data to, or receive data from, a PLC in a PLC/puter work. Intelligent device: This term describes any device equipped with its own CPU. I/O: This stands for inputs and outputs, which are modules that handle data to the PLC (inputs) or signals from the PLC (outputs) to an external device. Kbps: This stands for thousand bits per second, which is a rate of measure for electronic data transfer. Mbps: This stands for million bits per second. Node: This term is applied to any one of the positions or stations in a work. Each node incorporates a device that can municate with all other devices on the work. Protocol: The definition of how data is arranged and coded for transmission on a work. Ring topology. This is a LAN arrangement, as shown in Fig. 2C, in which each node is connected to tw