【正文】 the market the basic reason, under us elaborated with emphasis several questions, and study its development tendency. 3 PLC hardware and software PLC classification PLC already formed in the 9039。s micro, slightly, center, is big, giant many kinds of PLC. Divides according to the I/O points, may divide into: Miniature PLC: 32 I/O Small PLC: 256 I/O Medium PLC: 1,024 I/O Largescale PLC: 4,096 I/O Giant PLC: 8,195 I/O In recent years had the single plane to support 300 return routes and above 65,000 I/O largescale system correspondence medium PLC, used 16 ~32 position CPU, micro, small PLC originally used 8 CPU, now according to aspect and so on munication requests, some also changed to 16 ~32 position CPU. Because I/O 64 below PLC sales volume occupies entire PLC 47%, 64 ~256 accounts for 31%, equals the position entire PLC sales volume 78%, therefore to micro, small PLC should study. PLC hardware The PLC controller itself hardware uses the building block system structure, various factories product structure is mostly the same except for minor differences. Take Japanese ohm dragon C200HE as an example, for the main line template frame type structure, the bare bone (the CPU female board) on is loaded with the CPU template, other positions are loaded with the I/O template。 If the I/O template are many time, may connect I/O by the CPU female board after the I/O expansion electric cable to expand the female board, installs the I/O template in above。 Another method is provides longdistance I/O from the station and so on. These all explained the PLC factory the hardware various parts to the user development, is advantageous for the user to select, disposes scale different PLC, moreover this kind of hardware disposes openness, for the manufacturer, retails business (business agent), system integration business, the enduser brings very many convenient es for the marketing supply to be very greatly convenient, this is a big success experience. In PLC I/O template, except general DI/DO, outside AD/DA template, but also has developed a series of special functions I/O template, this was PLC uses in all the various trades and occupations to open the outlet, like used in the bar code recognition ASCII/BASIC template, used in the reaction control PID template, used in the operating control, the 無 錫 職 業(yè) 技 術(shù) 學(xué) 院 畢業(yè)設(shè)計說明書（英文翻譯） 3 machinefinishing highspeed counting template, the single axle position control template, the two axle position control template, the cam locator template, the radio frequency recognition connection template and so on, this also will be able to have the very big development in later. Moreover in the input, the output related part, the situation input, the output electricity isolation, aspect and so on isolation also can be more perfect. In PLC CPU and the memory coordination, pletes the control function. It with DCS system parameter and so on processing temperature, pressure, current capacity system different, uses the fast tour scanning cycle, generally is ~, is quicker selects 50ms or the smaller scanning cycle. It is a digital sampling control system. Central Processing Unit (CPU) is the brain of a PLC controller. CPU itself is usually one of the microcontrollers. Aforetime these were 8bit , and now these are 16and 32bitmicrocontrollers. Unspoken rule is that you’ll find mostly Hitachi and Fuji microcontrollers in PLC controllers by Japanese makers, Siemens in European controllers, and Motorola microcontrollers in American ones. CPU also takes care of munication, interconnectedness among other parts of PLC controllers, program execution, memory operation, overseeing input and setting up of an output. PLC controllers have plex routines for memory checkup in order to ensure that PLC memory was not damaged (memory checkup is done for safety reasons).Generally speaking, CPU unit makes a great number of checkups of the PLC controller itself so eventual errors would be discovered early. You can simply look at any PLC controller and see that there are several indicators in the form. of light diodes for error signalization. System memory (today mostly implemented in FLASH technology) is used by a PLC for a process control system. Aside form. this operating system it also contains a user program translated forma ladder diagram to a binary form. FLASH memory contents can be changed only in case where user program is being changed. PLC controllers were used earlier instead of PLASH memory and have had EPROM memory instead of FLASH memory which had to be erased with UV lamp and programmed on programmers. With the use of FLASH technology this process was greatly shortened. Reprogramming a program memory is done through a serial cable in a program for application development. User memory is divided into blocks having special functions. Some parts of a memory are used for storing input and output status. The real status of an input is stored either as “1”or as “0”in a specific memory bit/ each input or output has one corresponding bit in memory. Other parts of memory are used to store variable contents for variables used in used program. For example, time value, or counter value would be stored in this part of the memory. PLC controller can be reprogrammed through a puter (usual way), b