PLC carries out the process of the program and characteristic

PLC carries out the process cent of the program to be 3 phase, input sampling phase, program to carry out level, output refresh phase namely, if pursue,1 is shown. Sketch map of process of executive order of graph 1 PLC 1. Input sampling phase is inputting sampling phase, the input condition that way of job of PLC Esau paint presses order to be carried to all inputs undertakes sampling, in stocking input image register, input image register right now by refresh. Enter a program to treat stage then, level or other phase are carried out in the program, although input condition happening change, the content that inputs image register also won't be changed, the change that inputs condition has the input in next scanning cycle to handle level ability to be gone to by sampling only. 2. The program carries out level to carry out level in the program, PLC undertakes scanning executive by order to the program. If the program uses echelon graph to express, always press after going up first, fall, first left hind right order undertakes. Should encounter a program to jump turn when the instruction, turn according to jumping the condition is contented will decide whether the program jumps turn. When position of input, output is involved in the instruction, PLC numerates from inside inputting image register and component image register, have operation according to user program, the result of operation stocks component image register again in. To component image register, its content can change along with the process that the program carries out. 3. Output refresh phase is carried out when all programs after ending, enter output to treat stage. In this one phase, the condition that concerns with output in register of image of PLC general output (output relay condition) turn put in outputting Suo Cunqi, pass certain kind output, drive is exterior and laden. Accordingly, PLC is inside cycle of a scanning, inputting sampling phase to undertake only to inputting the sampling of condition. Input end to will be blocked after PLC enters a program to carry out level, the input sampling phase till next scanning cycle just undertakes new sampling to inputting condition. This means calls concentration sampling, it is namely inside cycle of a scanning, concentration undertakes sampling to inputting condition for some time. If be opposite,in user program output assigns for many times as a result, criterion the last time is significant. Inside cycle of a scanning, just will output condition from inside outputting image register in output refresh phase only, have break to outputting interface. In other phase output condition protects existence to output image register all the time in. This kind of means calls concentration to output. To small-sized PLC, its I/O check the number is less, user course is shorter, use the working means that centers sampling, concentration to output commonly, although be in what the system reduced on certain level to answer speed, but time of the great majority when making PLC works and exterior input / output equipment segregation, from go up at all the interference rejection capacity that increased a system, enhanced systematic dependability. And to large and medium-sized PLC, its I/O check the number is more, control function is strong, user program is longer, answer speed to improve a system, can use means of fixed sampling, fixed output, or suspend input, output means and use a variety of means such as intelligent I/O interface. From afore-mentioned analysises knowable, when the input of PLC end inputs signal happening to change to output end to make response to should inputting change to PLC, need period of time, this kind of phenomenon calls PLC to input ／ output to answer lag. To ordinary industrial control, this kind of lag allows completely. What should notice is, because way of PLC scanning work is caused,this kind answers lag is not only, basically be the input defer that the filter wave link of PLC input interface brings more, and the behavioral time of parts of an apparatus of the drive in outputting interface brings output defer, still concern with programming at the same time. Lag time is a parameter that when designing PLC application system, should notice to hold. CNC Milling