1、 大学 毕业设计 （ 外文翻译材料 ） 2012 年 6 月 学 院： 专 业： 学生姓名： 指导教师： 学院 电气工程及其自动化 外文翻译（原 文） 1 Features of the S7-200 The S7-200 provides several special features that allow you to customize how the S7-200 functions to better fit your application. The S7-200 Allows Your Program to Immediately Read or Write the I/O T
2、he S7-200 instruction set provides instructions that immediately read from or write to the physical I/O. These immediate I/O instructions allow direct access to the actual input or output point, even though the image registers are normally used as either the source or the destination for I/O accesse
3、s. The corresponding process-image input register location is not modified when you use an immediate instruction to access an input point. The corresponding process-image output register location is updated simultaneously when you use an immediate instruction to access an output point. Tip The S7-20
4、0 handles reads of analog inputs as immediate data, unless you enable analog input filtering. When you write a value to an analog output, the output is updated immediately. It is usually advantageous to use the process-image register rather than to directly access inputs or outputs during the execut
5、ion of your program. There are three reasons for using the image registers: The sampling of all inputs at the start of the scan synchronizes and freezes the values of the inputs for the program execution phase of the scan cycle. The outputs are updated from the image register after the execution of
6、the program is complete. This provides a stabilizing effect on the system. Your program can access the image register much more quickly than it can access I/O points, allowing faster execution of the program. I/O points are bit entities and must be accessed as bits or bytes, but you can access the i
7、mage register as bits, bytes, words, or double words. Thus, the image registers provide additional flexibility. The S7-200 Allows Your Program to Interrupt the Scan Cycle 外文翻译（原 文） 2 If you use interrupts, the routines associated with each interrupt event are stored as part of the program. The inter
8、rupt routines are not executed as part of the normal scan cycle, but are executed when the interrupt event occurs (which could be at any point in the scan cycle). Interrupts are serviced by the S7-200 on a first-come-first-served basis within their respective priority assignments. The S7-200 Allows
9、You to Allocate Processing Time for Run Mode Edit and Execution Status You can configure a percentage of the scan cycle to be dedicated for processing a run mode edit compilation or execution status. (Run mode edit and execution status are options provided by STEP 7-Micro/WIN to make debugging your
10、program easier.) As you increase the percentage of time that is dedicated to these two tasks, you increase the scan time, which makes your control process run more slowly. The default percentage of the scan dedicated to processing run mode edits and execution status is set to 10%. This setting was c
11、hosen to provide a reasonable compromise for processing the compilation and status operations while minimizing the impact to your control process. You can adjust this value by 5% increments up to a maximum of 50%. To set the scan cycle time-slice for background communications: 1.Select the View Component System Block menu command and select Background Time. 2.In the Background tab, use the drop down box to select the communications background time. 3.Click OK to save your selection. 4. Download the modified system block to the S7-200.
