【正文】 you must follow the directives of your own pany’s procedures and the accepted practices of your own training and location. Partition Your Process or Machine Divide your process or machine into sections that have a level of independence from each other. These partitions determine the boundaries between controllers and influence the functional description specifications and the assignment of resources. Create the Functional Specifications Write the descriptions of operation for each section of the process or machine. Include the following topics: I/O points, functional description of the operation, states that must be achieved before allowing action for each actuator (such as solenoids, motors, and drives), description of the operator interface, and any interfaces with other sections of the process or machine. Design the Safety Circuits Identify equipment requiring hardwired logic for safety. Control devices can fail in an unsafe manner, producing unexpected startup or change in the operation of machinery. Where unexpected or incorrect operation of the machinery could result in physical injury to people or significant property damage, consideration should be given to the use of electromechanical overrides which operate independently of the S7200 to prevent unsafe operations. The following tasks should be included in the design of safety circuits: Identify improper or unexpected operation of actuators that could be hazardous. Identify the conditions that would assure the operation is not hazardous, and determine how to detect these conditions independently of the S7200. Identify how the S7200 CPU and I/O affect the process when power is applied and removed, and when errors are detected. This information should only be used for designing for the normal and expected abnormal operation, and should not be relied on for safety purposes. 北京郵電大學世紀學院畢業(yè)設計（論文） 30 Design manual or electromechanical safety overrides that block the hazardous operation independent of the S7200. Provide appropriate status information from the independent circuits to the S7200 so that the program and any operator interfaces have necessary information. Identify any other safetyrelated requirements for safe operation of the process. Specify the Operator Stations Based on the requirements of the functional specifications, create drawings of the operator stations. Include the following items: Overview showing the location of each operator station in relation to the process or machine Mechanical layout of the devices, such as display, switches, and lights, for the operator station Electrical drawings with the associated I/O of the S7200 CPU or expansion module Digital inputs: Each scan cycle begins by reading the current value of the digital inputs and then writing these values to the processimage input register. Analog inputs: The S7200 does not update analog inputs from expansion modules as part of the normal scan cycle unless filtering of analog inputs is enabled. An analog filter is provided to allow you to have a more stable signal. You can enable the analog filter for each analog input point. When analog input filtering is enabled for an analog input, the S7200 updates that analog input once per scan cycle, performs the filtering function, and stores the filtered value internally. The filtered value is then supplied each time your program accesses the analog input. When analog filtering is not enabled, the S7200 reads the value of the analog input from expansion modules each time your program accesses the analog input. Analog inputs AIW0 and AIW2 included on the CPU 224XP are updated every scan with the most recent result from the analogtodigital converter. This converter is an averaging type (sigmadelta) and those values will usually not need software filtering. Tip 北京郵電大學世紀學院畢業(yè)設計（論文） 31 Analog input filtering is provided to allow you to have a more stable analog value. Use the analog input filter for applications where the input signal varies slowly with time. If the signal is a highspeed signal, then you should not enable the analog filter. Do not use the analog filter with modules that pass digital information or alarm indications in the analog words. Always disable analog filtering for RTD, Thermocouple, and ASInterface Master modules. Executing the Program During the execution phase of the scan cycle, the S7200 executes your program, starting with the first instruction and proceeding to the end instruction. The immediate I/O instructions give you immediate access to inputs and outputs during the execution of either the program or an interrupt routine. If you use subroutines in your program, the subroutines are stored as part of the program. The subroutines are executed when they are called by the main program, by another subroutine, or by an interrupt