ABB DCS 系统 培训课程 系列讲座 Code sorting.doc

Course T530Chapter 18 Code sorting18.1 General Information 18.1.1 DescriptionThis chapter describes the different methods of code sorting and how to handle loops.18.1.2 ObjectivesOn completion of this chapter you will be able to: Describe the differences between different methods of code sorting Describe the advantages of each method Analyze and solve loops in the program code.18.1.3 Reference DocumentationUse the online help: Contents tab, Control Module Diagram Editor Working with Control Modules18.2 Lesson 1 - Code sorting18.2.1 DescriptionThis chapter describes the different methods of code sorting and how to handle loops.18.2.2 ObjectivesOn completion of this lesson you will be able to: Describe the differences between different methods of code sorting Describe the advantages of each method Analyze and solve loops in the program code.18.2.3 According to IEC 61131-3The execution order of function blocks follows the program flow as implemented by the programmer.18.2.4 Control Module Code Sorting- introductionFor Control Modules, the compiler analyzes each code block (ST, IL, FBD, and LD - SFC is treated somewhat differently, see remark below) separately with respect to which variables that are read and written, respectively, by each block. The compiler then determines the optimal execution order. A code block that assigns a value to a variable has to be executed before the block that reads the variable value (compare figure below in which the code block in control module 1 must be executed before the code block in control module 2).The technique is called code sorting and means that the optimal execution order will be based on data flow instead of program flow (as is the case for function blocks). Code sorting guarantees that the code will be executed in correct order. Code sorting suppresses time-delays and results in that a new control module automatically becomes correctly placed in the process.For any from control modules called Function Blocks, it is only the parameter interface (In, Out, or In_out) that affects the code-block sorting. That is, no analysis is made on how the Function Blocks actually use the parameters. In addition, Function Block references to external variables do not affect the execution order of control module code blocks.Code sorting has no effect on control modules connected to different tasks.Within a SFC code block only the N action parts (not P1 or P2 actions) are sorted.18.2.5 Loop handlingHowever, in case both the control modules use the same variable for both reading and writing, the compiler gives a warning message that a loop exists, meaning that the execution order cannot be resolved:This case yields the following error information:In the example above, the Request value determines the Deliver value, which, in turn, determines the Request value. This condition is shown in the automatically generated text file, where the figures within parentheses refer to which code block the respective parameter depends on. Provided that circular dependence actually exists (and is not merely a result of a programming error), the problem can be solved by assigning a State-qualifier to the Delivered variable and introducing a new code block in Control Module 1, Code_Block_12:The loop dependency is now eliminated; Delivered:old stores the value from the previous scan and Delivered:new contains the updated value from the current scan. Hence, the execution order becomes Code_Block_11 - Code_Block_21 - Code_Block_12. The approach is particularly valuable for complex applications, which are difficult to monitor manually.18.2.5.1 Remarks on StateState can only be specified for local variables of type bool, int, uint, dint, and real. If you for some reason want to override the sorting and thereby avoid the State implications, you can assign the nosort attribute to the variable.18.2.5.2 Remarks on NoSortIncorrectly used, the nosort attribute may cause improper execution and application failure. Nosort should typically be used only when the code block connections themselves unambiguously determine the execution order Use nosort only if you know the data flow characteristics in detail.18.2.6 Code Sorting AdvantagesThe advantages of code sorting are best illustrated with a couple of examples: First consider a PID cascade loop (used for updating the PID integrators in case of manual operation), in which data flow in both directions between the master and the slave. This is handled by the Back code blocks at the end of the scan. This is not possible to do with function blocks if you want to work in an object-oriented way and avoid time delays.Second, now consider an application according to the illustration below. Lets say that some time after the application was set up, the need for a third conveyer arises. Because of the code sorting, a Control Module representing the functionality of the new conveyer becomes added and connected in the right place in the application logic (provided that the application has been properly designed in the specification phase).18.3 Exercise 18.1 Code sortingPlease do exercise 18.1Du kan tag bort denna osynliga text men ltnedanstende Page Break ligga kvar !Lt ven den efterfljande sidan ligga kvar !This page is intentionally left blankThis page is intentionally left blankOsynlig text:Nsta sida mste brja p UDDA sidnummerLmna EN eller TV tomma sidor hr OVANFR.Nedanstende Section Break fr INTE tas bort !Komplettera tom sida vid behov med nedanstende textThis page is intentionally left blank 3BSE 022 868/A 2000/12/5 16:27:00 18-