Subroutines rev01 fa16
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Subroutines are groups of program code that perform specific tasks and can be called from the main routine or other subroutines. They make programs more manageable by breaking them up into smaller tasks. The JSR instruction calls a subroutine, the SBR marks the start of a subroutine, and the RET returns from a subroutine. Parameters can be passed between routines but are not covered in this course. Subroutines improve readability and maintainability and can be reused in other programs.
Subroutines rev01 fa16
- 1. Chapter 08 – Program Control Instructions Sections 8-3-3 to 8-3-5
- 2. Subroutines Subroutine(s) is a group of program code that performs a specific task. The subroutine(s) are part of a program (in our case, the MainProgram) and are not part of the MainRoutine. A subroutine can be invoked, (called), from anyplace in the MainRoutine or from another subroutine. When a subroutine is called from within another subroutine it is referred to as nesting subroutines. Most, if not all, computer programs and PLC/PAC programs contain subroutines. Subroutines are used to make what would be an enormous program more manageable by breaking up the code into smaller tasks. Specific functions within a task should be placed in subroutines. Organizing a program by using subroutines makes the code easier to read, understand and maintain. Subroutines can also be reused in other programs that require the same task be performed.
- 3. Subroutines The ControlLogix uses separate routines to store and execute subroutines. The number of routines is limited to 32 per program. The MainRoutine is the main ladder and is the one that has been used for all previous labs. Scan starts at rung 0 in the MainRoutine and ends at the End statement in the MainRoutine.
- 4. Creating a Subroutine Step 01 In the Controller Organizer, right click on MainProgram and select New Routine… from the context specific menu.
- 5. Creating a Subroutine Step 02 When the New Routine dialog box opens, enter the name of the subroutine into the Name field, enter any description (opetional) and click the down arrow of the Type drop down box and select Ladder Diagram. If you want the new routine to open upon creation, place a check mark in the Open Routine check box. Routines other than Ladder Diagram can be created. The Type Drop Down shows that, Sequential Function Chart, Function Block Diagram and Structure Text routines can also be created. This course will only use Ladder Diagram.
- 6. Creating a Subroutine Step 03 The subroutine will appear listed under the program that it was created in. Double click the subroutine to open it in the ladder editor.
- 7. Program Control Instructions Instruction Instruction JMP – Jump to Label UID – User Interrupt Disable LBL – Label UIE – User Interrupt Enable JXR – Jump to External Routine SFR – Reset Sequential Chart JSR – Jump to Subroutine SFP – Pause Sequential Chart RET – Return from Subroutine EVENT – Trigger Event Task SBR – Subroutine Label EOT – End of Transition TND – Temporary End AFI – Always False MCR – Master Control Reset NOP – No Operation This course will cover the instructions shown in Red. The instructions shown in Green can be used, but will not be discussed in class.
- 8. Program Control Instructions JSR – Jump to Subroutine The JSR is an output instruction that is used to “call” a subroutine. The instruction can be conditional or unconditional. JSR instructions can have several parameters. Subroutine name to be “called” (required). Parameter to pass to the subroutine (optional). Parameter to accept a value returned from the subroutine (optional).
- 9. Program Control Instructions Entering JSR parameters: Routine Name (Required) – Double click this parameter field, click the drop down arrow and select the subroutine name from the drop down list. The subroutine name will only be in the list if the subroutine has been created. Input Par and Return Par (Optional) – These parameters will not be used in this course. When a parameter is not used it must be removed. Right click on the parameter field and select Remove Instruction Parameter from the context specific menu. Do this for each parameter that is not required.
- 10. Program Control Instructions JSR operation When the rung containing the JSR instruction is true, the processor scan jumps to the subroutine referenced in the Routine Name parameter and begins program execution at the first rung in that subroutine. A jump cannot be made into the middle of a subroutine. Executions will always start at the first instruction on the first rung in that ladder routine. This rung will unconditionally jump to the subroutine named: HeatControlFurnace01
- 11. Program Control Instructions SBR – Subroutine Label (optional, but recommended) The SBR is an input instruction that is always true and marks the beginning of a subroutine. It must be the first instruction on the first rung of the subroutine. The input parameter field(s) is used to reference tags whose data is to be used (passed) to the subroutine. (This course will not be using these parameters). SBR instruction can have several parameters. Parameter that passes data to the subroutine (optional)
- 12. Program Control Instructions For this course the Input Par parameter will need to be removed from the SBR instruction. To remove the parameter, right- click on the parameter and select Remove Instruction Parameter from the context specific menu.
- 13. Program Control Instructions RET – Return from Subroutine The RET instruction is an output instruction that is used to stop executing the subroutine and return to the ladder file that originally “called” the subroutine. The instruction can be conditional or unconditional and can contain several parameters. (This course will not use these parameters). The RET instruction is not always required. If the entire subroutine file is to be scanned, the End statement will also function as the RET. Parameter that passes data back to the ladder file that originally “called” the subroutine (optional)
- 14. Program Control Instructions For this course the Input Par parameter will need to be removed from the RET instruction. To remove the parameter, right- click on the parameter and select Remove Instruction Parameter from the context specific menu.
- 15. Program Control Instructions Several conditional RET instruction can be present in a subroutine. As an example: If an RET instruction becomes true that is on rung-6 in a subroutine containing 45-rungs, the program scan will return to the ladder file that originally called the subroutine and rung 7 through 45 will not execute. If the RET instruction on rung-6 becomes false and another RET instruction at rung-10 becomes true, the program scan will return to the ladder file that originally called the subroutine and rungs 11 through 45 will not execute. If the entire subroutine ladder file is to be always scanned, placing a RET instruction on the last rung of the program is optional. If an RET instruction is not found in the subroutine ladder file, the End statement performs the return.
- 17. Program Control Instructions - Siemens Siemens processors do not use subroutines per say. Siemens uses: Organizational Blocks Function Blocks Functions Data Blocks These will not be covered in this course.
- 18. Program Control Instruction - CCW The AB CCW does not have program control instructions. Subroutines can be created and will always execute in the order they appear in the project organizer window. The order of the routines can be moved by dragging them to a new location. In the picture shown, the order of execution is: Prog4 then Prog2 then Prog3 and finally Prog1. When Prog1 has finished executing the scan returns to Prog4 and repeats.