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How to use Sub Routines with Allen Bradley PLC

Sub routines are sub programs in a PLC project. We can split the whole program by its function or sequence. We can call the sub programs from main page using instructions.

Sub Routines with Allen Bradley PLC

AB PLC Sub Routines

In the above picture, there are totally 8 files listed. In that SYS 0 and SYS 1 are default files.

  • LAD 2-Main is the main page of the program, that means PLC starts executing IO’s from this page only.
  • LAD-3-IO MAPPING, LAD 4-SCALING, LAD 5-PID, LAD 6-PLANT 1 and LAD 7 PLANT 2 are sub programs categorized by project function and sequence.

PLC Instruction Description

  1. JSR-Jump to Sub routine

JSR-Jump Instruction in Allen Bradley PLC

We can use this instruction using condition by adding some input before the instruction like,

JSR-Jump Instruction Ladder Logic

In the SBR File number, we should give the ladder number i.e. which sub routine we should call at this place as shown in below figure.

Jump Instruction Address in PLC

  1. RET: Return to Main Page

RET Instruction in Allen Bradley PLC

Return instruction should add at the end of the sub routine program.

Return instruction AB PLC Logic

PLC Program

In this program, Sub routine instructions are explained using simple example.

 LAD 2-MAIN PAGE:

Allen Bradley PLC Example with Sub routines

LAD 3–IO MAPPING

AB PLC IO Mapping Logic

Note: Since its for example, LAD 4, LAD 5,LAD 6,LAD 7 pages are not included in this program

Program Description:

Case 1 :

If Sw2 is not turned on, In LAD 3-IO Mapping, no operations will be happen.

LAD 2-MAIN PAGE:

Ladder Logic example for JSR Instruction

LAD 3–IO MAPPING:

Even though, sw6 is turned ON, Move block is not doing any operation because in main page, JSR instruction is not activated

Move block in PLC

Case 2 :

If Sw2 is turned on, In LAD 3-IO Mapping, operations will be happen.

LAD 2-MAIN PAGE:

AB PLC Logic Mapping

LAD 3–IO MAPPING:

sw6 is turned ON, Move block is doing it s operation because in main page, JSR instruction is activated

AB PLC Example Ladder

Conclusion:

The above explained concept for subroutine is for example only. We can use this example program to understand the working of JSR and RET function in AB PLC ladder logic programming.

Author : Hema Sundaresan

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How to use Sub Routines with Allen Bradley PLC

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