CH.12 | PLC SUBROUTINE FUNCTIONS

INTRODUCTION

• Subroutines

  • programs that contain additional ladder logic diagram files that work with the main ladder logic diagram.

  • Execution in a PLC program scan depends on the state of an input device or the result of a compare instruction.

  • Placed in Program files 3 through 255 (SBR3 - SBR255)

  • Number and size restricted by PLC memory.

12.3 RETURN FROM SUBROUTINE

• First rung must have SBR instruction, which indicates this program represents a subroutine

• SBR must be connected to an OTE like B3:0.

  • Unless stated otherwise, using internal bits is recommended so that the limited # of outputs isn’t wasted.

• The Return (RET) Instruction causes the instruction pointer to go back and point to the next rung in the main PLC program file.

  • SBR file must have a RET instruction.

12.4 SUBROUTINE APPLICATIONS

• In most industrial applications

  • Lengthy LLDs are required

  • Usually divided into several SBR files

    • Each SBR is written and tested separately.

    • A shorter Main program used to call SBR for different conditions.

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12.5 INTERRUPT ROUTINES

• Interupt Routine

  • is one that interrupts the scan of the main PLC ladder logic diagram (LAD2) in order to execute an interrupt subroutine.

  • Similar to SBR and must reside in LAD3-LAD255

  • INT instruction replaces SBR at the beginning of SBR, indicating Interrupt Subroutine.

  • Two Types

    • Selectable Timed Interrupt (STI)

      • Used to interrupt the scan of a main program file at specific time intervals in order to scan a specified subroutine file.

      • Time intervals are specified by placing a time interval value in Status file register S:30.

    • I/O Interrupt (ISR)

      • Allows an I/O module to interrupt the processor operation cycle in order to scan a specified interrupt subroutine.

      • CANNOT use a standard discrete I/O module to accomplish I/O interrupt.

      • Module must be designed for ISR features.