Page 210 - Programmable Logic Controllers, Fifth Edition - Mobile version

JSR
JUMP-TO-SUBROUTINE the RET instruction may be conditional if this rung
precedes the end of the subroutine. In this way, the
SBR file number U:3
processor omits the balance of a subroutine only if its
rung condition is true.
SBR
SUBROUTINE
The jump to subroutine (JSR), subroutine (SBR), and
RET return (RET) instructions are used to direct the controller
RETURN to execute a subroutine file. Figure 9-11 shows a materials
conveyor system with a flashing pilot light as a subrou-
Figure 9-10 Allen-Bradley subroutine-related instructions. tine. The operation of the program can be summarized as
follows:

When a subroutine is called from the main pro- • If the weight on the conveyor exceeds a preset
gram, the program is able to escape from the main value, the solenoid is de-energized and pilot light
program and go to a program subroutine to perform PL1will begin flashing.
certain functions and then return to the main program. • When the weight sensor switch closes, the JSR is
In situations in which a machine has a portion of its activated and directs the processor scan to jump to
cycle that must be repeated several times during one the subroutine U:3.
machine cycle, the subroutine can save a great deal of • The subroutine program is scanned and pilot light
duplicate programming. The sequence of rungs could PL1 begins flashing.
be programmed one time into a subroutine and just • When the weight sensor switch opens, the proces-
called when needed. sor will no longer scan the subroutine area and pilot
The subroutine concept is the same for all program- light PL1 will return to its normal on state.
mable controllers, but the method used to call and return
from a subroutine uses different commands, depending on The Allen-Bradley SLC 500 controller main program
the PLC manufacturer. The subroutine-related instructions is located in program file 2 whereas subroutines are as-
used in the Allen-Bradley PLCs shown in Figure 9-10 are signed to program file numbers 3 to 255. Each subroutine
the jump to subroutine (JSR) output instruction, the sub- must be programmed in its own program file by assigning
routine (SBR) input instruction, and the return (RET) out- it a unique file number. Figure 9-12 illustrates the proce-
put instruction. dure for setting up a subroutine and can be summarized
The subroutine instructions can be summarized as as follows:
follows:
• Note each ladder location where a subroutine should
Jump to Subroutine (JSR)—The JSR instruction be called.
redirects logic execution from the current ladder • Create a subroutine file for each location.
file to the specific subroutine file. When rung condi- Each subroutine file should begin with an SBR
tions are true for this output instruction, it causes instruction.
the processor to jump to the targeted subroutine file.
Each subroutine must have a unique file number • At each ladder location where a subroutine is called,
( decimal 3–255). program a JSR instruction specifying the subroutine
file number.
Subroutine (SBR)—The SBR instruction is the first
input instruction on the first rung in the subroutine • The RET instruction is optional.
file. It serves as an identifier that the program file is – The end of a subroutine program will cause a
a subroutine. This file number is used in the JSR in- return to the main program.
struction to identify the target to which the program – If you want to end a subroutine program before it
should jump. It is always true, and although its use is executes to the end of program file, a conditional
optional, it is still recommended. return (RET) instruction may be used.
Return (RET)—The RET instruction is an output Nesting subroutines allows you to direct program flow
instruction that marks the end of the subroutine file. from the main program to a subroutine and then to another
It causes the scan to return to the main program at the subroutine, as illustrated in Figure 9-13. Nested subrou-
instruction following the JSR instruction where it ex- tines make complex programming easier and program op-
ited the program. The scan returns from the end of the eration faster because the programmer does not have to
file if there is no RET instruction. The rung containing continually return from one subroutine to enter another.

Program Control Instructions Chapter 9 191

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