Transcript Program Control Instructions:

Program Control Instructions:
Using RSLogix 500 Software
Why these skills are important:
• Program control instructions allow you to
change the order in which the processor
scans a ladder program.
• These instructions are used to minimize
scan time, create a more efficient
program, and troubleshoot a ladder logic
program.
Program Control Instructions:
• Input and output instructions that, when
used together, change how and where an
SLC 500 processor scans.
• Program control instructions dictate the
sequence in which the program is
executed, thereby interrupting the normal
sequence of the processor operating cycle.
Processor Operating Cycle & Program
Control Instructions:
• The following graphic
provides a review of
how the processor
operating cycle,
without program
control instructions, is
divided:
Common Program Control Instructions:
•The following program control instructions are
commonly used:
Jump to Label (JMP) & Label (LBL)
Instructions:
• Jump to Label (JMP) Instructions: An
•
•
•
output instruction that directs the processor to
skip rungs (forward or backward).
Label (LBL) Instruction: An input instruction
that is the target of JMP instructions having the
same label number.
JMP and LBL instructions are used in pairs to
skip portions (zones) of the ladder program,
which reduces scan time.
Zone: a section of ladder logic.
The following graphic shows examples of
single and multiple JMP instructions:
When the JMP rung is true:
• The program skips from the rung containing the
•
•
•
JMP instruction to the rung containing the
designated LBL instructions.
The program continues executing.
The zone rungs between the JMP and LBL
instructions are not executed.
The zone outputs remain in their last state.
When the JMP rung is false:
• The program does not execute the JMP
instruction.
• The zone rungs will be executed.
• The zone outputs function normally.
The condition of the JMP rung has no
effect on rungs outside the zone.
Jump to Subroutine (JSR), Subroutine (SBR), and
Return from Subroutine (RET) Instructions:
• Subroutine: A program file that performs a
specific task.
• A subroutine will interrupt the program
scan based on the specified instruction.
For SLC 500 processors, program files 3 to
255 are designated for subroutines.
Subroutine are used in the following situations:
Loading a recipe
Performing a calculation
Cleaning a vat
Purging a system
Emptying a tank
Packing and sealing a carton
The following graphic represents subroutines in a processor operating
cycle:
• Jump to Subroutine (JSR): An output
instruction that directs the controller to
jump to a targeted subroutine file.
• Subroutine (SBR): An input instruction
that marks the beginning of a subroutine.
• Return (RET): An output instruction that
marks the end of a subroutine execution
or the end of the subroutine file.
•JSR and SBR instructions are
used together to direct the
processor to execute a separate
subroutine file within the ladder
logic program. The RET
instruction causes the processor
to resume program execution in
the program file that called the
subroutine. Program execution
continues at the instruction
following the JSR instruction
where it exited the program.
The following graphic shows
the instructions used to execute
a subroutine file:
• Nested Subroutine File: A subroutine that directs
program execution to another subroutine.
Master Control Reset Instruction: An output instruction that,
when used in pairs, defines program zones where all non-retentive
outputs may be disabled at the same time for the same length of time.
Rungs within the MCR zone are still scanned, but scan time is reduced
due to the false state of non-retentive outputs.
The true and false states for MCR instructions are summarized in the
following table:
Master Control Reset (MCR) Instruction:
•Non-Retentive
Outputs and the
MCR Instruction:
Non-retentive outputs
reset when rung
conditions go false;
retentive outputs do
not. MCR instructions
turn off all nonretentive outputs in a
section of ladder logic.
Examples of nonretentive and retentive
outputs are shown in
the following graphic:
Keep these points in mind when interpreting MCR
instructions:
• The MCR instruction is not a substitute for
a hard-wired master control relay that
provides emergency stop capability.
• TOF timers will activate when placed
inside of a false MCR zone.
• If an unmatched MCR instruction exists in
the project, the END instruction is used to
mark the end of the MCR zone.
Temporary End (TND) Instruction:
• An output instruction that controls the scanning
•
operation of a processor.
When the logic preceding this output instruction
is true, the TND:
• Stops the processor from scanning the rest of the
•
•
program file.
Updates the I/O
Resumes scanning at rung 0 of the main program.
If this instruction rung is false, the processor
continues the scan until the next TND instruction
or the END statement.
Suspend (SUS) Instruction:
• An output instruction that causes a
processor to enter the suspend idle mode
and stores a suspend ID number in word 7
(S:7) of the status file. All outputs are deenergized.
• This instruction can be used to identify
specific conditions for program debugging
and system troubleshooting.
Interpreting Jump to Label (JMP) and Label
(LBL) Instructions:
Interpreting Jump to Subroutine (JSR), Subroutine
(SBR), and Return from Subroutine (RET)
Instructions:
• The following graphic shows an example of JSR, SBR, and RET
instructions.
Interpreting Master Control Reset (MCR)
Instructions:
The following graphic shows
an example of an MCR
instruction.
When pushbuttons 5, 8, and 9
are pressed, coils 6 and 8 are
energized and the T4:6 timer
begins timing.
When the pushbutton 7 is
pressed, the MCR zone is
activated.
Coil 8 is de-energized, and the
TON timer stops timing.
Coil 6 remains on because it is
a retentive output.
The T4:7 timer begins timing.
Interpreting Temporary End (TND)
Instructions:
• The following graphic shows an example of a TND
•
•
instruction. In the example, note the following:
When switch 10 is closed, the TND instruction stops the
processor from scanning the rest of the program file.
I/O is updated, and the processor resumes scanning at
rung 0 of the main program file.
Interpreting Suspend (SUS) Instructions:
• The following graphic shows
an example of an SUS
instruction. In the example,
note the following:
• When switch 15 is closed and
the value in N7:0 is greater
than or equal to 75, the
processor is placed in the
Suspend Idle Mode.
• The suspend ID is placed in
word S:7 of the processor
status file.
• All outputs are de-energized.