1. Program
Control
Instructions

2. Program Control Instructions
Program control instructions are used to alter the program scan from its normal sequence. Sometimes referred to as override instructions, they provide a means of executing sections of the control logic if certain conditions are met. They allow for greater program flexibility and greater efficiency in the program scan.

3. Typical Program Control Instructions Based On The SLC 500 And Associated RSLogix Software
TND
TND Temporary End Makes a temporary end
that halts program
execution
RET
RET Return from Subroutine Exits current subroutine
and returns to previous
condition
MCR
MCR Master Control Reset Clears all set outputs
between the paired MCR
instruction
SUS
SUS Suspend Identifies specific
conditions for program
debugging and system
troubleshooting
JMP
JMP Jump to Label Jump forward/backward
to a corresponding label
instruction
JSR Jump to Subroutine Jump to a designated
subroutine instruction
JSR
SBR
SBR Subroutine Identifies the subroutine
program
LBL
LBL Label Specifies label location
Program Control

4. Hardwired Master Control Relay Circuit
Hardwired master control relays are used in relay circuitry to provide input/output power shutdown of an entire circuit.

5. Hardwired Master Control Relay Circuit

6. MCR Instruction
MCR
The master control reset (MCR) instruction can be programmed to control an entire circuit or to control only selected rungs of a circuit. When the MCR instruction is false, or de-energized, all nonretentive (nonlatched) rungs below the the MCR will be de-energized even if the programmed logic for each rung is true. All retentive rungs will remain in their last state. The MCR instruction establishes a zone in the user program in which all nonretentive outputs can be turned off simultaneously. Therefore, retentive instructions should not normally be placed within an MCR zone because the MCR zone maintains retentive instructions in the last active state when the instruction goes false.

7. MCR Instruction Programmed To Control An
Entire Circuit

8. MCR Instruction Programmed To Control A Fenced Zone
The Master Control Reset (MCR) instruction is used in pairs to disable or enable a zone within a ladder program and has no address. You program the first MCR with input instructions in the rung and the ending MCR without any other instructions in the rung.
Fenced
Zone

9. MCR Instruction Programmed To Control A Fenced Zone
MCR Zone True
Lab # 1

10. MCR Instruction Programmed To Control A Fenced Zone
MCR Zone False
Lab # 1

11. Programming MCR Instructions
If you start instructions such as timers and counters
in an MCR zone, instruction operation ceases when the
zone is disabled.
The TOF timer will activate when placed inside a false
MCR zone.
When troubleshooting a program that contains an MCR zone you need to be aware of which rungs are within zones in order to correctly edit the circuit.
MCR controlled areas must contain only two MCR
instructions – one to define the start and one to define the end.

12. JMP
Jump Instruction
As in computer programming, it is sometimes desirable
to be able to jump over certain program instructions.
The jump instruction (JMP) is an output instruction used for this purpose. The advantages to the jump instruction include:
the ability to reduce the processor scan time by jumping over instructions not pertinent to the machines operation at that instant
The PLC can hold more than one program and scan only the program appropriate to operator requirements
Sections of a program can be jumped when a production fault occurs

13. Jump-To-Label Instructions
In a SLC 500 the JMP will have a number from 0 to 255, and the corresponding LBL will have the same number. In the ControlLogix controller the JMP and LBL have the same name.
Avoid jumping backwards in the program too many times as this may increase the scan beyond the maximum allowable time. The processor has a watchdog timer that sets the maximum time for a total program scan. If this time is exceeded, the processor will indicate a fault and shut down.
You should never jump into an MCR zone. Instructions that are programmed within the MCR zone starting at the LBL instruction and ending at the end MCR instruction will always be evaluated as though the MCR zone is true, without consideration to the state of the start MCR instruction.

14. Jump Operation
By using the jump instruction, you can branch or skip to different portions of a program and freeze all affected outputs in their last state.
Jumps are normally allowed in both the forward and backward directions.
Jumping over counters and timers will stop them from being incremented.

15. Jump-To-Label
With Allen-Bradley PLCs the jump (JMP) instruction and the label (LBL) instruction are employed together so the scan can jump over a portion of the program.
The label is a target for the jump, it is the first instruction in the rung, and it is always true.
A jump jumps to a label with the same address. The area that the processor jumps over is defined by the locations of the jump and label instructions in the program.
If the jump coil is energized, all logic between the jump and label instructions is bypassed and the processor continues scanning after the LBL instruction.

16. Jump To Label Program Jumped program
Timers should be
programmed outside the jumped section
Input conditions are not examined and outputs remain in their last state
Jumped program
rungs (5,6,7) are not scanned by the processor
Lab # 2

17. Jump-To-Label From Two Locations20

18. Jump - To - Subroutine
Another valuable tool in PLC programming is to be able to escape from the main program and go to a program subroutine to perform certain functions and then return to the main program.

19. Allen-Bradley Subroutine-Related Instructions
The JSR instruction causes the scan to jump to the program file designated in the instruction. It is the only parameter entered in the instruction. When rung conditions are true for this output instruction, it causes the processor to jump to the targeted subroutine file. Each subroutine must have a unique file number (decimal 3-255).

20. Allen-Bradley Subroutine-Related Instructions
The SBR instruction is the first instruction on the first rung in the subroutine file. It serves as an identifier that the program file is a subroutine. This is the file number that is used in the JSR instruction to identify the target to which the program should jump. It always true and although its use is optional, it is still recommended.

21. Allen-Bradley Subroutine-Related Instructions
The RET instruction is an output instruction that marks the end of the subroutine file. It causes the scan to return to the main program at the instruction following the JSR instruction where it exited the program. The scan returns from the end of the file if there is no RET instruction. The rung containing the RET instruction may be conditional if this rung precedes the end of the subroutine. In this way, the processor omits the balance of a subroutine only if its rung condition is true.

22. Setting Up A Subroutine File

23. Flashing Pilot Light Subroutine
If the weight on the conveyor exceeds a preset value, the solenoid is de-energized and the alarm light will begin flashing.
Process
When the weight sensor switch closes, the JSR is activated and the processor scan jumps to the subroutine area. The subroutine is continually scanned and the light flashes.
When the sensor switch opens, the processor will no longer scan the subroutine area and will return to the on state.

24. Setting Up A Subroutine File
SLC-500 controller subroutines are located in different program files from the main program. The main program is located in program file 2, whereas subroutines are assigned to program file numbers 3 to 255.
Note each ladder location where a subroutine should be called.
Create a subroutine file for each location. Each subroutine file
should begin with an SBR instruction.
At each ladder location where a subroutine is called, program
a JSR instruction specifying the subroutine file number.
The RET instruction is optional as the end of a subroutine program will cause a return to the main program.

25. Nested Subroutines
5/02 Processors allow up to 8 nesting sub routines