Page 207 - Programmable Logic Controllers, Fifth Edition - Mobile version

ignores the inputs and de-energizes all nonretentive
Ladder logic program
outputs (that is, the output energize instruction, the L1 Inputs PB PL1 Outputs L2
on-delay timer, and the off-delay timer). 1
• All retentive devices, such as latches, retentive tim- PB
ers, and counters, remain in their last state. TOF Switch Q2:0 PL1
timers will start timing when the MCR goes false. 2 JMP
• Input A of the start rung is false so output A and Switch PL2
T4:1 will be false and output B will remain in its PB PL2
last state. 3 PL3
• The input conditions in each rung will have no
effect on the output conditions. Q2:0 PB PL3
A common application of an MCR zone control in- 4 LBL
volves examining one or more fault bits as part of the start
fence and enclosing the portion of the program you want
de-energized in case of a fault in the MCR zone. In case of Figure 9-6 Jump (JMP) operation.
a detected fault condition, the outputs in that zone would
be de-energized automatically.
If you start instructions such as timers or counters
in an MCR zone, instruction operation ceases when Some manufacturers provide a skip instruction, which
the zone is disabled. The TOF timer will activate when is essentially the same as the jump instruction.
placed inside a false MCR zone. When troubleshooting The program of Figure 9-6 illustrates the use of a jump
a program that contains an MCR zone, you need to be instruction in conjunction with Allen-Bradley SLC 500
aware of which rungs are within zones in order to cor- programmable controllers. In this example, Addresses
rectly edit the circuit. Q2:0 through Q2:255 are the addresses used for the jump
MCR-controlled areas must contain only two MCR (JMP) instructions. The Q2 is internal and provided by
instructions—one to define the start and one to define the the software as you program the JMP instruction. The Q2
end. Never overlap or nest MCR zones. Any additional simply identifies this as ladder file 2. A JMP instruction in
MCR instructions, or a jump instruction programmed ladder file 3 would be Q3. The label (LBL) instruction is
to jump to an MCR zone, could produce unexpected a target for the jump instruction.
and damaging results to your program and to machine • The jump instruction with its associated label in-
operation. struction (LBL) must have the same address.
In addition to controlling power to an entire system, • The area of the program that the processor jumps
MCRs are also used when only a portion of a program is over is defined by the locations of the jump and
required to be isolated. For example: label instructions in the program.
• Inhibiting zones of the program while loading recipes. • When the jump instruction is true, all logic between the
• Monitoring emergency stops. jump and label instructions is bypassed and the proces-
• Establishing preconditions to synchronize a ma- sor continues scanning after the LBL instruction.
chine on start-up. • The label instruction must be programmed as the
first instruction on the rung where it resides.
9.3 Jump Instruction • The label instruction is always true, and the remain-
In PLC programming it is sometimes desirable to be able ing instructions on the rung must make up a verifi-
to jump over certain program instructions when certain able rung.
conditions exist. The jump (JMP) instruction is an output • The instructions to the right of the LBL on the label
instruction used for this purpose. When the jump instruc- rung are outside the jump zone and as such are not
tion is used, the PLC will not execute the instructions of a affected by the jump.
rung that is jumped. The jump instruction is often used to The operation of the program can be summarized as
jump over instructions not pertinent to the machine’s op- follows:
eration at that instant. In addition, sections of a program
may be programmed to be jumped should a production • When the switch is open the jump instruction is not
fault occur. activated.

188 Chapter 9 Program Control Instructions

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