LBL Q2:0 JMP Q2:0 PL3 PL3 PL2 PL1 PB Switch Switch PL2 PB PB L1 Outputs Inputs 1 2 3 4 Ladder logic program PB 9 Program Control Instructions Chapter Objectives After completing this ch
Trang 1LBL Q2:0
JMP Q2:0
PL3
PL3 PL2 PL1
PB
Switch
Switch
PL2
PB
PB
L1
Outputs Inputs
1
2
3
4
Ladder logic program
PB
9
Program Control
Instructions
Chapter Objectives
After completing this chapter, you will be able to:
• State the purpose of program control instructions
• Describe the operation of the master control reset instruction and develop an elementary program illustrating its use
• Describe the operation of the jump instruction and the label instruction
• Explain the function of subroutines
• Describe the immediate input and output instructions function
• Describe the forcing capability of the PLC
• Describe safety considerations built into PLCs and programmed into a PLC installation
• Explain the differences between standard and safety PLCs
• Describe the function of the selectable timed interrupt and fault routine files
• Explain how the temporary end instruction can be used
to troubleshoot a program
The program control instructions covered in this
chapter are used to alter the program scan from
its normal sequence The use of program
con-trol instructions can shorten the time required to
complete a program scan Portions of the
pro-gram not being utilized at any particular time can
be jumped over, and outputs in specific zones in
the program can be left in their desired states
Typical industrial program control applications
are explained
Trang 29.1 Program Control
Several output-type instructions, which are often referred
to as override instructions, provide a means of
execut-ing sections of the control logic if certain conditions are
met These program control instructions allow for greater
program flexibility and greater efficiency in the program
scan Portions of the program not being utilized at any
particular time can be jumped over, and outputs in specific
zones in the program can be left in their desired states
Program control instructions are used to enable or
dis-able a block of logic program or to move execution of a
program from one place to another place Figure 9-1 shows
the Program Control menu tab for the Allen- Bradley
SLC 500 PLC and its associated RSLogix software The
program control commands can be summarized as follows:
JMP (Jump to Label)—Jump forward/backward to a
corresponding label instruction
LBL (Label)—Specifies label location.
JSR (Jump to Subroutine)—Jump to a designated
subroutine instruction
RET (Return from Subroutine)—Exits current
sub-routine and returns to previous condition
SBR (Subroutine)—Identifies the subroutine program TND (Temporary End)—Makes a temporary end
that halts program execution
MCR (Master Control Reset)—Clears all set
non-retentive output rungs between the paired MCR instructions
SUS (Suspend)—Identifies conditions for debugging
and system troubleshooting
Hardwired master control relays are used in relay con-trol circuitry to provide input/output power shutdown of
an entire circuit Figure 9-2 shows a typical hardwired
master control relay circuit In this circuit, unless the
master control relay coil is energized, there is no power flow to the load side of the MCR contacts
The equivalent PLC instruction to a Master Control
Relay is the Master Control Reset (MCR) instruction
This instruction functions in a similar manner to the hardwired master control relay; that is, when the instruc-tion is true, the circuit funcinstruc-tions normally, and when the instruction is false, nonretentive outputs are switched off
Figure 9-2 Hardwired master control relay
Source: This material and associated copyrights are proprietary to, and used with the permission of Schneider Electric.
CR2
MCR
Master stop Master start
CR4
CR
M1
OL
CR1
M2
OL M1
MCR
Figure 9-1 Program Control menu tab
JMP LBL JSR RET SBR TND
Program Control
MCR SUS
Ascii Control Ascii String Micro
Trang 3The programmed MCR instruction is not a substitute for
a hardwired Master Control Relay It is highly
recom-mended that all PLC systems include a hardwired MCR
and Emergency Stop switches to provide safe, effective
shutdown of I/O power
A Master Control Reset (MCR) instruction is an
out-put coil instruction that functions like a master control
relay MCR coil instructions are used in pairs and can
be programmed to control an entire circuit or to
con-trol only selected rungs of a circuit In the program of
Figure 9-3, the MCR is programmed to control an entire
circuit The operation of the program can be
summa-rized as follows:
• The section or zone being controlled begins with the
first MCR instruction and ends with the second MCR
• When the first MCR instruction is false, or disabled,
all nonretentive rungs below it, in this case, outputs
M and PL1, will be de-energized even if the
pro-grammed logic for each rung is true
• All retentive rungs, in this case SOL, will remain in
their last state
• Assume the motor M is running and the MCR
instruction becomes disabled The motor will
im-mediately become de-energized and stop operating
When the MCR instruction then becomes enabled,
the motor will not revert back to its previous
running state but will have to be restarted via the start pushbutton
• Assume the level switch is closed and the MCR in-struction becomes disabled Pilot light PL1 will im-mediately become de-energized even though the level switch instruction is true and the rung appears to have logic continuity When the MCR instruction then be-comes enabled, PL1 will automatically be energized, provided the level switch has remained closed
• Assume solenoid SOL has been latched energized, both limit switches LS1 and LS2 are open, and the MCR instruction becomes disabled Solenoid SOL will remain energized When the MCR instruction then becomes enabled, the SOL will remain ener-gized, provided both LS1 and LS2 remained open
• Assume solenoid SOL has been latched de-energized, both limit switches LS1 and LS2 are open, and the MCR instruction becomes disabled
Solenoid SOL will remain de-energized When the MCR instruction then becomes enabled, the SOL will remain de-energized, provided both LS1 and LS2 remained open
• Retentive instructions should not normally be placed within an MCR zone because the MCR zone maintains retentive instructions in the state last ac-tive when the instruction disabled
Figure 9-3 Master Control Reset (MCR) instruction
ON/OFF
Ladder logic program
L
U
Stop Start
M
PL1
When MCR
is de-energized, all nonretentive outputs de-energize.
When MCR
is de-energized, all retentive outputs remain
in last state.
Outputs
SOL
ON/OFF
Stop
Start
Level switch
LS1
LS2
MCR
MCR
Trang 4Allen-Bradley SLC 500 controllers use the master
con-trol reset instruction to set up single or multiple zones
within a program The MCR instruction is used in pairs
to disable or enable a zone within a ladder program, and
it has no address Figure 9-4 shows the programming of
an MCR fenced zone with the zone true The operation of
the program can be summarized as follows:
• The MCR zone is enclosed by a start fence,
which is a rung with a conditional MCR, and an
end fence, which is a rung with an unconditional
MCR
• Input A of the start rung is true so all outputs act
ac-cording to their rung logic as if the zone did not exist
Figure 9-5 shows the programmed MCR fenced zone with the zone false The operation of the program can be summarized as follows:
• When the MCR in the start fence is false, all rungs within the zone are treated as false The scan
Figure 9-5 MCR fenced zone with the zone false
End fence
L1
Inputs
Input C
Input D
Input E
Input A
Input B
L2 Outputs
Output A
Output B
Start fence
OFF
ON Input C
Input A Ladder logic program
T4:1 1.0 10 0
TON TIMER ON DELAY Timer
Time base Preset Accumulated Input D
Input E
Latch output B
Unlatch output B MCR
MCR L
EN DN
U
Figure 9-4 MCR fenced zone with the zone true
Outputs
Output A
Output B Inputs
Input C
Input C
Input A
Ladder logic program
T4:1 1.0 10 0
TON TIMER ON DELAY Timer
Time base Preset Accumulated
Input D
Input D
Input E
Input E
Latch output B
Unlatch output B
Start fence
End fence
Active
ON
ON
Input A
Input B
MCR
MCR
DN EN
L
U
Trang 5ignores the inputs and de-energizes all nonretentive
outputs (that is, the output energize instruction, the
on-delay timer, and the off-delay timer)
• All retentive devices, such as latches, retentive
tim-ers, and counttim-ers, remain in their last state TOF
timers will start timing when the MCR goes false
• Input A of the start rung is false so output A and
T4:1 will be false and output B will remain in its
last state
• The input conditions in each rung will have no
effect on the output conditions
A common application of an MCR zone control
in-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
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
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
cor-rectly edit the circuit
MCR-controlled areas must contain only two MCR
instructions—one to define the start and one to define the
end Never overlap or nest MCR zones Any additional
MCR instructions, or a jump instruction programmed
to jump to an MCR zone, could produce unexpected
and damaging results to your program and to machine
operation
In addition to controlling power to an entire system,
MCRs are also used when only a portion of a program is
required to be isolated For example:
• Inhibiting zones of the program while loading recipes
• Monitoring emergency stops
• Establishing preconditions to synchronize a
ma-chine on start-up
In PLC programming it is sometimes desirable to be able
to jump over certain program instructions when certain
conditions exist The jump (JMP) instruction is an output
instruction used for this purpose When the jump
instruc-tion is used, the PLC will not execute the instrucinstruc-tions of a
rung that is jumped The jump instruction is often used to
jump over instructions not pertinent to the machine’s
op-eration at that instant In addition, sections of a program
may be programmed to be jumped should a production
fault occur
Some manufacturers provide a skip instruction, which
is essentially the same as the jump instruction
The program of Figure 9-6 illustrates the use of a jump instruction in conjunction with Allen-Bradley SLC 500 programmable controllers In this example, Addresses Q2:0 through Q2:255 are the addresses used for the jump (JMP) instructions The Q2 is internal and provided by the software as you program the JMP instruction The Q2 simply identifies this as ladder file 2 A JMP instruction in
ladder file 3 would be Q3 The label (LBL) instruction is
a target for the jump instruction
• The jump instruction with its associated label in-struction (LBL) must have the same address
• The area of the program that the processor jumps over is defined by the locations of the jump and label instructions in the program
• When the jump instruction is true, all logic between the jump and label instructions is bypassed and the proces-sor continues scanning after the LBL instruction
• The label instruction must be programmed as the first instruction on the rung where it resides
• The label instruction is always true, and the remain-ing instructions on the rung must make up a verifi-able rung
• The instructions to the right of the LBL on the label rung are outside the jump zone and as such are not affected by the jump
The operation of the program can be summarized as follows:
• When the switch is open the jump instruction is not activated
Figure 9-6 Jump (JMP) operation
LBL Q2:0
JMP Q2:0
PL3
PL3 PL2 PL1
PB
Switch
Switch
PL2
PB
PB
L1
Outputs Inputs
1
2
3
4 Ladder logic program
PB
Trang 6• With the switch open, closing PB turns on all three
pilot lights
• When the switch is closed the jump (JMP)
instruc-tion will activate
• With the switch closed, pressing PB turns on pilot
lights PL1 and PL3 only
• Rung 3 is skipped over during the PLC program
scan so PL2 will remain in its last state before the
JMP was enabled
Figure 9-7 illustrates the effect on input and output
structions of jumped rungs in a program The label
in-struction is used to identify the ladder rung that is the
target destination but does not contribute to logic continu-ity For practical purposes the label instruction is always considered to be logically true The operation of the pro-gram can be summarized as follows:
• Rungs 1, 2, 3, 8, 9, 10 are programmed outside of the jumped section and will always be executed as normal rungs
• If rung 4, which contains the JMP instruction, is
false, the Jump instruction is false and the jump is
not executed
• Rungs 5, 6, and 7 are executed as normal and the label instruction on rung 8 is transparent
Figure 9-7 Effect on input and output instructions of jumped rungs
PL1
Outputs
M M
PL2 Inputs
TS1
LS4
SOL3 PL2 SOL2 SOL1
SOL1
SOL3 LS3
TS1
LS2
LS2
LS1
PS1
PB3
PB3
PB2 PB1
LS4
SOL2
SOL3
SOL4
1
2
3
4
5
6
7
8
9
10
T4:6 1.0 5 0
TON TIMER ON DELAY Timer
Time base Preset Accumulated
M
M
M
T4:6
DN
Heater
JMP Q2:1
Q2:1 LBL
Heater Heater
Timers should be programmed outside the jumped section.
Jumped program rungs are not scanned by the processor.
Input conditions are not examined, and outputs remain in their last state.
DN T4:6
Ladder logic program
LS3
PL2
PL1 EN
DN
Trang 7• When rung 4, containing the JMP instruction, is
true, the processor is instructed to jump to the LBL
target in rung 8 and continue to execute the main
program from that point
• Instructions to the right of the LBL are out of the
jump zone and are executed as a normal rung
• Jumped rungs 5, 6, and 7 are not scanned by the
processor
• Input conditions for the jumped rungs are not
exam-ined and outputs controlled by these rungs remain in
their last state
• Any timers or counters programmed within the
jump area cease to function and will not update
themselves during this period For this reason they
are usually programmed outside the jumped section
in the main program zone
• This is called a forward jump, as we are jumping
forward in the program
You can jump to the same label from multiple jump
locations, as illustrated in the program of Figure 9-8 In
this example, there are two jump instructions addressed
Q2:5 There is a single label instruction addressed
Q2:5 The scan can then jump from either jump
instruc-tion to label Q2:5, depending on whether input A or
input D is true.
It is possible to jump backward in the program, but this
should not be done an excessive number of times Care
must be taken that the scan does not remain in a loop too
long The processor has a watchdog timer that sets the
maximum allowable time for a total program scan If this
time is exceeded, the processor will indicate a fault and
shut down
The forward jump is similar to an MCR instruction
in that both permit an input logic condition to skip over
a block of PLC ladder logic The main difference be-tween the two is in how the outputs are handled when the instructions are executed The MCR instruction sets all nonretentive outputs to the false state and keeps the retentive outputs in their last state The JMP instruction leaves all outputs in their last state You should never jump into a Master Control Reset zone If you do, in-structions 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
In addition to the main ladder logic program, PLC pro-grams may also contain additional program files known
as subroutines A subroutine is a short program that is
used by the main program to perform a specific func-tion Large programs are often broken into subroutine program files, which are called and executed from the main program In the SLC 500 series PLCs, the main ladder logic program is in program file two (shown as LAD 2) Ladder logic programs for subroutines can be placed in file number three (LAD 3) through file number
255 (LAD 255)
Use of subroutines is a valuable tool in PLC program-ming At times it is better to construct programs that consist of several subroutines than a lengthy single pro-gram When programs are written with subroutines, each subroutine can be tested individually for functionality
These subroutines can then be called from the main pro-gram as illustrated in Figure 9-9
Figure 9-8 Jump-to-label from two locations
Q2:5 Input A
Ladder logic program
Output A Input B
Q2:5 Input D
Output C Input E
Output D Q2:5
LBL
Input F
JMP
JMP
Figure 9-9 Main program with a call from a subroutine
Main program rungs
Jumps
Unconditional return Subroutine area
Returns to next instruction after JSR JSR
SBR
RET
Trang 8When a subroutine is called from the main
pro-gram, the program is able to escape from the main
program and go to a program subroutine to perform
certain functions and then return to the main program
In situations in which a machine has a portion of its
cycle that must be repeated several times during one
machine cycle, the subroutine can save a great deal of
duplicate programming The sequence of rungs could
be programmed one time into a subroutine and just
called when needed
The subroutine concept is the same for all
program-mable controllers, but the method used to call and return
from a subroutine uses different commands, depending on
the PLC manufacturer The subroutine-related instructions
used in the Allen-Bradley PLCs shown in Figure 9-10 are
the jump to subroutine (JSR) output instruction, the
sub-routine (SBR) input instruction, and the return (RET)
out-put instruction
The subroutine instructions can be summarized as
follows:
Jump to Subroutine (JSR)—The JSR instruction
redirects logic execution from the current ladder
file to the specific subroutine file When rung
condi-tions 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)
Subroutine (SBR)—The SBR instruction is the first
input instruction on the first rung in the subroutine
file It serves as an identifier that the program file is
a subroutine This file number is used in the JSR
in-struction to identify the target to which the program
should jump It is always true, and although its use is
optional, it is still recommended
Return (RET)—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
ex-ited 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
The jump to subroutine (JSR), subroutine (SBR), and return (RET) instructions are used to direct the controller
to execute a subroutine file Figure 9-11 shows a materials conveyor system with a flashing pilot light as a subrou-tine The operation of the program can be summarized as follows:
• If the weight on the conveyor exceeds a preset value, the solenoid is de-energized and pilot light PL1will begin flashing
• When the weight sensor switch closes, the JSR is activated and directs the processor scan to jump to the subroutine U:3
• The subroutine program is scanned and pilot light PL1 begins flashing
• When the weight sensor switch opens, the proces-sor will no longer scan the subroutine area and pilot light PL1 will return to its normal on state
The Allen-Bradley SLC 500 controller main program
is located in program file 2 whereas subroutines are as-signed to program file numbers 3 to 255 Each subroutine must be programmed in its own program file by assigning
it a unique file number Figure 9-12 illustrates the proce-dure for setting up a subroutine and can be summarized
as follows:
• 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
– The end of a subroutine program will cause a return to the main program
– If you want to end a subroutine program before it executes to the end of program file, a conditional return (RET) instruction may be used
Nesting subroutines allows you to direct program flow from the main program to a subroutine and then to another subroutine, as illustrated in Figure 9-13 Nested subrou-tines make complex programming easier and program op-eration faster because the programmer does not have to continually return from one subroutine to enter another
Figure 9-10 Allen-Bradley subroutine-related instructions
JSR JUMP-TO-SUBROUTINE SBR file number U:3
SBR SUBROUTINE
RET RETURN
Trang 9Solenoid
Weight sensor Pilot light
Figure 9-11 Flashing pilot light subroutine (a) Process (b) Program
OFF/ON
Sensor
Sensor Sensor
SBR SUBROUTINE
Main program file 2
Subroutine file 3
(b)
Inputs
Stop Stop
M1
Outputs
PL1
PL1
SOL Motor
Motor
Motor T4:1/EN
T4:0/DN
T4:1/DN
JSR JUMP-TO-SUBROUTINE SBR file number
T4:0 1.0 1 0
TON TIMER ON DELAY Timer
Time base Preset Accumulated
DN EN
T4:1 1.0 1 0
TON TIMER ON DELAY Timer
Time base Preset Accumulated
DN EN
RET RETURN
U:3
Sensor
PL1
Start
Start
Trang 109.5 Immediate Input and Immediate Output Instructions
The PLC input scan normally records the inputs before the program scan, and the output scan normally updates the
outputs after the program scan Immediate I/O
instruc-tions allow you to update data prior to the normal input scan as illustrated in Figure 9-14
Immediate I/O instructions interrupt the normal pro-gram scan to update the input image table file with cur-rent input data or to update an output module group with the current output image table file data Allen-Bradley SLC 500 PLC’s immediate I/O instructions are called
immediate input with mask (IIM) and immediate output with mask (IOM).
• Masking is a means of selectively screening out data
• Masking allows the programmer to specify which
of the 16 bits are to be copied from an input module
to the input image data table (or from the output image table to an output module)
• The other bits in the input image table or output module are not affected by these instructions
The immediate input with mask (IIM) instruction is
shown in Figure 9-15 The IIM instruction operates on
the inputs assigned to a particular word of a slot When the IIM rung is true, the program scan is interrupted, and data from a specific input slot are transferred through the mask to the input data file These data are then available
Figure 9-12 Setting up a subroutine file
JSR JUMP-TO-SUBROUTINE SBR file number 3
Main program file 2
SBR SUBROUTINE
RET RETURN
Subroutine file 3
Figure 9-13 Nested subroutines
JSR JUMP-TO-SUBROUTINE SBR file 3
JSR JUMP-TO-SUBROUTINE SBR file 4
Main program file 2
SBR SUBROUTINE
RET RETURN
Level 1 file 3
JSR JUMP-TO-SUBROUTINE SBR file 5
SBR SUBROUTINE
RET RETURN
Level 2 file 4
SBR SUBROUTINE
RET RETURN
Level 3 file 5
Programming nested subroutines may cause scan time
problems because while the subroutine is being scanned,
the main program is not Excessive delays in scanning the
main program may cause the outputs to operate later than
required This situation may be avoided by updating
criti-cal I/O using immediate input and/or immediate output
instructions