Petruzella F.D. Programmable Logic Controllers

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Programming Counters Chapter 8 173

As a result:

a. What is the accumulated count of counter CTU?

b. What is the accumulated count of counter CTD?

c. What is the state of output A ?

d. What is the state of output B ?

e. What is the state of output C ?

8. Write a program to implement the process il-

lustrated in Figure8-42 . An up-counter must be

programmed as part of a batch-counting operation

to sort parts automatically for quality control. The

counter is installed to divert 1 part out of every

• An emergency stop button is used to stop the pro-

cess at any time.

• If the sequence is interrupted by an emergency

stop, counter and timer are reset automatically.

7. Answer the following questions with reference

to the up/down-counter program shown in Fig-

ure8-41 . Assume that the following sequence of

events occurs:

• Input C is momentarily closed.

• 20 on/off transitions of input A occur.

• 5 on/off transitions of input B occur.

Figure 8-41 Program for Problem 7.

Input A

PB1

PB2

Reset

Input B

Input C

Ladder logic programInputs Outputs

Input A

Input B

Input C

C5:2

CTU

COUNT-UP COUNTER

Counter C5:2

Preset 10

Accumulated 0

CTD

COUNT-DOWN COUNTER

Counter C5:2

Preset 10

Accumulated 0

C5:2

Output B

Output C

Output A

RES

Output A

Output B

Output C

Figure 8-42 Control process for Problem 8.

Quality control line

Gate

solenoid drive

Parts conveyer

line

Proximity

switch

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174 Chapter 8 Programming Counters

kit. The controller must stop the take-up spool

at a predetermined amount of resistors (100). A

worker on the  oor will then cut the resistor strip

and place it in the kit. The circuit operates as

follows:

• A start/stop pushbutton station is used to turn the

spool motor drive on and off manually.

• A through-beam sensor counts the resistors as

they pass by.

• A counter preset for 100 (the amount of

resistorsin each kit) will automatically stop

the take-up spool when the accumulated count

reaches 100.

• A second counter is provided to count the grand

total used.

• Manual reset buttons are provided for each

counter.

11. Write a program that will latch on a light 20 s after

an input switch has been turned on. The timer will

continue to cycle up to 20 s and reset itself until

the input switch has been turned off. After the third

time the timer has timed to 20 s, the light will be

unlatched.

12. Write a program that will turn a light on when a

count reaches 20. The light is then to go off when a

count of 30 is reached.

13. Write a program to implement the box-stacking

process illustrated in Figure8-44 . This applica-

tion requires the control of a conveyor belt that

feeds a mechanical stacker. The stacker can stack

various numbers of cartons of ceiling tile onto

each pallet (depending on the pallet size and the

preset value of the counter). When the required

number of cartons has been stacked, the conveyor

is stopped until the loaded pallet is removed and

an empty pallet is placed onto the loading area. A

photoelectric sensor will be used to provide count

pulses to the counter after each carton passes by.

In addition to a conveyor motor start/stop station,

a remote reset button is provided to allow the

operator to reset the system from the forklift after

an empty pallet is placed onto the loading area.

1000 for quality control or inspection purposes.

The circuit operates as follows:

• A start/stop pushbutton station is used to turn the

conveyor motor on and off.

• A proximity sensor counts the parts as they pass

by on the conveyor.

• When a count of 1000 is reached, the counter’s

output activates the gate solenoid, diverting the

part to the inspection line.

• The gate solenoid is energized for 2 s, which

allows enough time for the part to continue to the

quality control line.

• The gate returns to its normal position when the

2-s time period ends.

• The counter resets to 0 and continues to

accumulate counts.

• A reset pushbutton is provided to reset the

counter manually.

9. Write a program that will increment a counter’s

accumulated value 1 count every 60 s. A second

counter’s accumulated value will increment 1

count every time the  rst counter’s accumulated

value reaches 60. The  rst counter will reset when

its accumulated value reaches 60, and the second

counter will reset when its accumulated value

reaches 12.

10. Write a program to implement the process illus-

trated in Figure8-43 . A company that makes elec-

tronic assembly kits needs a counter to count and

control the number of resistors placed into each

Figure 8-43 Control process for Problem 10.

Spool

motor drive

Through-beam

sensor

Figure 8-44 Control process for Problem 13.

Reflector

Cartons of

ceiling tile

Pallet

Sensor

Stac

ker

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Programming Counters Chapter 8 175

14. Write a program to operate a light according to the

following sequence:

• A momentary pushbutton is pressed to start the

sequence.

• The light is switched on and remains on for 2 s.

• The light is then switched off and remains off

for2 s.

• A counter is incremented by 1 after this

sequence.

• The sequence then repeats for a total of 4 counts.

• After the fourth count, the sequence will stop and

the counter will be reset to zero.

The operation of this system can be summarized

as follows:

• The conveyor is started by pressing the start

button.

• As each box passes the photoelectric sensor, a

count is registered.

• When the preset value is reached (in this

case12), the conveyor belt turns off.

• The forklift operator removes the loaded pallet.

• After the empty pallet is in position, the forklift

operator presses the remote reset button, which

then starts the whole cycle over again.

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176

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 speciﬁ c zones in

the program can be left in their desired states.

Typical industrial program control applications

are explained.

Chapter Objectives

After completing this chapter, you will be able to:

9.1 State the purpose of program control instructions

9.2 Describe the operation of the master control reset

instruction and develop an elementary program

illustrating its use

9.3 Describe the operation of the jump instruction and the

label instruction

9.4 Explain the function of subroutines

9.5 Describe the immediate input and output instructions

function

9.6 Describe the forcing capability of the PLC

9.7 Describe safety considerations built into PLCs and

programmed into a PLC installation

9.8 Explain the differences between standard and safety

PLCs

9.9 Describe the function of the selectable timed interrupt

and fault routine  les

9.10 Explain how the temporary end instruction can be

used to troubleshoot a program

Program Control Instructions

Image Used with Permission of Rockwell Automation, Inc.

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Program Control Instructions Chapter 9 177

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) —Identi es 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 mas-

ter control relay coil is energized, there is no power  ow

to the load side of the MCR contacts.

PLC manufacturers offer a form of a master control relay

as part of their instruction set. These instructions function

in a similar manner to the hardwired master control relay;

that is, when the instruction is true, the circuit functions

normally, and when the instruction is false, nonretentive

outputs are switched off. Because these instructions are not

hardwired but programmed, for safety reasons they should

not be used as a substitute for a hardwired master control

relay, which provides emergency I/O power shutdown.

A Master Control Reset (MCR) instruction is an out-

put coil instruction that functions like a master control

9.1 Master Control Reset Instruction

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  exibility and greater ef ciency in the program

scan. Portions of the program not being utilized at any

particular time can be jumped over, and outputs in speci c

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) —Speci es 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) —Identi es the subroutine program.

Figure 9-1 Program Control menu tab.

JMP LBL JSR RET SBR TND

Program Control

MCR SUS

Ascii Control Ascii String Micro

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

MCR MCR

Master stop

Master start

CR4

CR1

L1 L2

MCR

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178 Chapter 9 Program Control Instructions

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

according 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 ig-

nores 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 counters, remain in their last state.

• 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.

relay. MCR coil instructions are used in pairs and can

be programmed to control an entire circuit or to control

only selected rungs of a circuit. In the program of Fig-

ure 9-3 , the MCR is programmed to control an entire

circuit. The operation of the program can be summarized

as follows:

• When the MCR instruction is false, or de-energized,

all nonretentive (nonlatched) rungs below 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.

• Retentive instructions should not normally be placed

within an MCR zone because the MCR zone main-

tains retentive instructions in the last active state

when the instruction goes false.

• An off-delay timer will start timing when in a de-

energized MCR zone.

Allen-Bradley SLC 500 controllers use the master

control 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

Figure 9-3 Master Control Reset (MCR) instruction.

ON/OFF

Ladder logic program

Inputs

Level switch

PL1

LS1

SOL

LS2

SOL

Stop Start

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

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Program Control Instructions Chapter 9 179

Figure 9-5 MCR fenced zone with the zone false.

End fence

Inputs

Input C

Input D

Input E

Input A

Input B

Outputs

Output A

Output B

Start fence

OFF

Input C

Input A

Ladder logic program

T4:1

1.0

TON

TIMER ON DELAY

Timer

Time base

Preset

Accumulated

Input D

Input E

Input B

Output A

Latch output B

Unlatch output B

MCR

Figure 9-4 MCR fenced zone with the zone true.

Outputs

Output A

Output B

Inputs

Input C

Input A

Ladder logic program

T4:1

1.0

TON

TIMER ON DELAY

Timer

Time base

Preset

Accumulated

Input D

Input E

Input B

Output A

Latch output B

Unlatch output B

Start fence

End fence

Active

Input A

Input B

MCR

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 correctly edit the circuit.

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180 Chapter 9 Program Control Instructions

MCR-controlled areas must contain only two MCR

instructions—one to de ne the start and one to de ne 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.

9.2 Jump Instruction

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 instructions 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. Addresses Q2:0

through Q2:255 are the addresses used for the jump

(JMP) instructions. The label (LBL) instruction is a

target for the jump instruction. In addition, the jump

instruction with its associated label must have the same

address. The area of the program that the processor

jumps over is de ned by the locations of the jump and

label instructions in the program. If the jump coil is en-

ergized, all logic between the jump and label instruc-

tions is bypassed and the processor continues scanning

after the LBL instruction.

The operation of the program can be summarized as

follows:

• When the switch is open the jump instruction is not

activated.

• 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 is not turned on.

Figure9-7 illustrates the effect on input and output in-

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:

• When rung 4 has logic continuity, the processor is

instructed to jump to rung 8 and continue to execute

the main program from that point.

• 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

should be programmed outside the jumped section

in the main program zone.

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

Q2:20. There is a single label instruction addressed

Q2:20. The scan can then jump from either jump instruc-

tion to label Q2:20, 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 between

Figure 9-6 Jump (JMP) operation.

LBL

Q2:0

JMP

Q2:0

PL3

PL2

PL1

Switch

PL2

PL1

OutputsInputs

Ladder logic program

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Program Control Instructions Chapter 9 181

the two is in how the outputs are handled when the in-

structions are executed. The MCR instruction sets all

nonretentive outputs to the false state and keeps the reten-

tive 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, instructions that

are programmed within the MCR zone starting at the

LBL instruction and ending at the end MCR instruc-

tion will always be evaluated as though the MCR zone is

true, without consideration to the state of the start MCR

instruction.

9.3 Subroutine Functions

In addition to the main ladder logic program, PLC pro-

grams may also contain additional program  les known as

subroutines. A subroutine is a short program that is used

by the main program to perform a speci c function. Large

programs are often broken into subroutine program  les,

which are called and executed from the main program. In

the SLC 500 series PLCs, the main ladder logic program

is in program  le two (shown as LAD 2). Ladder logic

programs for subroutines can be placed in  le number

three (LAD 3) through  le number 255 (LAD 255).

Figure 9-7 Effect on input and output instructions of jumped rungs.

PL1

Outputs

PL2

Inputs

TS1

LS4SOL3

SOL4

SOL3

PL2

SOL2

SOL1

SOL3

LS3

TS1

LS2

LS1

PS1

PL1

PS1

LLS1

PB3

PB2

PB1

PB1 PB2

LS4

SOL2

SOL3

SOL4

T4:6

1.0

TON

TIMER ON DELAY

Timer

Time base

Preset

Accumulated

T4:6

Heater

JMP

Q2:1

LBL

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.

T4:6

Ladder logic program

LS3

PL2

PL1

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182 Chapter 9 Program Control Instructions

Use of subroutines is a valuable tool in PLC program-

ming. At times it is better to construct programs that con-

sist of several subroutines than a lengthy single program.

When programs are written with subroutines, each sub-

routine can be tested individually for functionality. These

subroutines can then be called from the main program as

illustrated in Figure9-9 .

When a subroutine is called from the main program,

the program is able to escape from the main program

and go to a program subroutine to perform certain func-

tions 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 sub-

routine 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 is

an output instruction that causes the scan to jump to

the program  le 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  le.

Each subroutine must have a unique  le number

( decimal 3–255).

Subroutine (SBR) —The SBR instruction is the  rst

input instruction on the  rst rung in the subroutine

 le. It serves as an identi er that the program  le is

a subroutine. This  le 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  le.

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

 le 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  le. Figure9-11 shows a materials

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

Figure 9-10 Allen-Bradley subroutine-related instructions.

JSR

JUMP-TO-SUBROUTINE

SBR file number U:3

SBR

SUBROUTINE

RET

RETURN

Figure 9-8 Jump-to-label from two locations.

T4:1

1.0

Q2:5

Input A

Ladder logic program

Output A

Input B

Input C

Q2:5

Input D

Output C

Input E

Output D

Q2:5

LBL

Input F

TON

TIMER ON DELAY

Timer

Time base

Preset

Accumulated

JMP

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