Petruzella F.D. Programmable Logic Controllers

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Sequencer and Shift Register Instructions Chapter 12 263

The difference between FIFO and LIFO stack opera-

tion is that the LIFO instruction removes data in the re-

verse of the order they are loaded (last in,  rst out). An

example of the LIFO instruction pair is shown in Fig-

ure12-36 and the operation of this function can be sum-

marized as follows:

• The load and unload of the LIFO stack operates

similarly to that of the FIFO stack, except that the

last word in the LIFO stack is the  rst word that is

unloaded from the stack.

• Words can be added to the LIFO stack without dis-

turbing the words already loaded on the stack.

• Otherwise, LIFO instructions operate the same as

FIFO instructions.

• A false-to-true transition of input B causes all data

in the FIFO  le to shift one position toward the

starting address of the  le, with the data from the

starting address of the  le shifting to the destination

address, N7:11.

The FIFO instruction is often used for inventory con-

trol. One example is where different parts need to be re-

moved from inventory to be used in production. Each part

is assigned a unique code, which is loaded into a FIFO

stack, and parts are removed in the order prescribed by

the stack. This type of control ensures that the oldest part

in the inventory is used  rst as the  rst part entered is the

 rst part removed.

The opposite principle—where the last data to be

stored are the  rst to be retrieved—is known as LIFO

(Last In, First Out). The LIFO instruction inverts the order

of the data it receives by outputting the last data received

 rst and the  rst data received last. A useful analogy is

a pile of work on your desk. As new work arrives you

drop it on the top of the stack. If your stack is LIFO, you

pick your next job from the top of the pile. If your stack

is FIFO, you pick your work from the bottom of the pile.

Figure12-35 shows how the FIFO and LIFO operations

work for container stacking operations.

Figure 12-35 FIFO and LIFO container stacking

operations.

Vertical storage area

Out

LIFO stackHorizontal storage area

Out

FIFO stack

2 13

Figure 12-36 LIFO instruction pair.

LFL

LIFO LOAD

Source

LIFO

Control

Length

Position

N70:1

#N70:3

R6:61

LFU

LIFO UNLOAD

LIFO

Dest

Control

Length

Position

#N70:3

N70:2

R6:61

Word

64 words allocated for

LIFO stack at #N70:3

LIFO unload

removes data

from stack in

reverse order.

Destination N70:2

Transfer of data

N70:1 Source

LIFO load enters

data into stack at

next position.

File #N70:3

BSL BSR SQC SQL SQO FFL

File Shift

Sequencer

File/Misc Program Control

FFU LFL LFU

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264 Chapter 12 Sequencer and Shift Register Instructions

1. Describe the operation of a drum switch.

2. What type of operations are sequencers most

suitable for?

3. Why are PLC sequencers easier to program than

PLC discrete outputs?

4. Answer the following with regard to an SLC 500

PLC sequencer output instruction:

a. Where is the information for each sequencer

step entered?

b. What is the function of the output word?

c. Explain the transfer of data that occurs as the

sequencer is advanced through its various steps.

5. What is the function of the  le of a sequencer?

6. What is the function of the mask in the sequencer

instruction?

7. What is the relationship between the length and the

position in a sequencer instruction?

8. What output and step programming limits may be

placed on sequencer instructions?

9. Sequencer instructions are usually retentive.

Explain what this means.

10. Compare the operation of an event-driven and a

time-driven sequencer.

11. Explain the function of a sequencer input and com-

pare instruction.

12. What is the difference between SQI and SQC

instructions?

13. What is the purpose of using the SQI and SQO

instruction in pairs?

14. What is the primary application in which an SQL

instruction is used?

15. Explain the function of a sequencer load

instruction.

16. How does a bit shift register manipulate individual

bits?

17. List four common applications for bit shift

registers.

18. When using a sensor as the input to the bit address

of a BSL instruction, what is its function?

19. Compare the operation of the BSL and BSR bit

shift instructions.

20. A bit shift register is said to operate in a synchro-

nous manner. Explain what this means.

21. What is the function of the unload bit in a BSL

instruction?

22. What is the function of the unload bit in a BSR

instruction?

23. A  rst in,  rst out word shift register operates in an

asynchronous manner. Explain what this means.

24. Why are both FFL and FFU instructions needed to

perform a FIFO function?

25. Compare the operation a FIFO register and a LIFO

CHAPTER 12 REVIEW QUESTIONS

1. Construct an equivalent sequencer data table for

the four steps of the mechanical drum-operated

sequencer drawn in Figure12-37 .

CHAPTER 12 PROBLEMS

2. Answer the following with reference to the

sequencer  le #B3:0 shown in Figure12-38 :

a. Assume that output bit addresses O:2/0 through

O:2/15 are controlling associated output pilot

Figure 12-37 Drum-operated sequencer for Problem 1.

Steps

switch

Motor

Peg

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Sequencer and Shift Register Instructions Chapter 12 265

3. Answer each of the following with reference to

the timer-driven sequencer program shown in

Figure12-39 :

a. How many bit outputs are controlled by this

sequencer?

lights PL1 through PL16. State the status of each

light for steps 1 through 4.

b. What output bit addresses could be masked? Why?

c. State the status of each bit of output word O:2 for

step 3 of the sequencer cycle.

Figure 12-38 Sequencer ﬁ le for Problem 2.

1015 14 13 12 11 10 9 8 7 6 5 4 3 2

Start

Positions

Step 1

Step 2

Step 3

Output

O:2

B3:0

B3:1

B3:2

B3:3

Step 4 B3:4

0000000000000000

Figure 12-39 Timer-driven sequencer program for Problem 3.

Ladder logic program Outputs

O:2/0

O:2/1

O:2/2

T4:1/DN

North/South

East/West

SQO

SEQUENCER OUTPUT

File

Mask

Dest

Control

Length

Position

#N7:0

00FFh

O:2

R6:0

TON

TIMER ON DELAY

Timer

Time base

Preset

Accumulated

T4:1

1.0

SQO

SEQUENCER OUTPUT

File

Mask

Dest

Control

Length

Position

#N7:10

00FFh

T4:1.PRE

R6:0

Integer Table

Radix

Decimal

Value

N7:10 0

N7:11 25

N7:12 5

N7:13 25

N7:14 5

Integer Table

Radix

Binary

Table:

N7:Integer

15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0

N7:0/ 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

N7:1/ 0 0 0 0 0 0 0 0 0 1 0 0 0 0 0 1

N7:2/ 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 1

N7:3/ 0 0 0 0 0 0 0 0 0 0 0 1 0 1 0 0

N7:4/ 0 0 0 0 0 0 0 0 0 0 0 1 0 0 1 0

O:2/4

O:2/5

O:2/6

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266 Chapter 12 Sequencer and Shift Register Instructions

c. Assume that the sequencer is stepped to position

8; what bit outputs will be on?

d. Assume that the sequencer is at position 8 and a

true-to-false transition of one of the inputs occurs.

What happens as a result?

5. Using whatever PLC sequencer output instruction

you are most familiar with, develop a program that

will operate the cylinders in the desired sequence.

The time between each step is to be 3seconds.

The desired sequence of operation will be as

follows:

• All cylinders to retract.

• Cylinder 1 advance.

• Cylinder 1 retract and cylinder 3 advance.

• Cylinder 2 advance and cylinder 5 advance.

• Cylinder 4 advance and cylinder 2 retract.

• Cylinder 3 retract and cylinder 5 retract.

• Cylinder 6 advance and cylinder 4 retract.

• Cylinder 6 retract.

• Sequence to repeat.

6. Using whatever PLC sequencer output instruction

you are most familiar with, develop a program to im-

plement an automatic car-wash process. The process

is to be event-driven by the vehicle, which activates

b. What is the address of the word that controls the

outputs?

c. What is the address of the sequencer  le that sets

the states for the outputs?

d. What is the address of the sequencer  le that con-

tains the preset timer values?

e. For what length of time is the red light

programmed to be on?

f. For what length of time is the green light

programmed to be on?

g. For what length of time is the yellow light

programmed to be on?

h. What is the time required for one complete cycle

of the sequencer?

i. Assume that the decimal value stored in N7:13

is changed to 35. Outline the changes that this

new value will have on the timing of the traf c

lights.

4. Answer each of the following with reference to

the event-driven sequencer program shown in

Figure12-40 :

a. When does the sequencer advance to the next step?

b. Assume that the sequencer is at position 2, as

shown; what bit outputs will be on?

Figure 12-40 Event-driven sequencer program for Problem 4.

Inputs Ladder logic program

#B3:0

F0FF

O:2

R6:0

SQO

SEQUENCER OUTPUT

File

Mask

Dest

Control

Length

Position

Output

module O:2

Current

step

Pos 2

015 1110 9 8

O:2

Mask

File

Pos

#B3:0

Destination

000 00001111

1111000011111111

Pos

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Sequencer and Shift Register Instructions Chapter 12 267

• If the product is defective, reject status lights come

on at stations 1, 2, and 3 to tell the assembler to

ignore the part.

• When a defective part reaches station 4, a diverter

gate is activated to direct that part to a reject bin.

• Using whatever PLC bit shift register you are most

familiar with, develop a program to implement this

process.

various limit switches (LS1 through LS6) as it is

pulled by a conveyor chain through the car-wash

bay. Design the program to operate the car wash in

the following manner:

• The vehicle is connected to the conveyor chain and

pulled inside the car-wash bay.

• LS1 turns on the water input valve.

• LS2 turns on the soap release valve, which

mixes with the water input valve to provide a

wash spray.

• LS3 shuts off the soap valve, and the water input

valve remains on to rinse the vehicle.

• LS4 shuts off the water input valve and activates

the hot wax valve, if selected.

• LS5 shuts off the hot wax valve and starts the air-

blower motor.

• LS6 shuts off the air blower. The vehicle exits the

car wash.

7. A product moves continuously down an

assembly line that has four stations, as shown in

Figure12-41 .

• The product enters the inspection zone, where its

presence is sensed by the proximity switch.

• The inspector examines it and activates a reject

button if the product fails inspection.

Figure 12-41 Assembly line program for Problem 7.

Station

Reject

lamp

status

Product

Proximity

switch

Inspection zone

Gate

Station

Reject

lamp

status

Reject

lamp

status

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268

PLC Installation Practices,

Editing, and Troubleshooting

Chapter Objectives

After completing this chapter, you will be able to:

13.1 Outline and describe requirements for a

PLCenclosure

13.2 Identify and describe noise reduction techniques

13.3 Describe proper grounding practices and preventive

maintenance tasks associated with PLC systems

13.4 List and describe speci c PLC troubleshooting

procedures

This chapter discusses guidelines for the instal-

lation, maintenance, and troubleshooting of a

PLC-controlled system. The chapter gives you

information on proper grounding that ensures

personal safety as well as correct operation of

equipment. Unique troubleshooting procedures

that apply speciﬁ cally to PLCs are listed and

explained.

Image Used with Permission of Rockwell Automation, Inc.

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PLC Installation Practices, Editing, and Troubleshooting Chapter 13 269

provides adequate room within the enclosure is usually

suf cient for heat dissipation. The temperature inside

the enclosure must not exceed the maximum operating

temperature of the controller (typically 60°C maximum).

Additional cooling provisions, such as a fan or blower,

may be required where high internal or ambient tem-

peratures are encountered. PLCs are always mounted

horizontally with the name of the manufacturer fac-

ing out and right-side up, as illustrated in Figure 13-2 .

Vertical mounting is not recommended due to thermal

considerations.

A hardwired electromechanical master control relay

(MCR) is normally included as part of the wiring for a

13.1 PLC Enclosures

A PLC system, if installed properly, should give years of

trouble-free service. The design of PLCs includes a num-

ber of rugged features that allow them to be installed in

almost any industrial environment. However, problems

can occur if the system is not installed properly.

Programmable logic controllers (PLCs) require protec-

tion against temperature extremes, humidity, dust, shock,

and vibration or corrosive environments. For these rea-

sons, PLCs are generally mounted within a machine or in

a separate enclosure as shown in Figure 13-1 .

An enclosure is the chief protection from atmospheric

conditions. The National Electrical Manufacturers Asso-

ciation (NEMA) has de ned enclosure types, based on

the degree of protection an enclosure will provide. For

most solid-state control devices, a NEMA 12 enclosure

is recommended. This type of enclosure is for general-

purpose areas and is designed to be dust-tight. Typically,

metal enclosures are used because metal enclosures pro-

vide shielding that helps minimize the effects of electro-

magnetic radiation that may be generated by surrounding

equipment.

Every PLC installation will dissipate heat from its

power supplies, local I/O racks, and processor. This heat

accumulates in the enclosure and must be dissipated

from it into the surrounding air. Excessive heat can

cause erratic operation of the PLC or PLC failure. For

many applications, normal convection cooling will keep

the controller components within the speci ed tempera-

ture operating range. Proper spacing of components that

Figure 13-2 PLCs are always mounted horizontally.

Source: Courtesy Rogers Machinery Company, Inc.

10.

2. 3.

10.

Power supply

PLC (programmable logic controller)

Digital input cards

Digital output cards

Analog input cards

Transient surge protectors

Circuit breakers

Relay switches

Operator interface terminal

NEMA 12 enclosure

Figure 13-1 Typical PLC control panel enclosure.

Source: Courtesy Aaron Associates.

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270 Chapter 13 PLC Installation Practices, Editing, and Troubleshooting

MCR is connected to interrupt power to the I/O rack in the

event of an emergency, but still allow power to be main-

tained at the processor. Figure 13-3 shows the typical wir-

ing for an AC power distribution with a master control

PLC system. The master control relay provides a means

of de-energizing the entire circuit that is not dependent on

software. The internally programmed MCR of a PLC is

not suf cient to meet safety requirements. The hardwired

Figure 13-3 Typical wiring for an AC power distribution with a master

control relay.

Source: Courtesy Pilz GmbH & Co. KG.

To V  of DC

input devices

DC power

supply

To V  of DC

output modules

To common

of DC input

modules

To common

of DC output

devices

To common (L1)

of 120 v AC

input devices

MCR

Multiple E-stops

Start

Fuse

Panel

ground bus

Grounding

electrode

120 V

Neutral

Disconnect switch

3-phase

supply

Step-down

transformer

Gnd

MCR

To L1

of 120 v AC

output modules

To neutral conn.

of 120 v AC

input modules

To neutral

of 120 v AC

output devices

L1 N

Gnd

DC commonV

Processor power supply

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PLC Installation Practices, Editing, and Troubleshooting Chapter 13 271

• Proper routing of wiring

• Proper suppression added to noise-generating devices

Noise suppression is normally needed for inductive

loads such as relays, solenoids, and motor starters when

operated by hard contact devices such as pushbuttons or

selector switches. When inductive loads are switched off,

high transient voltages are generated that if not suppressed

can reach several thousand volts. Figure 13-4 illustrates a

typical noise suppression circuit that is used to suppress

the high voltage spikes generated when a motor starter

coil is de-energized.

Lack of surge suppression on inductive loads may con-

tribute to processor faults and sporadic operation. RAM

can be corrupted (lost), and I/O modules can appear faulty

or can reset themselves. When inductive devices are ener-

gized or de-energized, they can cause an electrical pulse

to be back-fed into the PLC system. The back-fed pulse,

when entering the PLC system, can be mistaken by the

PLC for a computer pulse. It takes only one false pulse

to create a malfunction of the orderly  ow of PLC opera-

tional sequences.

Proper routing of  eld power and signal wiring to the

PLC enclosure as well as inside the enclosure helps to cut

down on electrical noise. The following are some general

guidelines for PLC wire routing:

• Use the shortest possible wire runs for I/O signals.

• When possible, conductors that are run from the

PLC enclosure to another location should be in a

metal conduit as the metal can serve as a shield

against EMI.

• Never run signal wiring and power wiring in the

same conduit.

• Segregate I/O wiring by signal type. Route AC and

DC I/O signal wires in separate wireways.

relay. The operation of the circuit can be summarized as

follows:

• A power disconnect switch is provided so that,

when required, the PLC can be serviced with the

power off.

• The step-down transformer provides isolation

from the main power distribution system and

decreases the voltage to the 120 volts required

for the controller power supplies and DC power

supplies.

• The momentary start button is pressed to energize

the master control relay.

• Pressing any one of the emergency-stop switches

de-energizes the master control relay and thus de-

energizes the I/O devices.

• Power to the processor of the PLC remains on so

status LEDs can continue to provide up-to-date

information.

• Emergency stop buttons use normally closed

contacts wired in series for fail-safe operation.

Inthe event a wire is broken or comes off a termi-

nal, the MCR relay is de-energized and power is

removed.

13.2 Electrical Noise

Electrical noise, also called electromagnetic interference,

or EMI, is unwanted electrical signals that produce unde-

sirable effects and otherwise disrupt the control system

circuits. EMI may be either radiated or conducted. Radi-

ated noise originates from a source and travels through

the air while conducted noise travels on an actual conduc-

tor, such as a power line.

When the PLC is operated in a noise-polluted indus-

trial environment, special consideration should be given

to possible electrical interference. To increase the operat-

ing noise margin, the controller should be located away

from noise-generating devices such as large AC motors

and high-frequency welders. Malfunctions resulting from

noise are temporary occurrences of operating errors that

can result in hazardous machine operation in certain ap-

plications. Noise usually enters through input, output,

and power supply lines. Noise may be coupled into these

lines by an electrostatic  eld or through electromagnetic

induction. The following reduce the effect of electrical

interference:

• Manufacturer design features

• Proper mounting of the controller within an

enclosure

• Proper equipment grounding

Figure 13-4 Motor starter noise suppression.

Source: Image Used with Permission of Rockwell Automation, Inc.

Noise

suppressor

Stop

L1 L2

Starter

coil

Start

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272 Chapter 13 PLC Installation Practices, Editing, and Troubleshooting

• Low-level signal conductors such as thermocou-

ples and serial communications should be run as

shielded twisted pair and routed separately.

• A  ber optic system, which is totally immune to all

kinds of electrical interference, can also be used for

signal wiring.

An important part of a PLC installation is clearly iden-

tifying each wire to be connected and the terminal to

which it is connected. A reliable labeling method, such

as the heat-shrinkable wire identi cation sleeves shown

in Figure 13-5 , should be used to label each wire. Wir-

ing connectors for input/output modules usually includes

spaces for labels used for identifying each I/O address and

device connected. Proper wire and terminal identi cation

will simplify the installation and aid in troubleshooting

and maintenance.

13.3 Leaky Inputs and Outputs

Many electronic devices with transistor or triac outputs

exhibit a small leakage current even when in the off state

that may need to be considered when they are connected

to PLC input modules. This so-called leakage is typi-

cally exhibited by two-wire proximity, photoelectric, and

other such sensors. Often, the leaky input will only cause

the module’s input indicator to  icker. However, a large

enough leakage current can activate the input circuit, cre-

ating a false input signal.

A common solution to the problem of leaky input cur-

rent is to connect a bleeder resistor across or in parallel

with the input, as shown in Figure 13-6 . The bleeder re-

sistor acts as an additional lower resistance load, which

allows the leakage current to  ow through the lower resis-

tance path. Typically a 10 kΩ to 20 kΩ resistor is used to

solve the problem.

Leakage current may also occur with the solid-state

switch used in many output modules. Problems similar to

that encountered with input modules can be created when

a high-impedance load device is used with these modules.

For example, a PLC output might supply a sound alert

device as illustrated in Figure 13-7 . In this case the leak-

age current could cause continuous false or intermittent

operation. A resistor can be connected as shown to bleed

off this current. An isolation relay could also be used to

solve this type of problem.

13.4 Grounding

Proper grounding is an important safety measure in

all electrical installations. The authoritative source on

grounding requirements for a PLC installation is the

National Electrical Code. The NEC specifies the types

Figure 13-5 Heat-shrinkable wire identiﬁ cation sleeves.

Source: Photo courtesy Tyco Electronics, www.tycoelectronics.com.

Figure 13-6 Bleeder resistor connection for input sensors.

Bleeder

resistor

Leakage

current

Input module

2-wire

proximity

switch

Common

Figure 13-7 Bleeder resistor connection for a high-

impedance output.

Leakage

current

Bleeder

resistor

Output

module

High-

impedance

load

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