Petruzella F.D. Programmable Logic Controllers

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Program Control Instructions Chapter 9 183

(a)

Solenoid

Weight

sensor

Pilot light

Figure 9-11 Flashing pilot light subroutine. (a) Process. (b) Program.

OFF/ON

Sensor

SBR

SUBROUTINE

Main program

file 2

Subroutine

file 3

(b)

Inputs

Stop

Outputs

PL1

OFF/ON

SOL

Motor

T4:1/EN

T4:0/DN

T4:1/DN

JSR

JUMP-TO-SUBROUTINE

SBR file number

T4:0

1.0

TON

TIMER ON DELAY

Timer

Time base

Preset

Accumulated

T4:1

1.0

TON

TIMER ON DELAY

Timer

Time base

Preset

Accumulated

RET

RETURN

U:3

Sensor

PL1

Start

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184 Chapter 9 Program Control Instructions

conveyor system with a  ashing 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  ashing.

• 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  ashing.

• 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  le 2 whereas subroutines are as-

signed to program  le numbers 3 to 255. Each subroutine

must be programmed in its own program  le by assigning

it a unique  le 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  le for each location. Each sub-

routine  le should begin with an SBR instruction.

• At each ladder location where a subroutine is called,

program a JSR instruction specifying the subroutine

 le number.

• The RET instruction is optional.

– The end of a subroutine program will cause a re-

turn to the main program.

– If you want to end a subroutine program before it

executes to the end of program  le, a conditional

return (RET) instruction may be used.

An optional SBR instruction is the header instruction

that stores incoming parameters. This feature lets you

pass selected values to a subroutine before execution

so the subroutine can perform mathematical or logi-

cal operations on the data and return the results to the

main program. For example, the program shown in Fig-

ure9-13 will cause the scan to jump from the main pro-

gram  le to program  le 4 when input A is true. When

the scan jumps to program  le 4, data will also be passed

from N7:30 to N7:40. When the scan returns to the main

program from program  le 4, data will be passed from

N7:50 to N7:60.

Nesting subroutines allows you to direct program  ow

from the main program to a subroutine and then to another

subroutine, as illustrated in Figure9-14 . 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.

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.

9.4 Immediate Input and Immediate

Output Instructions

The immediate input and immediate output instructions

interrupt the normal program scan to update the input

image table  le with current input data or to update an

output module group with the current output image table

 le data. These instructions are intended to be used only

for time-critical I/O data.

The immediate input (IIN) Allen-Bradley PLC-5 in-

struction is used to read an input condition before the I/O

update is performed. This operation interrupts the pro-

gram scan when it is executed. After the immediate input

instruction is executed, normal program scan resumes.

This instruction is used with critical input devices that re-

quire updating in advance of the I/O scan.

Figure 9-12 Setting up a subroutine ﬁ le.

JSR

JUMP-TO-SUBROUTINE

SBR file number 3

Main program

file 2

SBR

SUBROUTINE

RET

RETURN

Subroutine

file 3

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Program Control Instructions Chapter 9 185

Figure 9-13 Passing subroutine parameters.

RETURN ( )

Return parameter N7:50

RET

JSR

Input A

Execution resumes

JUMP-TO-SUBROUTINE

Program file 4

Input parameter N7:30

Return parameter N7:60

SUBROUTINE

Input parameter N7:40

Subroutine

file 4

Main ladder program

Return parameter returned to the

address that you specified in the

JSR instruction when execution

returns to the main ladder program.

SBR

Input parameter passed to the

SBR instruction when execution

jumps to the subroutine file.

Figure 9-14 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

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186 Chapter 9 Program Control Instructions

The operation of the immediate input instruction is il-

lustrated in Figure9-15 . When the program scan reaches

the immediate input instruction, the scan is interrupted

and the bits of the addressed word are updated. The

immediate input is most useful if the instruction asso-

ciated with the critical input device is at the middle or

toward the end of the program. The immediate input is

not needed near the beginning of the program since the

I/O scan has just occurred at that time. Although the im-

mediate input instruction speeds the updating of bits, its

scan-time interruption increases the total scan time of the

program. The operation of the program can be summa-

rized as follows:

• When the scan reaches a true IIN instruction, the

scan is interrupted.

• The processor updates 16 bits in the input

image table at the location indicated on the IIN

instruction.

• The two-digit address on the IIN instruction is com-

posed of the rack number ( rst digit) and the I/O

group number (second digit) containing the input or

inputs and needs immediate updating.

The immediate output (IOT) Allen-Bradley PLC-5 in-

struction is a special version of the output energize in-

struction used to update the status of an output device

before the I/O update is performed. The immediate output

is used with critical output devices that require updating in

advance of the I/O scan. When the program scan reaches

the immediate output instruction, the scan is interrupted

and the bits of the addressed word are updated. The opera-

tion of the immediate output instruction is illustrated in

Figure9-16 and can be summarized as follows:

• When the program scan reaches a true IOT instruc-

tion, the scan is interrupted and the data in the

output image table at the word address on the in-

struction are transferred to the real-world outputs.

• In this example, the IOT instruction follows the out-

put energize instruction.

• Thus, the output image table word is updated  rst,

and then the data are transferred to the real-world

outputs.

Figure 9-15 Immediate input instruction.

Ι/O scan

Program scan

Immediate input instruction

interrupts program scan

and examines bits in word

I:012 here in program

Returns to

program

scan

Word I:012

Module

group

(input)

Rack 1

IIN

Immediate output instruction

interrupts program scan

Word O:013

Returns to

program

scan

Module

group

(output)

Rack 1

IIN

IOT

O:013

I:012

I/O scan

Program scan

Figure 9-16 Immediate output instruction.

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Program Control Instructions Chapter 9 187

• This allows the programmer to update only sections

of the inputs to be used throughout the rest of the

program.

The immediate output with mask (IOM) instruction is

shown in Figure 9-18 . The IOM operates on the physi-

cal outputs assigned to a particular word of a slot. When

the IOM rung is true, the program scan is interrupted to

update output data to the module located in the slot speci-

 ed in the instruction. These data are then available to the

commands in the ladder following the IOM instruction.

The parameters entered are basically the same as those

entered for the IIM instruction.

Processor communication with the local chassis is

many times faster than communication with the remote

chassis. This is due to the fact that local I/O scan is

synchronous with the program scan and communication

is in parallel with the processor, whereas the remote

I/O scan is asynchronous with the program scan and

communication with remote I/O is serial. For this rea-

son, fast-acting devices should be wired into the local

chassis.

9.5 Forcing External I/O Addresses

The force function is essentially a manual override con-

trol function. Forcing allows the PLC user to turn an ex-

ternal input or output on or off from the keyboard of the

programming device. This is accomplished regardless of

the actual state of the  eld device. The forcing capability

allows a machine or process to continue operation until a

faulty  eld device can be repaired. It is also valuable dur-

ing start-up and troubleshooting of a machine or process

to simulate the action of portions of the program that have

not yet been implemented.

Forcing inputs manipulates the input image table  le

bits and thus affects all areas of the program that use those

bits. The forcing of inputs is done just after the input scan.

When we force an input address, we are forcing the sta-

tus bit of the instruction at the I/O address to an on or

The Allen-Bradley SLC 500 PLC’s immediate I/O

instructions contain a few improvements over those of

the PLC-5. The SLC 500’s instructions, which are called

immediate input with mask (IIM) and immediate out-

put with mask (IOM), allow the programmer to specify

which of the 16 bits are to be copied from an input mod-

ule 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. In addition, the SLC 500 instructions

allow you to input or output a series of data words from

a single input module or output a series of data words to

an output module.

The immediate input with mask (IIM) instruction is

shown in Figure9-17 . 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 speci c input slot are transferred through

the mask to the input data  le. These data are then avail-

able to the commands in the ladder following the IIM

instruction. The following parameters are entered in the

instruction:

Slot Speci es the slot and word that contain the data

to be updated. For example, I:3.0 means the input of

slot 3, word 0.

Mask Speci es either a hex constant or a register

address. For the mask, a 1 in the bit position passes

data from the source to the destination. A 0 inhibits

or blocks bits from passing from the source to the

destination.

Length Used to transfer more than one word per

slot.

The program operation of the instruction is summarized

as follows:

• The IIM instruction retrieves data from I:1.0 and

passes it through the mask.

• The mask permits only the four least signi cant bits

to be moved to the input register I:1.0.

Figure 9-17 Immediate input with mask (IIM) instruction.

IIM IOM SYC MSG IIE IID

Input/Output

Compare

Compute/Math

RPI REF

Move/Logic

IIM

Immediate Input w/ Mask

Slot

Mask

Length

I:1.0

000Fh

Figure 9-18 Immediate output with mask (IOM) instruction.

IIM IOM SYC MSG IIE IID

Input/Output

Compare

Compute/Math

RPI REF

Move/Logic

IOM

Immediate Output w/ Mask

Slot

Mask

Length

O:4.0

0FFFh

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188 Chapter 9 Program Control Instructions

bit of the output instruction at the address is usually not

affected. Figure 9-20 illustrates how an output is forced

on. The operation of the program can be summarized as

follows:

• The processor ignores the actual state of solenoid

output O:2/5.

• The programming device sets the force state in the

output force data  le and the PLC implements the

force to turn solenoid output O:2/5 on even though

the output image table  le indicates that the user

logic is setting the point to off.

• M output O:2/6 remains off because the status

bit of output O:2/5 is not affected by the force

instruction.

• Not all brands of PLCs operate this way. For ex-

ample, forcing an output with a GE Fanuc controller

will cause the contacts that have the same address as

the output to also change to the appropriate state.

off state. Figure 9-19 illustrates how an input is forced

on. The operation of the program can be summarized as

follows:

• The processor ignores the actual state of input limit

switch I:1/3.

• Although limit switch I:1/3 is off (0 or false) the

processor considers it as being in the on (1 or true)

state.

• The program scan records this, and the program is

executed with this forced status.

• In other words, the program is executed as if the

limit switch were actually closed.

Forcing outputs affects only the addressed output ter-

minal. Therefore, since the output image table  le bits are

unaffected, your program will be unaffected. The forcing

of outputs is done just before the output image table  le is

updated. When we force an output address, we are forcing

only the output terminal to an on or off state. The status

OFF

Input image table

Input module

I:1/3

Field input

device

Programming terminal

forces the state of

input I:1/3 ON (1)

O:2/5

O:2/5 O:2/6

Ladder logic program

I:1/3

OutputsInput

OFF

Force> ON

O:2/5

O:2/6

I:1/3

Ladder

logic

program

Forces Exist

Forces Enabled

ONLINE

Actual state of input

device (0) ignored

Figure 9-19 Forcing an input on.

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Program Control Instructions Chapter 9 189

 le online. With RSLogix 500 software, the steps are as

follows:

1. Open the program  le in which you want to force

the logic on or off.

Overriding of physical inputs on conventional relay

control systems can be accomplished by installing

hardwire jumpers. With PLC control hardwire jumpers

are not necessary because the input data table values

can be forced to an on or off state. The force func-

tion allows you to override the actual status of external

input circuits by forcing external data bits on or off.

Similarly, you can override the processor logic and

status of output data file bits by forcing output bits

on or off. By forcing outputs off, you can prevent the

controller from energizing those outputs even though

the ladder logic, which normally controls them, may

be true. In other instances, outputs may be forced on

even though logic for the rungs controlling those out-

puts may be false.

Figure9-21 shows the forces version of the data table

with bit I:1/3 forced on. You can enter and enable or

disable forces while you are monitoring your  le off-

line, or in any processor mode while monitoring your

Figure 9-20 Forcing an output on.

O:2/5

O:2/6

Ladder logic program

I:1/3

OutputsInput

OFF

Force> ON

O:2/5

O:2/6

I:1/3

Ladder

logic

program

Forces Exist

Forces Enabled

ONLINE

Output image table

Status of bit

O:2/5 remains

at 0

Field output

devices

O:2/5

O:2/6

OFF

Output module

Figure 9-21 Forces version of the data table with bit I:1/3

forced on.

Offset 15

I:1.0

I:2.0

Data File I1 (bin) . . INPUT Forces

Radix:

Columns:

Symbol:

Desc:

Enable

Remove All

Data File

Help

I:1.0/3

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190 Chapter 9 Program Control Instructions

example, if maintenance personnel are performing rou-

tine maintenance on a de-energized motor, the machine

may suddenly become energized by someone forcing the

motor to turn on. This is why a hardwired master control

circuit is required for the I/O rack. The hardwired circuit

will provide a method of physically removing power to

the I/O system, thereby ensuring that it is impossible to

energize any inputs or outputs when the master control

is off.

9.6 Safety Circuitry

Suf cient emergency circuits must be provided to stop ei-

ther partially or totally the operation of the controller or

the controlled machine or process. These circuits should

be hardwired outside the controller so that in the event of

total controller failure, independent and rapid shutdown

is available.

Figure9-23 shows typical safety wiring requirements

for a PLC installation. The safety requirements of this in-

stallation can be summarized as follows:

• A main disconnect switch is installed on the

incoming power lines as a means of removing

power from the entire programmable controller

system.

• The main power disconnect switch should be lo-

cated where operators and maintenance personnel

have quick and easy access to it. Ideally, the discon-

nect switch is mounted on the outside of the PLC

enclosure so that it can be accessed without opening

the enclosure.

• In addition to disconnecting electrical power, you

should de-energize, lock out, and tag all other

sources of power (pneumatic and hydraulic) before

you work on a machine or process controlled by the

controller.

• An isolation transformer is used to isolate the con-

troller from the main power distribution system and

step the voltage down to 120 VAC.

• A hardwired master control relay is included to pro-

vide a convenient means for emergency controller

shutdown. Because the master control relay allows

the placement of several emergency-stop switches

in different locations, its installation is important

from a safety standpoint.

• Overtravel limit switches or mushroom head emer-

gency stop pushbuttons are wired in series so that

when one of them opens, the master control is

de-energized.

• This removes power to input and output device

circuits. Power continues to be supplied to the

2. With the right mouse button, click the I/O bit you

want to force.

3. From the menu that appears, select Go to Data Table

or select Force On or Force Off.

4. From the associated data table that appears, click on

the Forces button.

5. The Forces version of the data table appears with

the selected bit highlighted. Click on this bit with

the right mouse button.

6. From the menu that appears, you can force the se-

lected bit on or off.

Exercise care when you use forcing functions. If

used incorrectly, force functions can cause injuries

to persons working around a system, and/or equip-

ment damage. For this reason, forcing functions should

be used only by personnel who completely understand

the circuit and the process machinery or driven equip-

ment ( Figure9-22 ). You must understand the potential

effect that forcing given inputs or outputs will have on

machine operation in order to avoid possible personal

injury and equipment damage. Before using a force

function, check whether the force acts on the I/O point

only or whether it acts on the user logic as well as on the

I/O point. Most programming terminals and PLC CPUs

provide some visible means of alerting the user that a

force is in effect.

In situations in which rotating equipment is involved,

the force instruction can be extremely dangerous. For

Figure 9-22 Exercise care when you use forcing functions.

Source: Courtesy Givens Engineering Inc.

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Program Control Instructions Chapter 9 191

electromechanical component must not be dependent on

electronic components (hardware or software). Any part

can fail, including the switches in a master control relay

circuit. The failure of one of these switches would most

likely cause an open circuit, which would be a safe power-

off failure. However, if one of these switches shorts out, it

no longer provides any safety protection. These switches

should be tested periodically to ensure that they will stop

machine motion when needed. Never alter these circuits

to defeat their function. Serious injury or machine dam-

age could result.

controller power supply so that any diagnostic

indicators on the processor module can still be

observed.

• Note that the master control relay is not a substitute

for a disconnect switch. When you are replacing

any module, replacing output fuses, or working on

equipment, the main disconnect switch should be

pulled and locked out.

The master control relay must be able to inhibit all

machine motion by removing power to the machine I/O

devices when the relay is de-energized. This hardwired

Power

mains

Main disconnect switch

Step-down

isolation transformer

Fuse

120

VAC

Emergency stop switches

Emergency

stop

Overtravel

limit

switch

Stop Start

MCR

Machine

start/stop

buttons

Master

control

relay

PLC Control Panel

PLC power

supply

GND

MCR

L1 2

MCR

L1 L2

PLC output module

PLC input module

L287654321L1

Figure 9-23 Safety wiring requirements for a PLC installation.

Source: Courtesy Minarik Automation & Control.

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192 Chapter 9 Program Control Instructions

• Safety PLCs use power supplies designed speci -

cally for use in safety control systems and redun-

dant backplane circuitry between the controller and

I/O modules.

Safety considerations should be developed as part

of the PLC program. A PLC program for any applica-

tion will be only as safe as the time and thought spent

on both personnel and hardware considerations make

it. One such consideration involves the use of a motor

starter auxiliary seal-in contact, shown in Figure9-25 ,

in place of the programmed contact referenced to the

output coil instruction. The use of the  eld-generated

starter auxiliary contact status in the program is more

costly in terms of  eld wiring and hardware, but it is

safer because it provides positive feedback to the pro-

cessor about the exact status of the motor. Assume,

for example, that the OL contact of the starter opens

under an overload condition. The motor, of course,

would stop operating because power would be lost to

the starter coil. If the program was written using an

examine-on contact instruction referenced to the out-

put coil instruction as the seal-in for the circuit, the

processor would never know that power had been lost

to the motor. When the OL was reset, the motor would

restart instantly, creating a potentially unsafe operating

condition.

Another safety consideration concerns the wiring

of stop buttons. A stop button is generally considered

a safety function as well as an operating function. As

such, all stop buttons should be wired using a nor-

mally closed contact programmed to examine for an on

Safety PLCs, such as the one shown in Figure9-24 ,

are now available for applications that require more ad-

vanced safety functionality. A safety PLC is typically

certi ed by third parties to meet rigid safety and reliabil-

ity requirements of international standards. Both stan-

dard and safety PLCs have the ability to perform control

functions but a standard PLC was not initially designed

to be fault tolerant and fail-safe. That is the fundamental

difference.

Some of the differences between standard and safety

PLCs include the following:

• A standard PLC has one microprocessor that

executes the program, Flash memory area that

stores the program, RAM for making calcula-

tions, ports for communications, and I/O for

detection and control of the machine. In contrast,

a safety PLC has redundant microprocessors,

Flash and RAM that are continuously moni-

tored by a watchdog circuit, and a synchronous

detection circuit. Redundancy is duplication. The

probability of hazards arising from one malfunc-

tion in an electrical circuit can be minimized

by creating partial or complete redundancy

(duplication).

• Standard PLC inputs provide no internal means for

testing the functionality of the input circuitry. By

contrast, safety PLCs have an internal output circuit

associated with each input for the purpose of testing

the input circuitry. Inputs are driven both high and

low for very short cycles during runtime to verify

their functionality.

71246

Number Feature

1 Module status indicators

2 Alphanumeric display

3 Node address switches

4 Baud rate switches

5 USB port

6 DeviceNet communication connector

7 Terminal connectors

8 Input status indicators

9 Output status indicators

10 IP address desplay switch

11 Ethernet connector

12 Service switch

Figure 9-24 Safety PLC.

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

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