Introduction to Industrial Automation

ings, (b) timing diagram of the overall operation.

PLC

Inputs

Outputs

0 V

+24 V DC 

0 V

+24 V DC

I0.0

Q2.0

I0.1

I0.2

Q2.3

Q2.6

C

1

C

2

C

3

I0.3

I0.4

I0.5

STOP 1

START 1

STOP 2

START 2

STOP 3

START 3

Figure 7.56  Connection status of I/O devices in a PLC for START-STOP operation of three motors.

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Basic Programming Principles of PLCs 

◾

 

325

(C

1

, C

2

, and C

3

) with their corresponding START-STOP control buttons. Because START-STOP 

automation is exactly the same for the three machines, a simple FC can be used instead of a single 

unified program. The first step is the FC programming that includes the declaration of variables, 

the FC naming, the introduction of comments, and the automation program implemented by the 

FC. These actions will have the result that follows:

Program

A(

O

SR T

O

MRT

)

A

ST P

=

MRT

BE

Variable declaration

Name

Declaration

Type

Comment

SR T

INPUT

Boole

Starting motor

ST P

INPUT

Boole

Stopping motor

MRT

OUTPUT

Boole

Power relay of motor

FC3

The second step is to write the general program that will call the FC3 in series for the three 

machines, and which will have the following form:

CAL L

FC 3

SR T:= I0.0

ST P:=

I0.1

MRT: = Q2.0

CAL L

FC 3

SR T:= I0.2

ST P:=

I0.3

MRT: = Q2.3

CAL L FC 3

SR T:= I0.4

ST P:= I0.5

MRT: = Q2.6

BE

Main program

Of course, the number of instructions included in the FC3 is small due to the simplicity of 

the example. The use of an FC is greatly facilitated when the repetitive operation requires a large 

number of instructions and repetitions. For example, let’s assume that FC3 has 80 instructions 

and 25 motors. Then the number of the required instructions and thus the size of a single unified 

program without using FCs can be easily imagined.

Both the main program and the FC3, as well as any other POU, are programmed, as expected, in 

the PLC software environment that will be utilized. Since the main program and FC3 are declared, 

programmed, and stored, the software environment is the one that will establish their interface without 

the need for any additional programming action by the user. In Figure 7.57, the I/O devices of two 

motors with inversion of rotation (from the overall 10 available, whose operation will be programmed 

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326

 

◾

  Introduction to Industrial Automation

using an FC) are shown. The same figure shows the FC1 that implements the inversion function of 

motors as a subroutine, the main program, and the instruction calls among them.

The programming of an FB that is directly linked to one or more data blocks is also addressed 

in a similar way. Figure 7.58 shows a production process consisting of three conveyor belts (M

1

, M

2

, 

and M

3

) for transferring different objects, three processing machines (W

1

, W

2

, and W

3

), a central 

conveyor belt (M

4

) and a finishing, assembly, and packaging station (S). The four conveyor belts 

have a similar function (e.g., ON, OFF, counting, etc.), but because they carry different objects, 

certain technical parameters and data are different for each conveyor. This may be, for example, 

the transport speed, the number limits of objects that each conveyor transfers, etc. Therefore, the 

operation of the conveyor belts can be controlled by a single FB, which will be associated with four 

data blocks (DB1,..., DB4) in which the corresponding parameter values for each conveyor belt will 

be stored. The processing machines W

1

, W

2

, and W

3

 have a completely similar function and the 

only element in which they differ also belongs in the START-STOP logic function. For example, if 

W

1

, W

2

, and W

3

 are coloring machines, only the paint color they apply can vary, and therefore the 

output that will be activated in each one. It is understood that each machine can paint with three 

different colors. If these are labeling machines, they can only differ in the type of label and so on; 

many similar examples can be assumed. For this reason, the processing machines W

1

, W

2

, and W

3

 

...

...

Outputs

...

...

PLC

Inputs

0 V

+24 V DC

0 V

+24 V DC

I 1.0

I 1.4

C

1L

Q 0.4

I 1.2

I 2.0

I 2.2

Q 0.0

STOP 1

STOP 2

I 1.6

C

1R

C

2L

C

2R

1

st

 call

2nd ca

ll

10th 

cal

l

Q 0.2

Q 0.6

START 1

Left

START 1

Right

START 2

Left

START 2

Right

FC1

Variable declaration

Name

Type

Declaration

Comment

STARTL

Input

Bool

Start left

STARTW

Input

Bool

Start right

STOP

Input

Bool

Stop operation

OUTPUTL

Output

Bool

OUTPUTW Output

Bool

Right rotation relay

Program

Α

STARTL

AN STARTW

S

OUTPUTL

R

OUTPUTW

A

STARTW

AN STARTL

S

OUTPUTW

R

OUTPUTL

A

STOP

R

OUTPUTL

R

OUTPUTW

END FUNCTION

Main program

CALL  FC1

STARTL:=

I1.0

STARTW:=

I1.2

STOP:=

I1.4

OUTPUTL:=

Q0.0

OUTPUTW:=

Q0.2

CALL FC1

STARTL:=

I1.6

STARTW:=

I2.0

STOP:=

I2.2

OUTPUTL:=

Q0.4

OUTPUTW:=

Q0.6

(Next calls of FC1 for the

operation of other motors) 

ΒΕ

Left rotation relay