Introduction to Industrial Automation

Problems

  7.1. Write a program in LAD language so that a machine can operate or stop permanently via 

two push buttons START and STOP respectively, but also to operate momentary via a third 

push button JOG (i.e., to operate as long as the JOG button is pressed).

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  Introduction to Industrial Automation

  7.2. In programmable automation, it is possible to use an NO or NC button for stopping the 

operation of a machine. Suppose that, during machine operation, the wire connecting the 

STOP button to the power source is broken due to a fault action as shown in the figure. In 

such a case, which of the two configurations is the most appropriate and why?

PLC

Inputs

Outputs

0 V

C

+24 V DC

0 V

+24 V DC

Q0.0

I0.0

STOP

PLC

Inputs

Outputs

0 V

C

+24 V DC

0 V

+24 V DC

Q0.0

I0.0

STOP

 7.3. An On-Delay timer can be programmed to be self-activated when a measured time is 

elapsed. Then, the timer output (T2) will present the response shown in the figure. Combine 

the timer operation with a digital output (Q0.0) in order to have an alternation (ON-OFF) 

of Q0.0 with time period double of that presented by the T2 pulse train. Write the required 

program in LAD language.

Q0.0

One scan

cycle 

T

2

T

2

T

T

  7.4. A digital input (I2.0) of a PLC is a pulse train with a frequency T. Write a program in LAD 

language so that a digital output (Q4.0) has a pulse train with frequency 2T, as shown in the 

figure. 

Q4.0

T

2T

I2.0

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Basic Programming Principles of PLCs 

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349

  7.5. Translate the conventional automation circuit, shown in the left-hand figure, in a LAD 

program for the I/O connections in PLC shown also in the right-hand figure.

R

N

PLC

Inputs

Outputs

0 V

C

1

+24 V DC

0 V

+24 V DC

Q0.0

I1.0

I1.4

S

1

S

3

C

2

Q0.4

S

2

I1.2

C

1

C

2

C

1

S

1

S

2

S

3

  7.6.  The example for counting and arithmetic operations described in Section 7.5.6 was addressed 

using a Boolean program. Program the same automation logic in LAD language.

  7.7. The arithmetic value stored in memory word MW200 of a PLC expresses a temperature 

measurement in 

°

C. Write a program to convert this value in 

°

F and store it in a different 

memory word.

  7.8. The ventilation of a road tunnel is achieved with the continuous operation of four ventilators. 

In the control room of the tunnel, there is suitable signaling provided by a PLC so that the 

responsible operator is informed of the ventilation status according to the following plan:

 

a.  When three or four ventilators are in operation, the green traffic light is ON and the cars 

freely enter the tunnel.

 

b.  When only two ventilators are in operation, the yellow traffic light is flashing (green 

light is OFF) and the operator has to be on standby to take necessary action.

 

c.  When only one ventilator is in operation or all have stopped, the red traffic light is ON 

and the operator has to intervene by limiting the traffic outside the tunnel.

 

  Detection of whether or not ventilators are operating is achieved by four corresponding 

sensors which are digital inputs to the PLC. After defining the kind of input contacts 

in relation to ventilators’ operation and their I/O addresses, write the required program 

that has the minimum number of instructions in any language you wish.

  7.9. Complete the RLO which is created by the CPU of the PLC during the execution of the 

instructions of the program in the empty rectangles (dash line) shown in the figure at the first 

scan cycle and at the first scan cycle after the time T2 (60 sec) has elapsed. The input devices 

have the statuses shown in the PLC figure.

0 V

+24 V DC

0 V

S

2

C

1

S

1

PLC

Inputs

Outputs

+24 V DC

I1.0

Q7.0

I1.1

1

1

1

1

I1.0

Μ1.1

Q7.0

Ι1.1

Q7.0

Q7.0

I1.1

M2.0

M6.0

M2.0

60 s

T2

ON delay

1

T2

M6.0

M3.0

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