Introduction to Industrial Automation

Figure 6.55  Measurement of the voltage that an input device must supply in the corresponding 

digital input of the PLC.

PLC

INPUTS

OUTPUTS

0 V

C

1

C

2

C

3

+24 V DC 

0 V

+24 V DC 

Q 0.0

I 1.0

I 1.4

S

1

S

2

S

3

Q 0.2

Q 0. 4

I 1.2

V

PLC

Inputs

Outputs

Q 0.0

I 1.0

I 1.4

Q 0.2

Q 0. 4

I 1.2

V

Figure 6.56  Measurement of the voltage of a digital output with a connected load.

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  Introduction to Industrial Automation

Review Questions and Problems

  6.1.  Explain how the status of a digital input signal 230 V is stored in the memory of a PLC.

  6.2. What is the so-called “scanning cycle” of a PLC? Is the time duration of scanning cycle a 

constant or variable parameter and why? 

  6.3. Describe the advantages, if there are any, of using a modular PLC instead of a compact PLC 

in an industrial application.

  6.4. One of the PLC’s advantages is the easy detection of equipment faults. The figure shows the 

conventional automation circuit of a pump (C2) which has been translated in a program and 

runs in a PLC. Suppose that the float level switch has been damaged and its contact remains 

open, although the liquid level in the tank is above the float switch. Explain why the detec-

tion of this fault will be easier in the case of programmable automation.

C

1

C

2

C

3

C

1

Float

Float

switch

C

2

PLC

INPUTS

OUTPUTS

0 V

C

1

C

2

C

3

+24 V DC 

0 V

+24 V DC 

Q 0.0

I 1.0

I 1.4

S

1

S

2

S

3

Q 0.2

Q 0. 4

I 1.2

PLC

INPUTS

OUTPUTS

Q 0.0

I 1.0

I 1.4

Q 0.2

Q 0. 4

I 1.2

V

10 KΩ

Voltage measurement in

an output without load

PLC

Inputs

Outputs

Q 0.0

I 1.0

I 1.4

Q 0.2

Q 0. 4

I 1.2

V

Figure 6.57  Measurement of the voltage of a digital output including a triac as a switching ele-