Introduction to Industrial Automation

ment and without a connected load.

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Basic Operating Principles of PLCs 

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265

  6.5.  What type of digital output (DO) module do you have to select for the activation of a solid 

state output device in order to avoid operating problems due to leakage current?

  6.6. The figure shows a typical simplified electric circuit of a digital input module. Describe the 

operations that take place inside the module between the input signal and the logic signal 

which is transferred to a CPU.

+

_

Logic signal

to CPU

5 V DC 

R

R

R

R

R

R

Input signal

220 V AC 

+

_

  6.7.  The classical scanning cycle of three steps (input scanning, program execution, and out-

put updating) is interrupted when the user program contains instructions for a periph-

eral read–write. For example, an instruction for a direct-in-the-module reading of an 

input will give the result shown in the figure. In your opinion, when is the scanning 

cycle faster? When the user program contains instructions for peripheral read–write, or 

included no instructions?

Scanning

of inputs

Program

execution

11

22

33

44

1

2

3

4

L1

Return

Updating

of outputs

Program instruction for

 direct in the module

reading of an input

Input module

1

2

3

4

L1

  6.8. An analog valve for flow control accepts an analog signal of –10 V DC up to +10 V DC from 

an analog output of a PLC. The PLC controls the liquid flow based on a proper algorithm. 

The analog output uses 12 bits for a digital value representation and 1 bit for sign. Complete 

a three-column table which will give (for each opening position of the valve from 0% up to 

100% in steps of 10%) the corresponding voltage value and its digital representation that is 

created by the analog output of the PLC.

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  Introduction to Industrial Automation

  

Analog voltage control signal

–10 V DC up to +10 V DC

Valve opening

0–100%

Liquid flow

 

  6.9.  In a liquid flow pipe network, a flow valve, similar to that of problem 6.8, has been inserted 

and permits the flow of 100 L/min when it is at its maximum opening. What will be the 

stored digital value of the analog output adjusted for 60 L/min flow?

 6.10. A small industrial process is controlled via a modular PLC. The automation task controls 

a system with 32 digital inputs, 28 digital outputs, 4 analog inputs, and 4 analog out-

puts. Given that peripheral equipment is not required due to the small dimensions of the 

industrial building, determine the required local equipment by selecting the PLC from the 

three available in the market, which will have the major number of empty places for future 

installation of I/O modules. The technical characteristics of the three available PLCs are 

shown in the table. Please create a diagram showing the structure and interconnection of 

the equipment units.

    The digital inputs and outputs are further categorized as follows:

 

32 DI = 20 DI with nominal voltage 230 V AC + 12 DI with nominal voltage 24 V DC

  28 DO = 16 DO with nominal voltage 230 V AC + 12 DO with nominal voltage 

24 V DC

    The power supply, CPU and communication modules each occupy a single place in the 

installation rack of any PLC.