Introduction to Industrial Automation

Figure 6.25  Block diagram of operations inside an analog output module for converting a digi-

tal value signal to analog.

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Basic Operating Principles of PLCs 

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existence of a fault (e.g., cut wire). The most significant bit of the word, expresses the sign (+, –) of 

the numerical value. A similar utilization of bits takes place in the case where the digital word has a 

shorter length than 16 bits, an operation that affects negatively the accuracy of the A/D converter.

When the digital word d

0

–d

7

 reaches the D/A converter, a corresponding constant electrical 

voltage is produced at the output of the converter. This means that the output of the converter 

continues to be discrete (1024 levels of voltage for a word of 10 bits) and when the digital input 

word is varying, the result is to have a corresponding piecewise constant voltage in the output of 

the converter which is also varied, as indicated in Figure 6.25. In order to have the varying output 

signal, but with the piecewise constant like a real analog signal, a specific circuit of an amplifier 

is driven and filtered, to smoothen the stepped voltage (or current). The accuracy of the analog 

output is dependent on the frequency update of the digital word input. This update is mainly 

dependent on the scanning cycle of the PLC, since the data produced from running the program 

related to the analog output module is transmitted only once in every scan cycle. In Figure 6.26, 

the functional block diagram of a real analog output module is depicted. As in the case of analog 

input modules, this part of the digital logical circuits is isolated from the part of the analog ones 

Μ = earth terminal

L+

M

Power supply

Clock

Address decoder

Α

D

Microprocessor

Counter

Multiplexer

–

  

+

  

–

  

+

  

+ 24 V

M

R

R

R

I

Εξοδ.

V

Εξοδ.

0–20 mA

  

  

–

  

+

  

M

R

R

–10–+10 V

...

Data

PLC–CPU

Electric isolation 

Control

signals

Address

bus

Data

bus

Analog output module

Accumulator

Opto-isolator