Introduction to Industrial Automation

6.6.3   Analog Output Modules

The analog output modules receive numerical data in digital form from the central processing unit 

of the PLC, transforming them into an analog voltage or current, in order to control the operation 

of an analog device. The digital values of the analog signals are usually words of 16 bits that are 

created from the processing of the program instructions, and are temporarily stored in the input/

output memory of the PLC, before being transmitted to the analog output modules for their con-

version into analog signals usually of 4–20 mA current or −10 V DC to +10 V DC. This process is 

highlighted in Figure 6.25. Even if the digital word can have a length of 16 bits, from these bits, 

only 11 are utilized for the digital representation of the analog signal and consist of the useful 

converted data, while the rest are utilized for the transfer of other general information or timing 

signals. A similar strategy is also followed in the digital representation of the analog input signals. 

For example, one bit can express the situation of a digitized measured value over a range or the 

d

7

d

6

d

5

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1

d

0

D/A

Output signal of 

D/A converter

Filter

Amplifier

Output 

Analog

Signal

Output

analog

device

(triode vane)

Data bits

in parallel

8–14 bits

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7

d

6

d

5

d

4

d

3

d

2

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1

d

0

Output signal of

D/A converter

Filter

Amplifier

Output

analog

signal

D/A