Introduction to Industrial Automation

Figure 6.17  Simplified electronic circuit of a digital output module with a triac as a switching ele-

ment (a) and an external electric connection of an output device requiring AC voltage supply (b).

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  Introduction to Industrial Automation

for AC supply voltages, do not suffer from wear, but are more sensitive to impulse currents and 

overvoltage, and thus are always equipped with a special fuse.

As in the case of digital input modules, digital output modules may also have a separate or a 

common power supply. I

characteristics and remarks as for the digital input modules, whil
cal groupings of digital outputs with a DC power supply.

From the previous description of the digital input/output modules, it is obvious that there are some 

specific technical features that characterize these units, while the selection of the necessary I/O modules 

is made for each individual application. These technical characteristics can be summarized as follows:

  1. The nominal operational voltage of the modules (value and kind)

  2. The number of inputs and/or outputs per module

  3. The kind of switching components that each digital output module contains (relay or elec-

tronic switching components)

LED

indicator

5 V DC

0 V

Logic signal from

CPU/memory 

Opto-isolator

0 V

+

Relay as switch

Protection

diode 

(C) L

(1) N

Output

device

External power

source DC or AC 

0 V 

0 V 

(1)

5 V DC

+ V

R

D1

D2

LED

indicator

Opto-Isolator

Logic signal from

CPU/memory

Protection 

diode

Output

device

Transistor as switch

External power

source DC 

(a)

(b)