Introduction to Industrial Automation

Figure 6.16  Block diagram of operations inside a digital output module.

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Basic Operating Principles of PLCs 

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therefore the output device. It is possible to have one fuse per individual digital output or a fuse 

every two or three or more outputs. Typical output devices are:

 

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Power or auxiliary relays

 

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Solenoids of electrovalves

 

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Indicating lights

 

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Devices for alarm indication

 

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Coils of electro-pneumatic valves

 

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Electronic devices that require an activation input or a change in their ON-OFF state

A typical external connection diagram of an output device to a digital output module is shown 

i
power transistors as switching elements are shown. The digital outputs with relay are generally 

suitable for AC and DC operating voltages of the output devices, which are able to provide addi-

tional electrical isolation and are resistant to impulse currents and overvoltages. The disadvantages 

are typical for all relays, e.g., the possibility of mechanical damage, the wear of the relay’s contacts, 

and their limited mechanical life.

Digital outputs with a power transistor as switching element are suitable only for DC operat-

ing voltages of the output devices, have no mechanical parts, and therefore do not suffer from wear 

or noise, while they are characterized by a relatively high switching frequency. However, these are 

sensitive to impulse currents and overvoltage. Finally, the digital outputs with triac are suitable 

LED

indicator

5 V DC

0 V

Logic signal from 

CPU/memory

Opto-isolator

L1

(1)

Output

device

External power

source DC or AC

AC

L2

Fuse

Rs

Cs

VAR

Triac

Logic signal

Isolation

Signal level conversion

Power signal

Output module

4 DO

Output

device

L1

L2

Nominal voltage of output

module

1

2

3

4

L1

L2

LED

 indicator

(a)

(b)