Introduction to Industrial Automation

Each group contains non-isolated inputs.

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  Introduction to Industrial Automation

periodically by the microprocessor to the output module (scanning process), the latter should 

have some kind of memory to retain or store the signals until the next transmission of signals 

from the microprocessor. The maintenance of these signals is performed by an integrated cir-

cuit that it is called “Latch” or “Hold” (e.g., the CD4508BE CMOS 4-Bit Latch Logic IC by 

Texas Instruments). As presented i
it follows the latch unit that is not included in the digital input module. This is normal, since a 

potential change of the input signals’ state and therefore a change in the input devices will be 

detected in the next scan cycle. If the change occurs in the dead time between two sequential 

scans, then the operation of the microprocessor is not affected, since it regulates when to take 

account of the change of status for the input devices. In contrast, in the digital output module, 

it is not possible to change the state of the output signals, and therefore the output devices, 

during the dead time between two sequential scans. The microprocessor’s operating logic for 

the PLCs is based precisely on the principle that it changes the state of the output devices at 

regular intervals (e.g., per scan cycle) and that each output state will remain unchanged in the 

dead time between two scans.

The remaining units of the digital output module (e.g., the electrical isolation, the optical dis-

play, and conversion ones) are similar to the corresponding ones of a digital input module in terms 

of design and function. However, in the realization of the units, the manufacturers pay special 

attention, since the digital output modules control devices with relatively larger power levels and 

current. Thus, the digital output modules comprise switch power elements at current levels typi-

cally up to 1 A or 2 A, the function of which (ON-OFF) causes electrical and magnetic noise that 

is sufficient to destroy the other sensitive electronic components. I
circuitry of a digital output that implements the above basic functions is shown in simplified 

form. In the conversion circuit of the output signals, weak signals are coming from the proces-

sor (after the electrical isolation) and are converted into power signals with current and voltage 

levels capable of driving output devices that demand power consumption. Therefore, every digital 

output module requires an external power source, which supplies the output devices through the 

switching components of the converter circuit. As switching power elements, someone can find a 

power transistor, a triac, a printed circuit board relay, a reed switch, etc., depending on the type 

of digital output module and the quality of manufacturing. When the triac, presented i

damage to the electronic circuit. This risk is addressed by the utilization of the R

S

C

S

 surge suppres-

sor. The varistor (Var) element also limits the transient phenomena and the noise. In series with 

the switch element, a fuse is introduced for protection from the overload of the output circuit, and 

220 V AC

Output devices: relays, electric-valves, indication lamps, etc.

Output

device

Status

indication

of outputs

Block of terminals

Signal level

conversion 

Optical

isolation

Multiplexing

electronics 

Latching

electronics

From CPU/

outputs’

memory 

5 V DC