Introduction to Industrial Automation

Figure 6.10  Block diagram of operations inside a digital input module.

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  Introduction to Industrial Automation

is presented in Figure 6.11, which contains four switching contacts that can be opened or closed 

depending on the position of the switch, i.e., according to the number selected. For example, in 

position 2, contact 2 is closed, while the rest of the others are open. The numerical switch in Figure 

6.11 is able to provide ten different choices (numbers 0–9) by utilizing only four digital inputs. 

The combination of two such numerical switches allows for 100 options (numbers 0–99) by using 

only eight digital inputs.

For performing the functionalities indicated in Figure 6.10 inside a digital input module, 

corresponding electronic circuits that are required in their simplified form are presented in 

Figure 6.12a. If the input signal is DC voltage, only the insertion of a resistor circuit for con-

verting the voltage level is required. If the input signal is AC voltage, then the input circuit 

contains an additional AC to DC converter or bridge rectifier. The smoothing capacitor C 

stabilizes the output voltage of the rectifier, while the resistors reduce the voltage to the desired 

low level, typically 5 V. The voltage of the Zener diode determines the minimum voltage value 

that could be detected. This low-level DC voltage is applied to the light emitting diode (LED) 

that is structurally integrated with a phototransistor. The last element is an electronic circuit 

powered by an independent internal power source (5 V DC) of the module. When light is 

emitted from the LED and falls on the phototransistor, the latter is conductive and allows 

the current path to the conductor to indicate the state of the logic signal. In conclusion, the 

existence of a voltage signal in the circuit entrance—e.g., the ON state of the input device—is 

transferred as DC voltage of a low or logic level to the microprocessor. Simultaneously with 

the creation of the logic signal, the indicative LED for the input device status is also supplied 

with power.

With the electrical isolation through the optical isolator, a complete decoupling is achieved 

between the electronics section for the input signal reading and the corresponding microprocessor. 

In this way, it is ensured that a possible short circuit, incorrect connection, transient spikes of the 

grid voltage, or even an additional noise that it is not removed from the modulation unit, will not 

damage the microprocessor. Less reliable methods use a transformer or a reed switch as an isolator. 

In this case, a transformer presents thermal losses that create thermal temperature problems, while 

Inputs to PLC

R

1

2

3

4

Thumbwheel switch

R

1

2

3

4

0

0

0

0

1

0

0

0

0

1

0

0

1

0

0

1

Position 0

Position 1

Position 2

Position 9

Status of switches per position

0 = open contact, 1 = closed contact

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