Digital Input-Output (Digital I/O)
Digital inputs and outputs are signal terminals that carry only two states (present/absent). Buttons, limit switches and sensor triggers connect to inputs; relays, warning lamps and driver triggers connect to outputs. On industrial boards these terminals are usually adapted to work with a 12-24 V line voltage.
Designing a digital input correctly is not just a matter of running a wire to a controller pin.
- Level adaptation: a 24 V field signal is brought down to the 3.3 V or 5 V level at which the controller operates, with a voltage divider or an optocoupler.
- Protection: transient pulses arriving at the input terminal and reverse polarity are handled with a series resistor and protection diodes.
- Undefined state: a pull-up or pull-down resistor is fitted so that the input does not float when nothing is connected.
- Bounce: mechanical contacts chatter for milliseconds as they close; this chatter is filtered out with a short delay in software.
On the output side, the type of load to be driven is decisive. With inductive loads, if the high voltage generated at the moment of interruption is not handled with a snubber diode or a similar element, the driving element fails. In field wiring, too, inputs cause far fewer problems when their signal cables are not run in the same duct as the power cables.
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