DAQ Introduction - Digital Inputs

Data Acquisition devices provide an interface between electrical signals a computer can read or write to control things in the real world.

A computer is equipped with various communication ports to access the outside world, but it needs software to read and write to them, as well as display data or accept user input.

In the early days of PC’s a user could use a simple programming language to write a program that would access the ports using direct port-in and port-out commands and read or set the value. Examples of digital inputs or outputs could be found in computers with joystick and trigger inputs, and in the parallel port control line outputs such as strobe, and control line inputs for busy, paper error, ack, and the 8 bits of data output. With a joystick port, there were digital inputs for trigger and analog inputs for joystick position.

With a digital input we can read two states, a high or low. A switch can be open or closed.
With a digital output, we can turn a light on or off, control a relay or turn a motor on or off. If we can turn a digital output on/off rapidly enough, we can vary the on/off time for PWM (Pulse Width Modulation) and effectively develop a average DC voltage anywhere between full on, full off, or with a 50% duty cycle, half the source voltage.