On this page we'll describe the 10 "DIO" Digital I/O connections on the RoboRIO.  
{{ :roborio-dio.jpg?nolink&250|}}

====== Introduction ======


By "digital" or "binary" we mean that each connector can convey a piece of information that is in one 
of two conditions.  There are several ways of saying this:
  * on / off
  * one / zero
  * Positive voltage / zero volts

When acting as an input, the RoboRIO interprets any voltage between 0 and 0.8v 


By "IO" we mean that the direction can be either:
  * Input: from an external switch or sensor to the RoboRIO
  * Output: from the RoboRIO to an external light or other device




The direction is controlled by the software that runs on the roborio (see below).

Each "DIO" consists of a header with 3 pins.  Typically, we connect a 3-wire "PWM" cable to each header.
  * Ground  (black)
  * +5v power (red)
  * Signal (white)

The interesting pin is the "signal" pin.  
When configured as an output, the signal pin is driven to 0 volts for a "zero", and 3.3 volts for a "one".  More on outputs later.

When acting as an input, the RoboRIO interprets the voltage seen on the "Signal" pin as:
  * any voltage between 0 and 0.8v is a "zero"
  * a voltage between 2.0v and 5.5v i a "one."
  * A voltage in between those ranges results in either a zero or a one being delivered to the software program.  We generally want to avoid this indeterminate region.

There is a "pullup" or "pulldown" resistor internal to the RoboRIO that helps ensure that the voltage on a disconnected DIO does not drift into that "unknown" region.  For the ten basic DIOs on the RoboRIO, the resistor is a pullup, so an unconnected DIO will be read by software as a "one".

So, to cause a DIO to be read as "zero" by software, all we have to do is connect the signal pin to ground, for example with a switch:

{{ :pwm-switch-cable.png?400 |}}


====== Software ======

Here's a Java code fragment:
<code java>
 package edu.wpi.first.wpilib.templates;

 import edu.wpi.first.wpilib.DigitalInput;
 import edu.wpi.first.wpilib.SimpleRobot;
 import edu.wpi.first.wpilib.Timer;

 public class RobotTemplate extends SimpleRobot {

 	DigitalInput mySwitch;
 	
 	public void robotInit() {
 		mySwitch = new DigitalInput(1);
 	}

 	public void operatorControl() {
 		// more code here	
 		switch_now = mySwitch.get();
 		if(switch_now) {
 			// do somthing if the switch is open ("one");
 		} else {
 			// do somthing else if the switch is closed ("zero");
 		}
 		// more code here	
 	}
 }
</code>


Notice somthing there that is true of most simple test programs:  there's a whole lot more "java stuff" then there is specifics about what we're trying to do.  There are really only two interesting lines.  The first one is:
    mySwitch = new DigitalInput(1);

This creates an object which represents our DIO pin in software.  The "1" in DigitalInput(1), means that its
associtated with DIO number 1.

The other interesting line is:
    switch_now = mySwitch.get();
which reads the current state of the DIO and stores it in a boolean variable called ''switch_now''.
We can then use switch_now in an ''if()'' or other statements.








There's some more software discussion here:
 * https://wpilib.screenstepslive.com/s/3120/m/7912/l/599687-using-limit-switches-to-control-behavior





What if we want to select between more than two choices?   See "fun with switches" on the page [[Switches as Digital IO]]

====== Conclusion ======


This discussion also applies to the 16 additional DIO pins located on the roborio expansion connector.