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Electrical & Wiring·Lesson 25 of 34

Reading the Lights: A Diagnostic Decision Tree

Turn the roboRIO, PD, and motor controller LEDs into a fast diagnosis of what is actually wrong.

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Before you touch a wire, read the lights. FRC hardware is covered in status LEDs that tell you exactly what stage of the system has failed, and WPILib publishes an exact reference for every one.

roboRIO Power LED is your first stop:

  • Solid green = power is good.
  • Amber = brownout protection tripped, outputs disabled. This is the single most common 'my robot stopped moving' cause. Go to the brownout lesson.
  • Red = power fault; check the user rails (6V/5V/3.3V) for a short circuit. Unplug things on the user rails one at a time.

roboRIO Status LED (separate from Power):

  • Off after boot = normal.
  • 2 blinks = software error, reimage the roboRIO.
  • 3 blinks = safe mode; restart and reimage if it persists.
  • 4 blinks = code crashed twice without rebooting; reboot.

roboRIO Comm LED:

  • Off = no communication with the Driver Station (radio/Ethernet problem).
  • Red solid = talking to the Driver Station but no user code running (deploy code).
  • Red blinking = E-stop triggered.
  • Green solid = good comms with the DS.

Robot Signal Light (RSL): solid ON when the robot is powered but disabled, blinking when enabled, OFF if the robot is unpowered or the RSL is miswired. An RSL that is solid when you expect blinking means your code is not enabling, not an electrical fault.

Power Distribution:

  • CTRE PDP: a green strobe/blink indicates enabled with no fault, slow green indicates disabled with no fault, orange indicates a sticky fault, and red indicates no CAN comms.
  • REV PDH: solid green = comms with the roboRIO established; solid blue = powered but no comms; orange/red = fail-safe.

Motor controllers (Talon FX/Kraken): blinking orange = powered, on the bus, neutral output; green = driving forward; red = reverse; any red blip while jostling wires = loose CAN.

The decision tree:

  1. roboRIO Power amber -> brownout. Power red -> user-rail short.
  2. Power green but Comm off/red -> communications or no-code problem, not power.
  3. PD red/blue -> CAN comms lost to the PD.
  4. One motor dead while others work, controller LED solid blue (REV) or no LED -> that device lost power or CAN; jostle-test its harness.

Reading lights first turns a 30-minute teardown into a 30-second diagnosis.

Key takeaways

  • roboRIO Power LED amber = brownout; red = user-rail short; green = power healthy.
  • Comm LED red-solid means no user code is running, not a wiring fault; red-blinking means E-stop.
  • A red LED blip on a CAN device while jostling the harness pinpoints a loose connection at that segment.

Go deeper

Lesson quiz

Required

Answer all 3 questions correctly to complete this lesson.

1.While diagnosing a robot, you notice the roboRIO Power LED is solid amber (orange) instead of green. What does this indicate?

2.The roboRIO Comm LED is solid red. Following the diagnostic decision tree, what is the most likely cause?

3.The Robot Signal Light (RSL) is glowing solid rather than blinking. What state is the robot in?

Answer every question to submit.