CAN Bus Gremlins: Missing and Conflicting Devices

CAN bus issues are notorious because they are intermittent — a device works on the bench, then disappears when the robot vibrates on the field. CAN connects all your smart devices (SPARK MAX, TalonFX/Kraken, CANcoder, Pigeon 2.0, PDH) on a single twisted-pair daisy chain.

Symptom: a device is ‘not found’ or motors randomly stop. Diagnose systematically:

1. Check for duplicate IDs. Two devices sharing a CAN ID is the classic killer — SPARK MAX and many CTRE devices ship as ID 0 from the factory. Open the REV Hardware Client (for SPARK MAX) and Phoenix Tuner X (for CTRE devices); each scans the bus and lists every device with its ID. A device that flickers in and out, or two devices fighting over one ID, jumps out immediately. Reassign every device to a unique ID and persist it.
2. Check the wiring topology. CAN must be a single daisy chain (yellow to yellow, green to green) from the roboRIO through every device to the PDH. A common rookie mistake is wiring it as a star/parallel splice instead of a series chain. Tug every connector — vibration loosens cheap crimps, and one loose joint downstream drops every device after it.
3. Check termination. The bus needs a 120-ohm resistor at each physical end. The roboRIO has one built in; the PDH/PDP provides the other (often via a switch or jumper). Missing termination makes the bus marginal — it works at rest and fails under vibration.

Read the faults. The Driver Station and the device tools report sticky faults. A ‘No CAN Comm’ / loss-of-CAN fault means the device cannot talk over the bus. High CAN bus utilization (visible in the DS) means you are flooding the bus — reduce status-frame rates on devices you poll rarely.

The reliability habit veterans swear by: label every CAN ID on a wiring diagram and physically on the device, keep a spare 120-ohm terminator in the pit, and after any chassis work, re-scan the full bus in both tools before driving. A robot that loses CAN mid-match can lose the match outright, so treat clean CAN wiring as a competitive requirement, not a nicety.

Key takeaways

• Duplicate CAN IDs (SPARK MAX and many CTRE devices ship as 0) are the #1 cause of missing devices — give each a unique ID and persist it
• CAN must be a single daisy chain with 120-ohm termination at each end; a star topology or loose connector drops everything downstream
• Re-scan the full bus in REV Hardware Client and Phoenix Tuner X after any chassis work, and label every ID

This lesson was adapted from learnfrc.com.