Building a Reliable CAN Bus
The shape of the bus
Section titled “The shape of the bus”A CAN bus is a single chain (a ‘daisy-chain’), not a star or a loop. It must have exactly two endpoints, each capped with a 120-ohm terminating resistor. The roboRIO has one built-in terminator; the PD (PDH/PDP) contains the other. So the standard layout is:
roboRIO - > motor controller -> motor controller -> … -> PD
with the roboRIO and the PD at the two ends.
Wiring details
Section titled “Wiring details”- Use 22 AWG twisted pair - twisting CANH and CANL together rejects electrical noise.
- CANH (yellow) connects to CANH, CANL (green) connects to CANL, all the way down the chain.
- At the roboRIO spring terminals, strip about 5/16 in (~8 mm).
- Keep every junction solid - a single intermittent CAN connection can knock out every downstream device.
Device IDs
Section titled “Device IDs”Every device on the bus needs a unique CAN ID :
- CTR devices (Talon FX/FXS, Talon SRX, Victor SPX): set IDs in Phoenix Tuner X.
- REV devices (SPARK MAX, SPARK Flex, PDH): set IDs in the REV Hardware Client.
- Defaults to remember: PDP = ID 0 , PDH = ID 1.
Duplicate IDs cause erratic behavior, so plan an ID scheme (for example, drivetrain motors 1-4, arm 5-6) and label each controller.
Termination check
Section titled “Termination check”With the robot off, measure resistance across the CANH/CANL pair. A healthy, properly terminated bus reads about 60 ohms (two 120-ohm resistors in parallel). If you read ~120 ohms, one terminator is missing or disconnected; if you read very high resistance, the bus is open somewhere.
Common CAN problems
Section titled “Common CAN problems”- Intermittent device dropouts - usually a loose connector or a cold crimp.
- Whole bus (or downstream segment) down - a break in the chain, or a shorted device.
- Wrong/duplicate IDs - devices fight over the same address.
- Missing termination - flaky communication at high bus traffic.
Sources
Section titled “Sources”Key takeaways
Section titled “Key takeaways”- CAN is a daisy-chain with the roboRIO and PD at the two terminated ends (120 ohms each).
- Use 22 AWG twisted pair; CANH-to-CANH (yellow) and CANL-to-CANL (green).
- A healthy bus measures ~60 ohms across CANH/CANL with the robot off.
- Give every device a unique ID via Phoenix Tuner X or REV Hardware Client (PDP=0, PDH=1).
This lesson was adapted from learnfrc.com.