Building a Reliable CAN Bus

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

• 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

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

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

• 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

• WPILib Introduction to FRC Robot Wiring
• WPILib Power Distribution Module