Why a Ground Connection Matters on a CAN Bus (And How to Wire It Correctly)

When working with the Controller Area Network (CAN) bus—whether in automotive, industrial automation, robotics, or embedded systems—you’ll quickly notice that most CAN connectors include a ground pin. At first, this may seem unnecessary because CAN uses differential signaling. If the data is transmitted as a difference between CAN-H and CAN-L, why bother with a ground?

In reality, that extra ground pin plays a crucial role in keeping the network stable, reliable, and safe. In this post, we’ll break down why CAN requires a ground reference, what the CAN ground actually does, and how to properly wire grounding and shielding in real-world applications.

Why CAN Needs a Ground Reference

Even though CAN is differential, the transceivers still need a shared reference point. Here’s why:

1. Differential signals still depend on common-mode voltage

CAN transceivers can only operate within a certain common-mode voltage window (typically –2 V to +7 V for ISO11898-2 devices). If two nodes’ grounds differ by more than this, the transceiver may no longer recognize CAN-H and CAN-L levels.

A shared ground ensures that both nodes interpret the differential pair within valid voltage ranges.

2. Ground equalizes potential differences

Separated power supplies or long cable runs can cause the ground at one node to float higher or lower than another.

A CAN ground wire equalizes these potentials, preventing:

communication failures
excessive common-mode offset
bus errors
transceiver damage

3. Safety: prevents destructive return currents

If there is no shared ground, return currents can inadvertently flow through:

the shield
the CAN-H/CAN-L wires
sensitive electronics

A proper CAN ground line provides a safe, low-impedance reference path.

CAN Ground vs. CAN Shield: What’s the Difference?

Many beginners confuse CAN ground (logic ground) with the cable shield. They serve very different purposes:

✔ CAN Ground (GNDCAN / Logic Ground)

Carries electrical reference potential for the transceivers
Ensures common-mode compatibility
Helps prevent voltage drift between nodes
Usually part of the internal logic / power return path

The CAN ground is a signal reference, not an EMI protection layer.

✔ CAN Shield (Shield / Drain Wire)

Protects the bus from electromagnetic interference (EMI)
Functions as a Faraday cage around the twisted pair
Often connected to chassis ground at one end (or both under controlled conditions)
Does not carry signal return current

Shield ≠ signal ground.
Ground ≠ shield.

Should You Use Chassis Ground or Logic Ground?

This depends on the system design and whether your CAN interface is isolated.

✔ Use Logic Ground When:

The CAN transceiver is not galvanically isolated
Both ends must share a clean reference
Devices have separate power supplies
Most automotive and embedded boards follow this model

Logic ground ensures consistent signaling.

✔ Use Chassis Ground When:

Connecting or terminating shielding
Reducing noise and EMI in industrial or automotive environments
Bonding cable shields to earth for safety
Following standards like CANopen, DeviceNet, or automotive OEM guidelines

Chassis ground is typically used for shield termination, not for logic reference.

How to Wire CAN Grounding in Vehicles

Vehicles are electrically noisy environments (alternators, ignition, injectors, motors, high-current loads). Proper grounding is essential.

Standard automotive grounding guidelines:

Connect GNDCAN between all ECUs.
→ Usually included in the OEM 3-wire or 5-wire CAN harness.
Connect the cable shield to chassis ground at ONE location
→ Usually at the gateway module or main ECU
→ Prevents ground loop formation
Avoid tying shield directly to logic ground.
→ Keeps noise out of the transceivers
Avoid grounding the shield at both ends unless OEM design requires it.
→ Otherwise, the shield may carry high currents from chassis ground differences.
Keep CAN ground as clean as possible.
→ Use star grounding where practical
→ Avoid sharing ground with high-current loads like starters or compressors

How to Wire CAN Grounding in Industrial Systems

Industrial systems often have:

isolated power domains
long cable runs
heavy motors
noisy switching equipment
multiple racks or cabinet grounds

Standard recommendations (CANopen, DeviceNet, IEC guidelines):

Use logic ground between all nodes

Even isolated nodes need a controlled reference point.

Bond cable shield to chassis ground

Methods vary:

One-end shield grounding (low-noise or short networks)

Tie shield to chassis ground at the master/controller
Leave shield floating at other nodes
Most common in small or moderate-length networks

Both-end shield grounding (long runs or high EMI)

Bond shield to chassis at both ends
May require:
- drain resistors
- anti-loop devices
- transient protection

Do NOT:

tie shield to logic ground
run shields into PCB ground planes
splice shields incorrectly
allow shield continuity to break at connectors

Best Practices for Long CAN Cable Runs

Long runs amplify every grounding and shielding issue. For distances above ~30 meters, follow these rules:

✔ Use twisted-pair shielded CAN cable (ISO11898-2 compliant)

This ensures correct impedance (120 Ω) and reduces EMI susceptibility.

✔ Use a dedicated GNDCAN wire

Do NOT rely on:

conduit
cable shield
chassis
earth
power returns

✔ Ensure both ends have termination (120 Ω each)

For long runs, use quality metal-film resistors with tight tolerance.

✔ Connect shield to chassis at least at one end

Use the cabinet ground bar or vehicle frame.

✔ Avoid running CAN next to:

AC mains
motor drive cables
solenoid power lines
relay bundles
high-current inverter outputs

✔ Add surge suppression for outdoor or harsh environments

Use:

TVS diodes
isolated transceivers
transient filters
common-mode chokes

✔ Use galvanic isolation when:

cable length > 100 m
different power domains exist
equipment may be hot-plugged
grounding is uncertain or noisy

Isolation fixes many grounding problems before they start.

Final Thoughts

Even though CAN is a differential signaling system, it absolutely requires a ground reference to function reliably. The CAN ground equalizes node potentials, ensures valid common-mode levels, and protects against electrical damage.

A properly wired CAN network should include:

Logic Ground (GNDCAN) for signal reference
Shield wired to chassis ground, not logic ground
Proper termination, shielding, and isolation practices

Get these right, and your CAN network will be stable—even in noisy automotive or industrial environments.

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