KUKA KSS00021 Bus Communication Error – Causes, Symptoms & How to Fix

Error Code Overview

• Error Code: KSS00021
• Category: Bus / Communication / Network / Module
• Severity: High
• System: KRC Controller / KSP/KPP Servo Drives / X20 I/O / Fieldbus / EtherCAT
• Impact: Robot motion stops; drives, I/O modules, and controllers may be affected

Quick Fix for KUKA KSS00021

If KUKA KSS00021 Bus Communication Error appears, take these immediate actions:

1. Stop all robot motion immediately
2. Inspect communication cables and connectors
3. Verify all control modules are powered and online
4. Check bus/network settings in SmartHMI
5. Reset the alarm and restart the controller

Important: Temporary bus interruptions may clear after restart, but repeated KSS00021 alarms usually indicate hardware, wiring, or controller network faults.

What Does KUKA KSS00021 Mean?

KUKA KSS00021 indicates that the KRC controller has detected a communication failure somewhere within the robot’s internal bus system.

Th is bus system is responsible for transferring data between:

• Servo drives
• I/O modules
• Safety modules
• Motion control hardware
• Internal communication boards

In simple terms: The controller cannot reliably communicate with one or more critical control modules, so robot motion is stopped until communication is restored.

Common Symptoms of KSS00021

When th is fault occurs, operators may notice:

• Robot stops unexpectedly during startup or production
• Servo drives fail to enable
• I/O modules appear offline
• Communication status LEDs flash abnormally
• Multiple hardware modules show timeout alarms

These symptoms usually indicate a failure somewhere in the robot’s internal communication network.

Main Causes of KUKA KSS00021

1. Communication Cable or Connector Failure

Damaged wiring can interrupt bus communication:

• Broken cable
• Loose connector
• Bent or damaged pins
• Connector corrosion

This is one of the most common causes.

2. Failed Bus Module or Drive

A faulty hardware module may stop responding:

• Servo drive failure
• I/O module malfunction
• Communication board failure

3. Bus Configuration Error

Incorrect settings may disrupt communication:

• Wrong addressing
• Invalid module mapping
• Firmware mismatch
• Configuration file errors

4. Internal Controller Port Failure

Communication ports may fail due to:

• Electrical overload
• Physical wear
• Controller board damage

5. Electrical Noise / Grounding Issues

EMI interference may corrupt communication signals:

• Poor grounding
• Inadequate shielding
• High-current cable interference

How to Troubleshoot KUKA KSS00021

Step 1 – Identify the Faulted Module

Use SmartHMI diagnostics to determine:

• Which module lost communication
• Whether multiple devices are affected

Step 2 – Inspect Bus Wiring

Check:

• Cable integrity
• Connector seating
• Pin damage
• Shield condition

Reconnect all bus-related wiring.

Step 3 – Check Module Status

Verify:

• Module LEDs
• Drive startup status
• Hardware diagnostics

Replace non-responsive modules if necessary.

Step 4 – Verify Configuration Settings

Confirm:

• Bus topology is correct
• Module addresses match setup
• Firmware versions are compatible

Step 5 – Inspect Power and Grounding

Check:

• Voltage stability
• Grounding continuity
• Shield termination
• Nearby EMI sources

Step 6 – Reset and Retest

After repairs:

1. Clear KSS00021 in SmartHMI
2. Restart the KRC controller
3. Jog robot slowly
4. Monitor for repeat faults

Recommended Replacement Parts

Pro Diagnostic Tip

If multiple modules go offline at the same time, focuson:

• Main communication backbone
• Shared power supply issues
• Controller communication board faults

When several modules fail together, the root cause is often upstream—not the individual modules themselves.

How to Fix KUKA KSS00021

Dependingon the cause:

• Replace damaged cables/connectors
• Replace failed drives/modules
• Correct bus configuration settings
• Repair/replace controller communication board
• Improve grounding and EMI protection

How to Prevent Bus Communication Errors

To reduce recurrence:

• Inspect bus wiring regularly
• Keep connectors clean and secure
• Verify configuration after hardware replacement
• Maintain proper shielding and grounding
• Keep spare communication modules available

Related Errors

KUKA KSS00022 Fieldbus Communication Error – Module lost connectionon Fieldbus network.
KUKA KSS00023 Profinet Communication Error – Profinet interface communication lost.
KUKA KSS00024 EtherCAT Communication Error – EtherCAT network module offline.
KUKA KSS00410 X20 I/O Communication Error – X20 module not responding.
KUKA KSS01407 Fieldbus Communication Error – Controller detects communication timeout.

⚠️ Closely related to bus/network communication, module connectivity, and control reliability.

FAQ

What does KUKA KSS00021 mean?

It means the KRC controller detected a failure in internal bus communication with one or more modules.

Can the robot run with KSS00021 active?

No. Robot motion is typically blocked until communication is restored.

What causes KUKA KSS00021?

Typical causes include:

• Cable damage
• Failed module/drive
• Bus misconfiguration
• Controller board failure
• EMI interference

Is KSS00021 serious?

Yes. It can disable robot motion and may indicate larger controller communication issues.

Can ignoring KSS00021 damage the robot?

Yes. Repeated communication interruptions may cause:

• Unexpected shutdowns
• Drive faults
• Module stress/failure