In Universal Robots systems, C157 is a safety-integrity shutdown event.
It is not a standard servo alarm and not a typical motion fault.
C157 is triggered when the controller detects instability inside the safety validation process.
The fault usually involves:
Once detected, the controller immediately stops robot motion and enters a protected safety state.
In real production environments, C157 is commonly related to electrical or communication instability rather than software corruption.
UR safety systems continuously validate synchronization between multiple layers:
Validation occurs cyclically at very short timing intervals.
The controller expects:
Even a brief interruption can trigger C157.
In production cells, the root cause is often physical rather than software-related.
Typical sources include:
These problems are especially common in integrated automation lines.
Typical behavior includes:
Field behavior usually appears as:
This differs significantly from a true mechanical collision event.
One of the most common early indicators.
Possible symptoms:
In many field cases, pendant instability appears shortly before C157 occurs.
Common behavior:
This pattern is common after repeated watchdog interruptions.
A very common field pattern:
Often correlated with:
This is typical unstable safety-loop behavior.
The most common trigger layer involves heartbeat timing instability.
Typical causes include:
Even very small interruptions can activate safety shutdown logic.
In real factory environments, heartbeat instability is usually caused by:
Most cases are electrical or mechanical in origin rather than firmware-related.
Very common in production systems.
Typical causes:
Many “random C157” events are actually caused by unstable external safety circuits.
Not controller failure.
UR safety architecture uses redundant dual-channel validation.
Both channels must switch inside a tightly controlled timing window.
If synchronization drifts:
Channel A ≠ Channel B
The controller immediately interprets the condition as unsafe.
Even very small timing differences can trigger C157.
This is frequently seen on older safety relays and aging production cells.
Possible contributors include:
Safety LEDs often reveal abnormalities earlier than log analysis.
Watch for:
Experienced field technicians often inspect safety indicators before deep diagnostic logging.
In integrated automation systems, network timing can also contribute to C157.
Possible triggers:
Heavy traffic can delay safety validation timing enough to trigger watchdog protection.
Most common in:
C157 can also originate from unstable 24V safety supply conditions.
Typical situations include:
Possible effects:
Monitor the 24V safety rail during:
Look for:
Many intermittent safety faults begin at the power layer.
Electromagnetic interference is extremely common in industrial environments.
High-risk areas include:
Symptoms often worsen:
Weak grounding can closely imitate safety communication failure.
Inspect:
Many intermittent C157 faults originate in external safety wiring.
Especially in older control cabinets.
Check:
If possible, test with an alternate pendant or cable assembly.
Teach pendant cable degradation is one of the highest-frequency field causes.
Observe live safety signals for:
Milliseconds matter in safety synchronization systems.
Check:
Safety communication must remain deterministic.
Packet delay can trigger watchdog timeout behavior.
Focus on events before the C157 shutdown:
The real trigger usually appears before the displayed stop event.
| Trigger Condition | Probable Cause | Priority Action |
| Power-on immediate C157 | External safety loop open | Check E-stop chain & terminals |
| Trigger when moving pendant | Pendant cable instability | Flex test / replace cable |
| Trigger when external machine starts | EMI / grounding issue | Inspect shielding & grounding |
| Random intermittent C157 | Internal safety board degradation | Cross-test controller / inspect safety module |
In real production environments, intermittent C157 faults are commonly linked to:
A common mistake is treating C157 as a software problem first.
In most field cases, the actual cause exists inside the electrical safety layer.
This is one of the most effective field diagnostic methods for intermittent safety faults.
If the fault disappears during isolation:
→ External safety-chain issue becomes highly likely
If the fault remains:
→ Internal controller or safety module becomes more probable
Simple isolation testing is often faster than extended log analysis.
No.
C157 is a safety integrity event.
Usually related to:
Not URScript logic.
Usually because the root problem is intermittent.
Common examples:
Reboot only resets the state temporarily.
Yes.
Very common.
Includes:
Especially in high-flex areas.
Yes.
Heavy PLC traffic or unstable fieldbus timing can delay safety validation cycles enough to trigger watchdog timeout behavior.
More common in large integrated automation systems.
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