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Fault 18
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Fault 18 — Intermittent IO-Link C/Q Conductor

What you'll learn

**Reported symptom:** The IO-Link sensor stays powered but repeatedly drops to COMLOST when the machine vibrates.

Lab time: ~14 minutes.

Lesson briefing

Intermittent IO-Link C/Q Conductor

Reported symptom: The IO-Link sensor stays powered but repeatedly drops to COMLOST when the machine vibrates.

The circuit was commissioned from a known-good drawing. Treat every terminal as suspect, but test from source to load instead of guessing. Use voltage mode on an energised control circuit and continuity only after the simulated circuit is isolated.

Your task

Use master port diagnostics and a controlled flex/wiggle test to separate device power from communication continuity.

Choose the fault category only when the measurements support it.

Hints

Hint 1

Stable L+ and L− with COMLOST narrows the fault to C/Q, configuration or the device.

Hint 2

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Hint 3

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This lesson uses 7 placed components on the lab canvas. Components are vendor-neutral (no proprietary trademarks); the wiring rules apply to any equivalent industrial part.

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Competency and practice field guide

Intermittent IO-Link C/Q fault lesson: practice plan

Direct answer

The learner can correlate physical movement and cable state with port events, device identity, process-data quality and application fallback.

Written for automation learners diagnosing an IO-Link device that disconnects, falls back or produces stale data intermittently.

an instrumentation calibration bench connecting pressure, temperature, load, level and smart sensors to PLC input channels and reference measurements while studying IO-Link C/Q intermittency, event evidence and robust recovery
System map / 02

NODE 01observable

Scope

Device and port identity, L+, L− and C/Q, connector, cable flex, port current, cycle time, event history, retry, SIO fallback, process-data validity and replacement policy.

NODE 02observable

Signal path

Physical connection through C/Q communications, master port state, identity, process-data update, quality, PLC application fallback and recovery.

NODE 03observable

Baseline practice

Repeated movement and operating cycles maintain identity, fresh valid data and defined application behavior.

NODE 04observable

Edge cases

Cable flex, vibration, contamination, voltage dip, overcurrent, connector movement, master restart, device replacement, reconnect and stale buffer.

NODE 05observable

Fault practice

A power, connector, cable, C/Q, port, identity, event, data-quality, fallback or recovery mismatch.

NODE 06observable

Transfer to the job

The installed cable, connector, master and device inspected and stress-tested under approved representative conditions.

Answer surface / 07

What is the C/Q wire in IO-Link?

It is the communication or switching-signal conductor used between an IO-Link device and master port alongside supply and return.

How do you diagnose an intermittent IO-Link device?

Preserve time-aligned port events, identity, supply, process-data validity and application state while reproducing one controlled movement or load condition.

Motor, wiring and VFD path

Connect the control command to physical motor behavior

Move from safe control wiring and contactors into VFD parameters, measurements, faults and PLC command paths.