Heidenhain TNC Alarms: The Field Engineer's Guide to iTNC 530 and TNC 640 Errors

Heidenhain controls are different from Fanuc or Siemens. The alarm system is built around a structured number range that tells you exactly which subsystem is throwing the fault before you even look at the message. Once you understand the structure, you stop guessing and start diagnosing.

This guide covers the alarm architecture on the iTNC 530 and TNC 640, the most common alarms you will see in the field, and how to work through them efficiently.

How Heidenhain Alarm Numbers Are Structured

Every Heidenhain alarm has a hex-based number in the format XXX-XXXX. The first three digits identify the module or subsystem. The last four digits identify the specific fault within that module.

Common module prefixes:

• 120-xxxx — Operating system and software configuration errors
• 125-xxxx — Communication and network faults
• 220-xxxx — Tool management errors
• 230-xxxx — Axis and encoder configuration faults
• 231-xxxx — Servo drive and motor faults (most common in field service)
• 237-xxxx — Position encoder and feedback faults
• 330-xxxx — PLC and machine interface errors
• 600-xxxx — NC program and cycle errors

When you see an alarm, the prefix tells you where to start looking. A 231-xxxx alarm is a drive or motor issue. A 330-xxxx alarm is a PLC ladder problem. This alone saves time on unfamiliar machines.

231-xxxx Servo Drive Alarms

These are the most common Heidenhain alarms in production environments. They point to servo amplifier faults, motor problems, or encoder feedback issues.

231-C350: Axis Module Not Ready

This alarm fires when a drive axis module fails to initialize or loses its ready signal during operation.

Most common causes:

• Drive module internal fault — check the LED status on the CC (Controller Card) unit
• 24V supply voltage drop to the drive module — measure at the module connector, not at the cabinet power supply
• Loose or corroded signal connector between CC and the axis module
• Drive module overtemperature — feel the module and check cabinet ventilation

Diagnostic steps:

1. Note which axis is flagged. The alarm message will show the axis name.
2. Power down and reseat the drive module connector at both ends.
3. Check the green/red LED on the axis module. Solid red means internal fault — module needs replacement.
4. Verify 24V at the module supply pins with a multimeter.
5. If intermittent, check cabinet temperature and clean any blocked air filters.

231-C370: Angle Error Motor Encoder

The control measured an error in the motor encoder commutation angle at the reference position. This is a critical safety alarm — the control cannot ensure accurate positioning.

Most common causes:

• Motor encoder cable damaged or shielding compromised
• Motor encoder replaced without proper commutation angle teach-in
• EMI interference on the encoder cable from nearby high-current conductors

Diagnostic steps:

1. Inspect the encoder cable from the motor to the CC unit for damage, kinking, or pinch points.
2. Check the connector at the motor end — this is the most common failure point on Heidenhain encoders.
3. If the motor was recently replaced, verify that the commutation angle teach-in procedure was completed per the Heidenhain documentation.
4. If cable looks good, substitute a known-good encoder cable as a test.

231-C3A0: Incorrect Reference Position

The axis reached its reference point but the position does not match the stored reference value within tolerance.

Most common causes:

• Reference switch contamination or misalignment
• Encoder glass scale contamination or damage
• Machine crash that moved the scale or reference switch
• Software parameter for reference offset incorrect after scale replacement

Diagnostic steps:

1. Clean the reference switch and check its mounting — it must be secure with no play.
2. Run a slow manual reference traverse and watch the position display during approach. It should be smooth and consistent.
3. Inspect the linear scale or rotary encoder for contamination, particularly on the read head.
4. Compare the reference position parameter (MP settings) against the original commissioning data.

237-xxxx Position Encoder Alarms

These alarms relate to the linear or rotary position feedback — scales, encoders, and their signal quality.

237-3801: Telegram Counter Error (UM-FSSW)

A serial communication error between the iTNC and the encoder interface. The data frame counter does not match expectations, indicating transmission errors.

Most common causes:

• Damaged HEIDENHAIN cable between the encoder and the IK/SEA interface board
• Incorrect termination on long cable runs
• EMI from servo cables running parallel to encoder cables in the cable chain
• Faulty encoder interface board (IK 220 or similar)

Diagnostic steps:

1. Check cable routing — encoder cables should never run parallel to power or servo cables in the cable chain.
2. Inspect the cable for damage at bend points, particularly where it enters the cable chain.
3. Measure cable length against maximum specs for the encoder type. Exceeding cable length causes reliable signal degradation.
4. Swap the encoder interface board if cable checks pass.

330-xxxx PLC and Machine Interface Alarms

These alarms come from the machine builder's PLC logic, not from Heidenhain directly. The messages are usually written by the machine builder in their language of choice.

A 330-001A alarm showing "Input (ES.B) not equal to 0" means the PLC is reading an input in an unexpected state. The diagnostic approach:

1. Open the PLC diagnosis in the TNC (MOD > PLC > Diagnosis or the machine-specific menu path).
2. Identify the specific input or marker referenced in the alarm message.
3. Trace the input back to the physical signal source — safety relay, limit switch, pressure switch, etc.
4. Check if the signal is correct at the source using a multimeter or the PLC input monitor.

The 330-xxxx range requires machine-specific documentation from the builder. Heidenhain publishes the base alarm descriptions, but 330-xxxx messages are almost always customized by the OEM.

Common Mistakes When Diagnosing Heidenhain Alarms

Replacing drives before checking cables. The majority of 231-xxxx alarms are cable or connector issues, not drive failures. Always check the cable first.

Ignoring the module LEDs. Every Heidenhain drive module has LED status indicators. They will tell you if it is an overcurrent fault, overtemperature, or communication fault before you waste time measuring voltages.

Skipping the power cycle. Some Heidenhain alarms are latched and will not clear until a full power cycle including the control cabinet main switch. A simple emergency stop reset is not sufficient.

Not checking the Heidenhain Service App. The iTNC 530 and TNC 640 have a built-in diagnostic tool accessible via the MOD key. It logs alarm history with timestamps, which is invaluable for tracking intermittent faults.

Using AxisMD for Heidenhain Alarm Lookups

AxisMD has a full searchable database of Heidenhain alarm codes with root cause analysis for every alarm in the 231-xxxx, 237-xxxx, and 330-xxxx ranges.

When you hit an alarm on the floor, look up the exact code in the Heidenhain alarm database to get field-tested causes and step-by-step procedures without digging through a 400-page manual.

The faster you diagnose, the faster the machine is back in cut.