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Inspect HSCI cable connections

Power off the system. Check all HSCI cable connections between TNC and inverter module. Verify the distinctive Heidenhain HSCI connectors (typically Sub-D or circular connectors with locking mechanism) are fully seated and locking tabs engaged. Inspect cable for physical damage, especially near connectors.

Always power down control and drives before disconnecting HSCI cables to prevent bus errors or component damage.

Verify cable shield continuity

Using a multimeter, verify cable shield continuity from end to end. HSCI cables require intact shielding for noise immunity. Check that shield is properly connected to backshells at both ends.

Heidenhain HSCI cables are directional in some configurations - verify 'IN' and 'OUT' markings if present on connectors match the signal flow direction.

Test with known-good cable

If available, temporarily replace the HSCI cable with a known-good spare cable of appropriate length. Heidenhain specifies maximum HSCI cable lengths - typically 10m per segment without repeaters.

Keep a spare HSCI cable in your service kit - these proprietary cables are a common failure point and cannot be field-fabricated reliably.

Check inverter status LEDs

Inspect the inverter module front panel for status LED indicators. Heidenhain inverters typically show green for normal operation, red for fault conditions. Verify at least the control power LED is illuminated.

Do not access inside inverter enclosures with power applied - lethal voltages present on DC bus capacitors.

Verify inverter supply voltages

Check that main 3-phase AC power is present at inverter input terminals. Verify DC bus voltage is within normal range (typically shown on inverter display if equipped). Confirm 24VDC control power supply is present and within specification (22-28VDC).

Inverters may appear powered but have failed DC bus pre-charge circuits - check DC bus voltage explicitly, not just AC input.

Review inverter fault history

If the inverter has its own diagnostic display or interface, check fault history for power-related faults or HSCI communication errors that occurred before or during TNC startup.

Inverters with undervoltage or overvoltage faults may disable HSCI communication as a protective measure.

Check TNC HSCI configuration

Enter MOD mode, navigate to System Parameters. Access the HSCI device configuration section (exact path varies by TNC model: on iTNC 530/TNC 620 check under 'General Parameters' > 'HSCI Configuration'). Note the device address configured for the inverter (typically addresses 1-15).

Document the entire HSCI device map while you're in MOD - this helps identify which specific device address is missing.

Verify inverter physical address

Locate the address setting mechanism on the inverter module. Older Heidenhain inverters use rotary DIP switches or BCD switches on the front panel or side. Newer models may have address set via parameter. Verify this matches the TNC configuration exactly.

Do not change addresses while power is applied - set addresses with system powered down.

Check for duplicate addresses

Verify no two devices on the HSCI bus share the same address. Duplicate addresses cause unpredictable communication behavior and may cause one device to appear missing.

Use the TNC PLC Diagnostics screen to view all responding HSCI devices - compare this list to your configuration to identify the specific missing address.

Correct and re-initialize

If address mismatch found, correct either the physical inverter address or the TNC configuration to match. After correction, perform a complete power cycle (not just NC reset) to force full HSCI enumeration.

After address changes, the TNC may require cycling through PLC Start mode - wait for full PLC initialization before declaring success.