231-8B40

Power down and inspect connectors

Turn off machine and control power. Inspect X116 (older systems) or X122 (newer iTNC/TNC640) connectors at both the TNC CC unit and the inverter side. Check for bent pins, corrosion, or loose locking tabs.

Disconnect main power and wait 5 minutes for inverter DC bus capacitors to discharge before touching connectors.

Measure continuity of enable lines

Using a multimeter, check continuity of drive enable signal wires (typically pin assignments documented in your specific inverter manual). Verify shield connections are intact.

On TNC640 systems with CC6xxx, the enable signals are on X122 connector pins 13-15. Refer to your inverter's TNC interface documentation for exact pinout.

Reconnect and verify signal

Clean connectors with contact cleaner, firmly reconnect, and power up system. Monitor inverter status during startup sequence to confirm enable signal is received.

Many Heidenhain inverters have diagnostic LEDs that indicate enable signal status—consult your specific inverter manual (UM1xx, UM2xx series) for LED patterns.

Check inverter status display

Examine the inverter front panel LED indicators or LCD display for fault codes. Document all active faults shown on the inverter itself, not just the TNC alarm.

Heidenhain UM and UMC inverters show specific error codes on their displays—these often provide more detail than the TNC alarm alone.

Review inverter error history

Access the inverter's internal error log (procedure varies by inverter model—consult service manual). Look for faults that occurred immediately before or during the drive enable attempt.

On UMC series inverters, error history can be accessed via the service menu on the inverter display by holding the ESC and ENTER buttons during power-up.

Clear inverter faults and retry

If a resettable fault is found (not a hardware failure), clear the inverter fault using the inverter's reset procedure, then attempt to enable the drive from the TNC again.

Do not repeatedly reset faults without identifying root cause—repeated faults indicate a real hardware problem requiring repair.

Test motor and feedback cables

If inverter reports motor or encoder faults, power down and inspect motor power cables and encoder cables for damage, proper shielding, and secure connections at both motor and inverter ends.

Encoder cable shield must be connected to the inverter housing at the gland—floating shields cause encoder noise that can prevent drive release.

Verify safety circuit status

Check that emergency stop is released, all safety door switches are closed, and safety relay (if present) is energized. Measure 24V DC at the safety relay output.

Never bypass safety circuits permanently—only use bypass for diagnostic purposes with machine guarding in place and proper lockout/tagout procedures.

Check STO inputs at inverter

Locate the STO terminals on the inverter (usually labeled STO1 and STO2 or STOA/STOB). Verify 24V DC is present at both inputs when safety circuit is closed. Check wiring schematic for proper connection.

STO inputs require both channels to be energized independently for redundancy. A single broken wire in either channel will prevent drive release.

Test with temporary STO bridge

If no safety system is required for your application, verify STO terminals are properly bridged according to inverter manual. For diagnostic purposes only, temporarily bridge STO inputs with appropriate rated jumpers and test drive enable.

Bridging STO for testing should only be done with machine completely secured and by qualified personnel. Remove bridge and restore proper safety circuit immediately after testing.