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Heidenhain Controller
System · Intermediate · 30-120 minutes
HEIDENHAIN 231-E250 - E250 Error in cut-out channel Ch: St: St-2ndCh: Sgn
This alarm originates from the control's safety circuit monitoring system detecting an error condition in one of the redundantly-monitored cut-out channels. The TNC uses dual-channel monitoring for safety-critical signals, and when the states between channels don't match or a channel fails to respond correctly, E250 is triggered. This is a hardware-level safety interlock failure requiring immediate attention.
231-E250
HEIDENHAIN 231-E250 - E250 Error in cut-out channel Ch: St: St-2ndCh: Sgn
E250 Error in cut-out channel Ch: St: St-2ndCh: Sgn
The safety self-test detected a fault in a cut-out channel. The error text identifies the affected channel (STO.A.x, STO.B.x, STO.A.G or STOS.A.G) and whether the channel is active when it should be inactive or vice versa.
Root Cause Summary
This alarm originates from the control's safety circuit monitoring system detecting an error condition in one of the redundantly-monitored cut-out channels. The TNC uses dual-channel monitoring for safety-critical signals, and when the states between channels don't match or a channel fails to respond correctly, E250 is triggered. This is a hardware-level safety interlock failure requiring immediate attention.
Safety
- Refer to Heidenhain service manual
- Ensure qualified technician performs repairs
Causes and Fixes
- 1
Document alarm parameters
Write down all parameter values shown: %1 (cut-out channel number), Ch:%4 (channel ID), St:%5 (state channel 1), St-2ndCh:%6 (state channel 2), Sgn:%7 (signal identifier). These are critical for identifying which specific safety circuit is failing.
Take a photo of the alarm screen - these parameters are often needed when contacting Heidenhain support or ordering replacement boards.
- 2
Access PLC status display
Press MOD function, select 'PLC Status' or navigate to Service > PLC > I/O Status. Look for the cut-out channel inputs (typically labeled CUT_OUT_x or EMERGENCY_STOP_x) and verify their actual state matches the machine condition.
Do not bypass or jumper safety circuits for testing without implementing equivalent safety measures.
- 3
Inspect safety circuit connectors
Power down the control and inspect X116 and X117 connectors (emergency stop/safety circuits) on the MC unit. Check for burned pins, loose contacts, or corrosion. Also verify X1 main power connector integrity as voltage issues here affect safety circuits.
Always disconnect mains power and wait 5 minutes for capacitors to discharge before inspecting internal connections.
- 4
Measure cut-out circuit voltages
Using a multimeter, verify 24VDC supply to the safety circuit inputs on the UVR or MC board. Check continuity of emergency stop loop when activated and deactivated. Voltage should be stable 24VDC ±10% without fluctuation.
Use an oscilloscope if available to check for noise or intermittent dropout on the 24VDC safety supply - this often reveals issues not seen with a standard multimeter.
- 5
Test emergency stop circuit
With control powered, activate and release the emergency stop while monitoring PLC status. Both redundant channels should change state simultaneously. If St:%5 and St-2ndCh:%6 don't match, this confirms redundancy circuit failure.
On iTNC 530 and TNC 640, you can enable extended diagnostics in MOD > System > Diagnosis to log state transitions with timestamps.
- 6
Replace MC or PLC board
If all external wiring and voltages are correct but the alarm persists, the cut-out monitoring circuit on the MC or CC board has failed. Order replacement board with your control's specific part number and software version. Transfer PLC configuration after installation.
Before replacing boards, create a full backup of machine parameters via TNCbackup utility or USB export.
Call a technician if:
- ·Safety circuit modifications required
- ·TNC safety module replacement needed
- ·Safety validation testing required
- ·Machine parameter changes affecting safety functions
Prevention
- Perform monthly safety circuit continuity checks
- Monitor safety relay contact wear during PM
- Keep safety circuit connections clean and tight
- Document all safety parameter changes
Common Mistakes
- Bypassing safety circuits permanently
- Ignoring intermittent safety alarms
- Modifying safety parameters without proper validation
- Using non-safety rated components in STO circuits
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Always verify with your machine manual or a certified technician before performing service procedures.