231-E190

Check Safety Relay Status

Navigate to MOD function → Machine Parameters → Safety Parameters and verify STO relay status indicators show proper operation.

Ensure machine is in safe state before accessing safety parameters

Measure STO Relay Outputs

Using multimeter, measure voltage at STO relay output terminals. Should read 0V when STO is active, 24V when inactive.

Check both normally open and normally closed contacts of the safety relay

Verify Relay Coil Voltage

Measure 24VDC supply to STO relay coil and confirm proper switching when MC sends enable/disable commands.

Work with qualified electrician when measuring live circuits

Replace Safety Relay Module

If measurements confirm relay failure, replace with identical Heidenhain-approved safety relay module and reconfigure parameters.

Document all parameter settings before replacement for quick restoration

Check Inverter STO Inputs

Verify STO1 and STO2 inputs on each axis inverter are receiving proper 24VDC signals from safety circuit using multimeter.

Both STO inputs must be active simultaneously for proper operation

Verify STO Feedback Signals

Check '-STO.A.G' feedback signals from each inverter at MC input terminals. Should switch from high to low when STO is activated.

Inverter must be powered for STO feedback signals to function

Test Individual Axis Isolation

Disconnect STO signals to each axis individually and monitor which specific axis is not providing proper feedback response.

Use PLC diagnostics screen to monitor real-time STO signal status

Replace Faulty Inverter

Replace inverter that fails to provide proper STO feedback and restore axis parameters from backup.

Ensure proper motor cable shielding and grounding after inverter replacement

Check MC Safety Input Status

Access PLC diagnostics via MOD function → PLC → Input/Output Status and verify STO feedback input states match physical signal levels.

Compare displayed input status with actual measured voltages at MC terminals

Verify Input Circuit Continuity

Check continuity of '-STO.A.G' signal wiring from inverters to MC safety input terminals using ohmmeter with power off.

Disconnect power and verify lockout before continuity testing

Test MC Input Module

Apply known good 24VDC test signal to MC safety inputs and verify proper recognition in PLC diagnostics.

Use isolated 24VDC source to avoid ground loop issues during testing

Replace MC Safety Input Card

If input module fails testing, replace safety input card and verify proper parameter configuration for STO monitoring.

Backup all safety parameters before replacing any MC modules

Verify STO Monitoring Parameters

Check machine parameter MP7300.x (Safety monitoring) and MP7310.x (STO monitoring time) for correct values per machine documentation.

Default STO monitoring time is typically 500ms but may vary by machine type

Check Axis Assignment Parameters

Verify MP1010.x (Axis assignment) matches actual hardware configuration and STO signal routing to correct axes.

Incorrect axis assignment can cause safety system malfunction

Validate Safety Circuit Logic

Review MP7320.x (Safety logic configuration) to ensure STO signal polarity and logic states match hardware implementation.

Document original parameter values before making any changes

Restore Default Safety Settings

If parameters appear corrupted, restore safety parameters from known good backup or factory defaults and reconfigure per machine specifications.

Safety parameter changes require machine validation testing before production use