Troubleshooting Siemens SIMODRIVE 611 Spindle Relay Function 1 (Terminal A11) on Gildemeister G-20

Engineers maintaining Gildemeister G-20 turning centers that pair a Siemens SIMODRIVE 611 spindle drive with a SIMATIC S5-115U controller will encounter a recurring fault pattern in which the spindle runs for approximately ten seconds and then drops out, even though every interlock and the infeed module enable read healthy. In nearly every reported instance of this pattern, the visible-but-no-voltage condition on free-programmable relay output (terminal A11 on control module 6SN1121-0BA11-0AA1) is the load-bearing clue. The article below works that fault from first symptom to verified re-start, with field-test steps that can be performed with a hand multimeter and the three push-buttons on the drive.

1. Problem Summary

The classic fault signature, observed in the field on Gildemeister G-20 platforms but applicable to any SIMODRIVE 611 / S5-115U pair wired to use Relay Function 1, is the following:

• Spindle run command is accepted from the operator panel or from the part program.
• Spindle accelerates, holds a setpoint, then drops out at roughly ten seconds after the run edge.
• 7-segment display on control module 6SN1121-0BA11-0AA1 shows the Relay Function 1 segment illuminated throughout the run.
• Voltage at terminal A11 to the SIMATIC S5-115U digital input measures 2 VDC instead of the nominal 24 VDC expected for a logic "1".
• Infeed module enable is verified present; machine interlocks read healthy; spindle enable from the PLC is high.

The 10-second window is the strongest tell. Spindle drops driven by thermal overload or encoder fault are stochastic and accumulate with load; a precise 10-second window points at a parameterized timer, a setpoint enable window, or a relay function condition that becomes false at that elapsed time. The 2 VDC residual on terminal A11 separates this fault from a PLC handshake problem, a wiring break, or a stuck enable.

2. SIMODRIVE 611 Architecture and the S5-115U Interface

The SIMODRIVE 611 family is a modular digital drive system in which power conversion, closed-loop control, and the operator interface are split across plug-in modules that share a common DC bus. The Gildemeister G-20 spindle axis uses the following slot pattern in the 611 rack:

The spindle enable handshake to the SIMATIC S5-115U is wired point-to-point through the NE module terminal block. Three signals form the critical path:

1. Spindle enable from PLC (input to NE module) — typically terminal on the X-axis or setpoint enable input.
2. Drive healthy / status word (output from NE module) — typically a free-programmable relay function such as Relay Function 1 on terminal A11.
3. Fault / alarm signal (output from NE module) — typically a second free-programmable relay function on terminal A12 or A13.

The PLC writes the spindle enable, monitors Relay Function 1 as the "spindle ready" handshake, and monitors the fault relay as the "spindle not faulted" handshake. If Relay Function 1 de-asserts while the run command is still active, the PLC will drop the spindle enable on the next OB1 scan, producing the controlled shutdown observed at 10 seconds.

3. Control Module 6SN1121-0BA11-0AA1 Identification

Before any parameter work, verify the module identity. The 6SN1121 family has many sub-variants that differ in firmware and option set; mis-identifying the part leads to looking at the wrong parameter ranges.

3.1 Nameplate Reading

The nameplate is on the front face of the NE module, below the 7-segment display. Confirm the following fields match the machine's electrical drawing:

3.2 7-Segment Power-Up Sequence

With the infeed module enabled and the DC link charged, the NE module runs a self-test then displays firmware version and module status. The exact sequence varies with firmware revision, but the pattern is: blank → dash segments → firmware version r.x.x → horizontal bars → idle state. If the display sticks on a number, letter, or a fault code at this point, do not proceed to parameter work — record the code and resolve the power-up fault first. Reference for fault codes is the SIMODRIVE 611 family documentation on the Siemens Industry Online Support portal.

4. Relay Function 1 and Terminal A11 Operating Logic

The free-programmable relay functions in the SIMODRIVE 611 firmware are software-defined output conditions, not hard-wired contacts. The NE module evaluates the assigned condition every controller cycle (typically 1 ms to 4 ms depending on configuration) and drives the hardware output (a transistor or relay contact, depending on the option variant) at the assigned terminal. The mapping from function to terminal is a parameter setting, not a wiring choice.

For a 6SN1121-0BA11-0AA1 used as the spindle NE module on a G-20, the typical mapping is:

If Relay Function 1 is configured for a condition such as "speed within tolerance window", and the spindle drops below the threshold for any reason — including a transient under-speed during a load peak, an encoder glitch, or a torque limit trip — the relay de-asserts, the S5-115U reads logic 0, and the PLC drops the spindle enable. The observed 10-second interval is then either:

• The configured setpoint enable window that expires at 10 s, or
• The PLC's debounce / scan delay between the relay de-assertion and the spindle enable drop, or
• A ramp-function generator time parameter set to 10 s in the original commissioning.

Field-proven resolution on identical G-20 platforms is to re-assign Relay Function 1 to a condition that remains true for the duration of the run — typically "drive healthy" or "pulses enabled" — and to address the upstream 10-second timer separately. Re-assignment alone does not cure a parameter-driven soft timeout.

5. Root Cause Analysis: Interpreting 2 VDC on Terminal A11

2 VDC on a sourcing output is the most informative measurement in this fault tree. Work the hypothesis table before any module removal:

5.1 Definitive Field Test

This five-minute procedure separates a module failure from a wiring or PLC failure:

1. With the drive idle and the infeed enabled, isolate the wire at terminal A11 at the PLC input card terminal block (do not disconnect at the NE module).
2. Connect a multimeter (DC volts, 50 V range) between the NE module terminal A11 and terminal 0V (or module frame ground).
3. Read the voltage. This is the unloaded module output.
4. Command a spindle run from the operator panel; observe the meter.
5. Compare the loaded (PLC connected) and unloaded (PLC disconnected) voltage readings.

5.2 Why 2 VDC Specifically

The 6SN5450 / 6ES5451 / 6ES5455 S5-115U digital input modules source a small wetting current through an optocoupler LED. When the NE module output transistor is off, this wetting current flows from the 24 V supply through the input optocoupler, into the module output pull-down network, and to 0V. The voltage divider across the off-state output produces the observed 2 VDC reading. The reading is therefore a fingerprint of "transistor off with PLC input still wired", not a measurement of the transistor leakage spec.

6. Reading and Logging Parameters via the Three-Button Menu

The NE module's three push-buttons (P, +, –) provide full access to the parameter tree without a PC. The standard navigation pattern is:

1. Press P to enter parameter mode. The 7-segment shows the current parameter number.
2. Use + and – to step through parameter numbers.
3. Press P a second time to display the parameter value (decimal integer by default).
4. Use + and – to change the value. The new value flashes; no commit yet.
5. Press P to commit and return to parameter number display. Hold P to abort.

6.1 Write Protection

By default the parameter set is read-only from the front panel. Write protection is enforced by an access-level parameter that defaults to a level that allows reads but not writes. The procedure to clear write protection varies by firmware version; the general pattern is:

1. Navigate to the access-level parameter (commonly a parameter in the P06xx or P65xx range, depending on firmware).
2. Read the current access level. Note the value.
3. Change the value to the appropriate expert or commissioning level for the firmware in use.
4. Commit. The 7-segment should now show a different "P" indicator or display the value without the read-only flag.
5. Restore the original access level after all writes are complete.

6.2 Logging Procedure (First-Time Documentation)

Maintenance crews that have never logged the parameter set should perform the following:

1. Power the drive and enable the infeed; allow the DC link to charge and the NE module to complete its power-up sequence.
2. Starting from parameter P0000 (or the parameter tree root for the firmware), walk every parameter number in sequence.
3. Tabulate number, value, units, and a short description in a spreadsheet. The units are documented in the parameter manual for the specific firmware version.
4. Pay specific attention to:
   1. Relay function assignment parameters (functions 1, 2, 3, and any others enabled)
   2. Setpoint enable / pulse enable timing
   3. Ramp-function generator times (accel, decel, initial/final round)
   4. Motor and encoder data block
   5. Speed and torque limits
   6. Watchdog and timeout parameters
5. Save the log with date, firmware version, hardware revision, and the technician's initials.
6. After any configuration change, re-walk the affected range and append a new dated log entry.

This log is the safety net for both in-house reconfiguration work and bench repair. Send the log with any module that is removed from the machine — a repair vendor with no parameter reference must default the module, which forces a full re-commissioning on return.

7. Field-Programmable Relay Function Reconfiguration

If the parameter log shows that Relay Function 1 is assigned to a condition that becomes false during the run cycle, the function can be re-assigned. The general procedure:

1. Identify the parameter that maps Relay Function 1 to its current condition. The parameter name is firmware-specific; in the SIMODRIVE 611 family it is typically in the relay function configuration group.
2. Read the current value. Note it for the log.
3. Select an alternative condition that remains true for the duration of a spindle run. Typical safe choices:
   1. "Drive healthy / no fault active"
   2. "Pulses enabled"
   3. "DC link OK AND no fault" (composite condition if supported)
4. Write the new value and commit.
5. Restore the access-level protection.
6. Cycle the drive (DC link discharge, re-enable) to load the new configuration into the runtime.
7. Verify with a controlled run that the S5-115U input corresponding to A11 reads 24 VDC throughout the run window.
8. Update the parameter log with the new value and the date.

Reassignment does not re-commission the drive. However, the 10-second shutdown root cause is often a separate timer parameter, and the parameter log must be walked for any other timing-related value set to 10 s, 10000 ms, or 10.0 s. A 10-second ramp, a 10-second enable window, and a 10-second setpoint hold are all common candidates; the value in milliseconds is the one to inspect.

8. Module Removal, Repair, and Refit Decision Matrix

If the 2 VDC reading persists with the PLC field wire disconnected and the relay function condition is verified to be true, the NE module output stage is damaged and the module requires bench repair. Use this matrix to choose the path:

8.1 Pre-Removal Checklist

1. Capture the parameter log per Section 6.
2. Photograph the nameplate, the wiring at A11, and the PLC input card terminal block.
3. Note the infeed module serial number and the power module serial number — the repair vendor may want the full rack context.
4. Label every field wire on the NE module terminal block before disconnection. A photograph is not a substitute for a wire tag.
5. Power down in the correct sequence: spindle stop → drive enable off → controller stop → infeed disable → DC link discharge confirmed → main disconnect open → LOTO applied.

8.2 Documentation to Ship with the Module

• Parameter log (printed, dated, signed)
• Machine model and serial number (Gildemeister G-20 plus the manufacturer serial plate)
• Description of the fault including the 2 VDC measurement and the 10-second shutdown behavior
• Request that the vendor back up the parameter set before any work and re-flash the firmware to the documented version on return
• Return shipping address and a contact for technical questions

8.3 Field Repair Considerations

For a 6SN1121-0BA11-0AA1 with a failed output transistor, field repair is technically possible for a technician with surface-mount rework capability and the Siemens component data, but it is not recommended:

• The PCB is multi-layer with internal ground planes; rework risk is high.
• Calibration of the relay output stage requires a test box that simulates the load.
• A failed repair voids the module for the next 20 years of service.
• Bench repair at an authorized center typically costs a fraction of a new module and returns the unit with a warranty.

9. Reinstallation and Commissioning Verification

After the module is returned (or replaced with a unit flashed to the documented firmware and pre-loaded with the parameter set), follow this verification sequence before releasing the machine to production:

9.1 Mechanical Refit

1. Confirm the module is fully seated in the rack and the rear connector is engaged.
2. Tighten the front-panel screws to the specified torque (hand-tight plus a quarter turn; do not over-torque).
3. Reconnect the field wires per the wire tag, tightening to the terminal block torque spec.

9.2 Power-Up

1. Remove LOTO and re-energize the main disconnect.
2. Enable the infeed; observe the DC link pre-charge on the infeed module indicators.
3. Watch the NE module 7-segment power-up sequence; confirm the firmware version matches the pre-removal log.
4. Confirm no fault code is displayed at idle.

9.3 Parameter Check

1. Re-walk the parameter tree and confirm the relay function assignments and timer parameters match the pre-removal log.
2. If the module is a replacement, re-load the parameter set from the log and re-flash the firmware to the documented version.
3. Restore the access-level protection.

9.4 Static Test

1. With the spindle disabled at the PLC, force Relay Function 1 to true via the parameter (a feature available on most firmware versions for output testing).
2. Measure voltage at A11 with a multimeter. Expect 24 VDC nominal.
3. Release the force; measure again. Expect 0 to 2 VDC.

9.5 Dynamic Test

1. Command a low-speed spindle run from the operator panel.
2. Monitor the A11 voltage on a clamp meter or a scope for the full window previously associated with the fault.
3. Confirm the voltage remains at 24 VDC throughout.
4. Command stop; confirm the voltage falls cleanly to 0 to 2 VDC.

9.6 PLC Handshake Verification

1. From the S5-115U programming device (or PG with Step 5), monitor the input status word bit corresponding to A11.
2. Confirm the bit transitions correctly at run start, run stop, and fault clear.
3. Confirm the PLC's spindle enable output remains high as long as the A11 bit is high.

9.7 Functional Test

1. Run a complete part program covering all spindle speed ranges used in production.
2. Confirm the spindle executes every range without the 10-second shutdown.
3. Run the program at least three consecutive times to catch any intermittent behavior.

10. Preventive Maintenance and Documentation Discipline

The reliability of a SIMODRIVE 611 / S5-115U pair on a Gildemeister G-20 depends on consistent documentation. The following practices are field-proven on identical platforms:

• Maintain a printed parameter log inside the electrical cabinet, updated on every change. The log must be dated, signed, and stored in a sealed sleeve to survive the cabinet environment.
• Photograph the module nameplate, the wiring at A11, and the PLC input card terminal at commissioning. Store the images in the machine's documentation folder with a checksum or hash to detect tampering.
• Schedule a 6-month verification of relay function outputs using the static test method in Section 9.4. A relay that reads 23.5 VDC today may read 22.0 VDC in six months as the output transistor degrades; catching the trend prevents the next unplanned stop.
• Keep a known-good spare 6SN1121-0BA11-0AA1 on the shelf, flashed to the matching firmware and pre-loaded with the parameter file. This minimizes MTTR when a module fails outside the repair center's turnaround window.
• Train at least two maintenance technicians on the three-button parameter navigation and write-protection procedure. A single wrong write during a fault diagnosis can extend the outage by days.
• Track firmware versions across the spare and the installed module. A spare flashed to a different firmware than the installed unit will parameter-mismatch on install.

11. Diagnostic Quick-Reference Matrix

12. Frequently Asked Questions