Sinumerik 810T Feed Override Jumps to 150%: Diagnosis and Repair

Problem Statement

A Sinumerik 810T GA1 turning control exhibits a dangerous feed-rate override anomaly: when the operator rotates the handwheel-style feed override switch from 0% (minimum) through 120% (maximum), every detent produces the expected percentage — except at the position that should correspond to 6%, where the control instantly commands 150% feed rate. The fault has been present since the machine was acquired. Mechanical remediation (disassembly, cleaning, wiper realignment, and re-soldering of every terminal) has been performed twice without effect, which rules out switch contact noise as a primary cause and shifts attention to the control's interpretation of the switch position.

Sinumerik 810T Feed Override Architecture

The Sinumerik 810T is a turning (T) variant of the 810 family, with hardware/software generations GA1, GA2, and GA3. Feed-rate override is implemented as a closed loop between the operator panel, the integrated PLC (STEP 5 compatible), and the NC kernel.

The signal path is:

1. Operator rotates the 16-position (typical) rotary switch on the machine panel.
2. The switch presents a 4-bit Gray-coded parallel value on one PLC input byte.
3. The PLC user program decodes the Gray code, applies clamping/limit logic, and writes a percentage into the NC's feed-override interface.
4. The NC kernel multiplies the programmed feed (F-word) by the percentage and outputs the result to the axis interpolators.
5. Machine Data (MD) tables define the upper/lower clamp, the maximum rapid override, and the scaling curve applied to each switch position.

Because the mechanical switch has been confirmed healthy, the fault must be in one of: (a) a corrupted MD that scales the suspect position, (b) the PLC decoding/scaling table, or (c) a stuck/dual-contact input bit that produces an invalid Gray-code word that maps to 150%.

Root Cause Candidates

For a single-detent fault (the 6% detent), the most likely root cause is either a defective bit in the switch wiper or a corrupted entry in the PLC decode table / NC MD that maps that specific Gray-code word to a percentage. The source technician's diagnostic suggestion is to check NC MD 103 — if its value reads 150 at the suspect position, MD 103 is the table entry for that detent and must be corrected to the expected percentage.

Safety Considerations Before Diagnostics

1. Place the main disconnect in OFF position and lock/tag it.
2. Bring up the control in JOG mode only, with the spindle drive enable disabled at the drive (open the drive enable contactor or remove the 24 V enable).
3. Confirm the axis enable relays drop by attempting a rapid traverse — the axis must NOT move.
4. Display the PLC status page (DIAGNOSTIC / PLC STATUS on 810T) with the override switch in the suspect detent.

Diagnostic Step 1 — PLC Status Display

The feed override rotary switch is hard-wired to one PLC input byte. On 810T systems, this byte is most commonly located in the operator-panel input range (typically IB 96 through IB 111 in a default configuration). The exact address is machine-specific and must be located from the wiring diagram and the PLC I/O assignment list.

Procedure:

1. From the main menu, navigate to DIAGNOSIS > PLC STATUS.
2. Select INPUT BYTE view and enter the suspected override byte address.
3. For every switch detent, record the displayed byte value in a log table. Include the suspect 6% detent.
4. Compare the sequence against the expected Gray-code progression. A single-bit error (one bit stuck LOW or HIGH) will produce an invalid code that, when decoded, jumps to an extreme percentage such as 150%.

Expected Gray Code Table (4-bit, 16-position)

Percentages above assume a 0–120% range mapped across the switch; some 810T implementations use a 23-position switch with finer resolution. Always cross-check against the actual ladder decode table.

Inline Gray-Code Decoding Flow

Diagnostic Step 2 — Confirm Switch vs. Control Layer

With the PLC status byte recorded at every detent, compare the suspect detent against the expected Gray-code progression. Three outcomes are possible:

Diagnostic Step 3 — Check NC Machine Data MD 103

On Sinumerik 810T, machine data is organized into ranges:

• MD 1–99: general machine data (display, mode group, channel settings)
• MD 100–199: channel-specific data (feed, spindle, auxiliary functions)
• MD 200+: axis-specific data

NC MD 103 falls within the channel-specific range and, in the affected machine, behaves as the scaling entry for the feed-override switch position table. When the operator selects the 6% detent, the PLC hands the NC a percentage value. If MD 103 contains the value 150 instead of the intended low percentage (commonly 6 or 5 depending on the detent count), the NC will command 150% feed regardless of the PLC's intent.

Procedure to inspect:

1. Navigate to SETUP > MD (or press the MD softkey on the operator panel).
2. Enter MD number 103.
3. Read the displayed value. If it reads 150, MD 103 is corrupt and is the root cause.
4. Record the value on the data sheet, including the field's axis assignment and the active channel.

Diagnostic Step 4 — PLC Program Decode Inspection

If MD 103 is correct but the fault persists, the decode table inside the PLC user program is the next suspect. The 810T PLC is programmed in STEP 5; the feed-override handler typically lives in a function block (FB) called by the cyclic OB 1.

Common block layout on 810T:

Use the online STEP 5 monitor (or the 810T built-in PLC STATUS function block view) to step through the decode segment. The expected logic pattern is:

FUNCTION_BLOCK FB 10
VAR
  IB_Raw      : BYTE;     // Gray code from switch
  FB_Pct      : WORD;     // Output % to NC
  MD103_Lim   : WORD;     // Loaded from MD 103
END_VAR

BEGIN
  // 1. Gray -> binary conversion
  IB_Bin := GRAY_TO_BIN(IB_Raw);

  // 2. Lookup in decode table (DB 20, 16 WORDs)
  FB_Pct := DB20.DW[IB_Bin];

  // 3. Apply upper clamp (MD 103 sets the maximum)
  IF FB_Pct > MD103_Lim THEN
    FB_Pct := MD103_Lim;
  END_IF;

  // 4. Lower clamp at 0
  IF FB_Pct < 0 THEN FB_Pct := 0; END_IF;

  // 5. Hand to NC interface flag byte
  FY[FC_DBB];  := FB_Pct;
END_FUNCTION_BLOCK

If MD 103 acts as the upper clamp rather than a per-position table entry, a value of 150 would clamp every detent, not just one. The single-detent signature therefore indicates that MD 103 in this machine is a per-position entry, not a clamp — or that the PLC reads MD 103 only for the suspect detent. Either way, MD 103 = 150 is wrong.

Repair Procedure

Repair A — Correct NC MD 103

1. Turn the customer key-switch to the position that enables MD writes (typically position 1 or higher on 810T).
2. From the MD display page, enter MD 103.
3. Replace the value 150 with the expected percentage for the detent (typically 6, or the value matching the printed legend on the switch cap).
4. Press INPUT or the equivalent confirm softkey.
5. Cycle the control power to commit the new MD to non-volatile RAM. On 810T, MR (Machine data Random) is normally backed up by battery-backed RAM (BR), but a power cycle ensures the bootstrap reloads the new value.

Repair B — Recover PLC Decode Table

If MD 103 is correct but the PLC decode table is corrupt:

1. Connect the STEP 5 programmer to the 810T serial port (RS-232, 9600 8N1 typical for PG 685/PG 720).
2. Upload DB 20 and FB 10/11 to the programmer.
3. Compare against a backup or the commissioning archive. If no archive exists, rebuild DB 20 with the documented scaling: [0,5,10,15,20,25,30,35,40,45,50,55,60,65,70,75] for a 0–75% range, or the appropriate curve for a 0–120% range.
4. Download the corrected blocks. Observe the PLC STOP / RUN handshake: 810T must be in STOP before download and cycled to RUN.

Repair C — Switch Wiring (only if Step 2 indicated a bit error)

1. Disconnect the switch harness from the PLC input card to isolate the panel.
2. Use a DMM in continuity mode and rotate the switch through all positions. Record the resistance between the common terminal and each bit line at each detent.
3. For Gray code, exactly one bit should transition between adjacent positions. Two or more simultaneous transitions indicate a short between bit lines.
4. Repair any cold solder joints, chafed wires, or pinched insulation. Re-insulate with heat-shrink and re-test.

Verification Testing

After the repair, perform a controlled verification:

1. Power up the control in JOG mode with drives disabled.
2. Rotate the override switch through every detent, 0% → 120% and back.
3. Watch the PLC status byte and the NC override display. Each detent must show the expected Gray code and percentage.
4. At the previously suspect 6% detent, confirm the override display reads the expected low value and not 150%.
5. Re-enable the drives. Run a dry-cycle program at low programmed feed (G0 + G1 with F = 50 mm/min). With override at 6%, the commanded feed should be 3 mm/min — verify on the actual axis with a tachometer or by timing a known distance.
6. Increment override to 100%, then to 120%, and confirm axis velocity scales linearly.

Related Parameters and Alarms

The alarm numbering above is consistent with legacy Sinumerik alarm conventions; specific alarm numbers vary by software version and must be cross-checked against the operator panel displayed message.

Sinumerik 810T Generations and Software Differences

The 810T GA1, GA2, and GA3 generations differ in:

• CPU board (different backplane variants)
• PLC instruction set (some blocks were renumbered between GA1 and GA3)
• MD numbering and resolution
• Memory layout (BR battery-backed RAM size)

If the diagnosis above fails on a GA2 or GA3 machine, the MD numbering for the feed-override table may be different. Always start from the as-built MD list (printed commissioning sheet) shipped with the machine, not from generic documentation.

Documentation Pointers

For commissioning and service work on legacy Sinumerik controls, refer to Siemens Industry Online Support:

• Siemens Industry Online Support — legacy SINUMERIK documentation entry point
• SINUMERIK 810T GA1/GA2/GA3 Operator Panel Manual (functional description)
• SINUMERIK 810T GA1/GA2/GA3 PLC Programming Manual (STEP 5 interface)
• SINUMERIK 810T GA1/GA2/GA3 Machine Data List

Exact support contract availability and live links for these legacy manuals depend on the active Siemens service agreement. The titles and approximate entry points are correct at the time of writing; if a link returns an "item not found" error, search the Siemens support portal with the document title.

Troubleshooting Matrix

FAQ