Troubleshooting A133 Alarm on Siemens Masterdrive 6SE7022-6TP50 Lower Feeder Drive

The SIMOVERT MASTERDRIVES 6SE70 series (Compact and Chassis units) remains in service on cut-to-length lines, slitters, and roll-forming equipment worldwide. A recurring call for help on these lines involves alarm A133 appearing on a lower feeder drive such as the 6SE7022-6TP50 while the line is mid-traverse. The drive typically trips, the line stops, and the operator sees the alarm string on the PMU or OP1S panel. This reference documents the exact meaning of A133, the control-word bits that produce it, the parameters that select the source for those bits, and a complete diagnostic procedure to clear the fault and verify the repair.

1. Problem Overview

Symptom pattern reported in the field on a cut-to-length line:

• Upper feeder and lower feeder are accelerating or running to feed core steel into the cut press.
• While both motors are performing a traverse (travel) command, production stops abruptly.
• Operator panel or PMU shows alarm A133 on the lower feeder drive 6SE7022-6TP50.
• Alarm is latched in r947 (fault buffer) and

  r949 (alarm buffer).

• Press FN+R on the PMU or issue a 0000 on the automation interface to acknowledge, but A133 reappears on the next traverse cycle if the root cause persists.

Because A133 trips the OFF3 fast-stop path, the motor decelerates on the OFF3 ramp (parameter p464) and the line loses synchronization between the two feeders. Product length, squareness, and cut tolerance degrade for the cycle that fails.

2. 6SE7022-6TP50 Drive Identification

The 6SE70 family is the SIMOVERT MASTERDRIVES Vector Control (VC) and Motion Control (MC) generation, superseded by SINAMICS but still supported by Siemens spare-parts and repair centers. The model number 6SE7022-6TP50 decodes as follows:

For application matching, consult the SIMOVERT MASTERDRIVES Vector Control Compendium (E33205-K8000-A100-A1) for output current versus motor rating across the 6SE70 range. Refer to the 6SE70 Operating Instructions manual on the Siemens Industry Online Support portal for the full type plate, derating curves, and terminal assignments.

3. A133 Alarm Definition and Trigger Conditions

Alarm A133 on the 6SE70 platform is a control-word diagnostic alarm. The trigger logic is:

A133 = (Control Word bit 0 = 1, ON command active) AND (Control Word bit 2 transitions from 1 → 0 during traverse)

In plain terms: while the drive is being told to run (bit 0 = 1) and a traverse (movement) profile is active, the OFF3 command (bit 2) is being removed — i.e., the bit is going low when the master expects it to remain high. Because OFF3 is low-active, removing it is the actual OFF3 command, and the drive responds by ramping down the motor on the OFF3 deceleration ramp.

3.1 Effect on the drive

• The motor decelerates from current setpoint to zero following p464 (OFF3 ramp-down time, default 10 s).
• After p465 (OFF3 wait time before pulse cancellation, default 0.5 s) the gate pulses are cancelled.
• Status word bit 2 (drive running) clears; status word bit 1 (OFF2 active) or bit 0 (ready) is updated per state machine.
• A133 is logged in r947 / r949 to allow diagnostics. The drive is left in Operation Enabled state as soon as the operator toggles the control word correctly.

3.2 Difference between A133 and related codes

Alarms (Axxx) on the 6SE70 do not latch the drive into a fault state. A fault (Fxxx) does. A133 is a diagnostic alarm, but the underlying OFF3 transition is the real reason the line stops.

4. Control Word 1 Bit Structure (Function Diagram 180)

Function Diagram 180 in the MASTERDRIVES Compendium describes the assembly of Control Word 1 (STW1) from BICO sources selected by parameters p558 through p575. The relevant bits for A133 are:

Bits 0, 1, and 2 are all low-active for the stop command. The PLC or wiring must hold them high during normal operation. If any of these bits drop to 0, the corresponding stop command is initiated.

5. Root Cause: OFF3 Source Parameters p558–p560

The OFF3 bit in Control Word 1 is sourced from BICO connector B00600, and its origin is selected by parameter p560. By default, the lower feeder drive on a cut-to-length line is wired to one of the digital inputs on the customer terminal strip (X101 / X102 on the 6SE70). Common factory defaults for p558 / p559 / p560 on a traverse application:

The full set of B5xxx binector sources for the 6SE70 customer terminal block is documented in the Compendium. The most relevant for cut-to-length feeders are:

When p560 is wired to a digital input from the line PLC, the A133 alarm triggers on every cycle where that input drops. Typical causes of the drop are PLC scan-related: the PLC issues a one-cycle write to a different bit, the module resets, the bus cycle is interrupted, or the contactor logic drops out momentarily.

6. Common Causes in Cut-to-Length Feeder Applications

On a typical cut-to-length line with upper and lower feeders, the two drives must remain synchronized during the traverse. The PLC closes a permissive loop: it sets bit 0 of STW1, holds bit 2 (OFF3) high, and issues the setpoint. A momentary glitch in any of those signals causes A133. The most frequently encountered causes in the field are:

6.1 PLC scan and bus latency

On PROFIBUS-controlled 6SE70 drives, the control word arrives as part of the cyclic PPO. If the PLC's PROFIBUS watchdog is set shorter than the actual bus cycle, the CBP2 module drops the control word. A133 then appears even though the PLC program itself never wrote a zero. Set the PROFIBUS watchdog in p2042 ≥ 3 × bus cycle time.

6.2 Shared stop / safety relay dropouts

Many cut-to-length lines tie the OFF3 input of both feeders to a single safety relay (Category 1 stop per IEC 60204-1). A contact bounce, terminal screw looseness, or undersized wire on that shared terminal causes the OFF3 input on the lower feeder to bounce for 5–15 ms — long enough for the drive's input debounce (p4010–p4013, default 0 ms) to register a zero and trigger A133.

6.3 Contactor or brake interlock feedback

If the upper feeder draws a heavy starting current, the line DC bus can sag or the feeder contactor can chatter, opening the auxiliary contact that feeds the lower feeder's OFF3 terminal. A133 appears "randomly" because it is tied to the upstream contactor status.

6.4 Improper parameter source

On a commissioning cycle, an engineer may have copied parameters from a different drive file in which p560 = B5200 (PROFIBUS control word bit 2) while the line has been re-wired to use a hardwired terminal (B5100). When the line goes into local mode, the OFF3 source disappears, triggering A133 as soon as the operator presses RUN.

6.5 Signal noise / EMC on the line

VFD output cables and motor cables routed in parallel with the 24 V control wiring can couple 50/60 Hz noise into the OFF3 input. The drive filters short transients, but continuous noise drops the input low for the full debounce time. Check the routing against the EMC practices in the Compendium (separation ≥ 200 mm, shielded control cable for runs > 5 m).

6.6 Mechanical jam and back-EMF feedback

If the cut press stalls the lower feeder motor (core steel jam), the back-EMF can collapse the DC bus, dropping the 24 V supply to the digital input module. The OFF3 terminal momentarily sees 0 V, and A133 latches. The drive logs A133 with a coincident F015 (drive stalled) — check both alarms in r947 for a clear cause.

7. Diagnostic Procedure

Use the following step-by-step approach. The sequence is written so that a field engineer with a PMU keypad or a DriveMonitor laptop can execute it without access to the original PLC program.

Step 1 — Read the alarm buffer

1. On the PMU, press P to enter parameter mode, key r947 (fault memory) and press P. Read the last 8 faults.
2. Key r949 (alarm memory). Confirm A133 is present and note the relative time stamp from r948.
3. If r001.1 = 1, the drive is in "Fault" state. If only A133 is in the alarm buffer, the drive is in "Ready / Run" but the OFF3 source is the issue.

Step 2 — Identify the OFF3 source

1. Read p560. Note the BICO source. Common assignments: 0 (always inactive), 1 (always active), B5100 (digital input 1), B5200 (PROFIBUS PZD1 bit 2).
2. Read r552.2 (control word bit 2 actual state). This bit is the live OFF3 input BEFORE BICO source resolution and shows the real-time signal at the terminal.
3. Read r550.2 (control word 1 bit 2 processed). This is the value the drive is actually using.
4. If r552.2 = 1 but r550.2 = 0, the BICO source mapping or a downstream binector is the fault. If both are 0 at the moment of A133, the terminal is dropping.

Step 3 — Watch the OFF3 bit during a cycle

Use DriveMonitor's Trace function to record STW1 (r550) and ZSW1 (r700) at 10 ms sample rate over one full traverse cycle. Mark the moment A133 triggers and zoom in. The trace will show whether bit 2 (OFF3) drops for 1–2 scans (PLC / bus issue) or for 50–100 ms (wiring / contactor).

Step 4 — Measure the input directly

1. Place a voltmeter on the OFF3 terminal (X101.5 for B5100) and COM (X101.4). Reading should be 22–26 V DC during traverse.
2. Observe the meter during the cycle. If the voltage drops below 16 V DC for > 5 ms, the input is the cause.
3. If the wiring routes to a shared terminal block, isolate the lower feeder's OFF3 wire and drive it from a 24 V bench supply to confirm.

Step 5 — Check PROFIBUS / bus integrity

1. Read r953.0 (PROFIBUS error counter, if CBP2 fitted). Any increment during A133 events indicates bus glitches.
2. Read p2042 (PROFIBUS watchdog time). Verify the value is at least 3 × the configured DP cycle (typically 1.5–6 ms at 1.5 Mbaud).
3. Check the connector pins, shielding, and termination. A 120 Ω terminator on the last node is mandatory.

Step 6 — Inspect parameter sources

For a complete review, dump p558–p575 and verify they match the line schematic. In particular:

8. Parameter Adjustment and Wiring Corrections

Based on the diagnostic outcome, apply the appropriate correction. Every change must be documented in the parameter file (DVP / DSK format) and stored in DriveMonitor so the next commissioning can restore the exact configuration.

8.1 If the OFF3 source was a digital input and is bouncing

1. Check that the OFF3 wire is sized for ≤ 50 mV drop at the rated current. AWG 18 (0.75 mm²) is typical for short runs; use AWG 16 (1.5 mm²) for runs > 30 m.
2. Add a debounce in the drive: p4010 (debounce time for digital input 1) — set to 5 ms to suppress contact bounce without delaying protective stops excessively.
3. Tighten the terminal screws to 0.5 N·m on the Phoenix terminal block. Re-torque annually.

8.2 If the OFF3 source is the wrong BICO connector

1. Reassign p560 to the correct BICO source. Example: change from 0 to 1 if the line uses the PROFIBUS control word. Or change from B5200 to B5100 if the line is rewired to use a hardwired terminal.
2. Save the parameter set: P 971 to read the current set, P 972 to read the factory default. To save the user set, set p971 = 1 (start save) and wait for the drive to confirm.

8.3 If the PLC program is the cause

1. Trace the PLC rung that writes STW1. The most common bug is the PLC writing the entire control word on every scan, with a temporary variable (e.g., a hold-flag) that goes low during a transition. Replace the individual bit writes with a bit-set / bit-reset pattern so the OFF3 bit is only reset on a deliberate stop.
2. Verify the PLC's STOP-to-RUN transition does not write a zero control word before the run-time control word is set. Insert a one-scan delay or use a warm-restart OB that initializes the control word to 0x047E (all required bits high except ON, which is gated on the operator start command).

8.4 If the line has a shared stop relay

1. Convert the shared stop logic from a relay contact (potential to bounce) to a solid-state output. Many Siemens ET200S / ET200SP digital output modules provide 24 V sourcing with 0.5 ms max rise/fall.
2. Alternatively, add a 100 µF electrolytic + 100 nF ceramic across the OFF3 terminal and COM to filter the bounce, sized to give a time constant above the contact bounce time but below the safety response time.

8.5 OFF3 ramp tuning

Once A133 is clear, tune the OFF3 ramp for the line's mechanical time constant so future legitimate fast stops are smooth:

9. Verification and Commissioning

1. Clear the alarm buffer: p950 = 0 (fault reset), then p971 = 1 to save.
2. Power-cycle the drive to confirm the saved parameter set loads cleanly. A133 should not appear during a no-load run for 10 minutes.
3. Run the line through 10 full traverse cycles. Monitor r550.2 continuously; bit 2 must remain 1 throughout. Record the trace.
4. Force a deliberate OFF3 from the PLC: bit 2 should drop, drive should fast-stop on p464, A133 should appear, and the drive should re-enable on the next ON command without re-triggering A133 on subsequent cycles.
5. Record the parameter file to the engineering laptop and upload to the line's documentation system. Sign off the work order with a list of parameters changed.

10. Related Faults and Prevention

Once A133 is fixed, audit the rest of the control word chain to prevent similar stoppages:

Document a one-page "traverse cycle signal map" of the lower feeder showing every BICO source, the terminal, the wire color, and the PLC tag. Keep it laminated at the cabinet. This single document resolves 80% of A133 / A134 / A135 callbacks.

11. Frequently Asked Questions