FM 351 Reference Lost: S7-300 Positioning Module Troubleshooting

FM 351 Reference Lost: S7-300 Positioning Module Troubleshooting

This field reference addresses an intermittent "reference lost" condition on a Siemens FM 351 positioning module operating in a SIMATIC S7-300 (CPU 315-2DP) controller. The fault is reproducible on a single channel during axis motion, but produces no module fault LED and no diagnostic buffer entry on the CPU, making it a classic "soft" reference-point error caused by encoder signal integrity, hardware configuration, or reference point approach parameters. The troubleshooting procedure below is derived from the official SIMATIC FM 351 Installation and Parameterization Manual and the S7-300 FM 351 Operating Instructions.

1. Problem Description

Two FM 351 modules are mounted in the S7-300 rack, providing four positioning channels. Three of those channels are wired to incremental encoders on a Senator-group pressure die casting machine. The fourth channel is unused. The system exhibits the following intermittent behavior:

• The axis in motion halts unexpectedly during positioning travel.
• The application reports "reference lost" in the FM 351 user data interface (UDT/FB status bits).
• The CPU diagnostic buffer contains no FM 351 diagnostic interrupt entry.
• The SF (group fault) LED on the FM 351 does not latch.
• Replacing the FM 351 module does not eliminate the fault.
• Replacing the incremental encoder does not eliminate the fault.
• Wiring a dedicated 24 V DC supply to the module's 1L+/2L+ terminals does not eliminate the fault.

This pattern is diagnostic of a non-fatal, runtime-only loss of the reference relationship between the FM 351's internal position counter and the actual mechanical axis. The module does not classify this as a hardware fault because the encoder continues to deliver a valid differential signal chain; instead, the FM 351's reference-point state machine has been invalidated.

2. FM 351 Reference Point Architecture

To diagnose the fault you must understand the three reference-point states defined in the FM 351 manual and reproduced in the FM 351 Getting Started commissioning guide:

The Reference Lost state is the third state in the table. It is reported through the STW1/STW2 status bits in the FM 351 user data interface and via the _JOB_ERR / _CHAN_ERR feedback words. Per the operating instructions, when this state is entered:

1. All active positioning jobs are aborted.
2. All outputs (including enable and rapid/creep speed commands) are cleared.
3. The axis comes to an uncontrolled stop or to a stop defined by the higher-level PLC logic.
4. The state persists until a new reference point approach is successfully executed, or the operator issues "Set Reference Point" in the parameterization tool.

3. Root Cause Categories

Per the S7-300 FM 351 Operating Instructions Error Handling section, Reference Lost is generated by one of three runtime conditions. Each is examined below in priority order for intermittent faults.

3.1 Encoder Signal Failure During Motion

The FM 351 continuously monitors the differential RS-422 signals (A, /A, B, /B, N, /N) on the encoder input. A loss of any of these signals during motion, or a violation of the minimum edge-to-edge timing (quadrature), triggers the encoder error handler. When encoder error handling is configured as "De-synchronize," the FM 351 immediately enters the Reference Lost state.

Encoder wiring checks:

1. Verify shielded twisted-pair cable is used end-to-end from encoder pigtail to FM 351 front connector (X1/X2). Siemens recommends 6XV1830-0EH10 or equivalent (LiYCY-CY TP) for incremental encoder runs in industrial environments.
2. Confirm shield is bonded to the FM 351 front connector shell and to the encoder body ground ring, not to the cable carrier or intermediate junction boxes.
3. Verify the encoder supply jumper on the FM 351 front connector supplies the correct voltage (5 V DC for 5 V encoders, 24 V DC for 24 V encoders). The 5 V supply is sourced from the module's 1L+/2L+; the 24 V supply uses the same terminals. Setting the jumper incorrectly burns the encoder or causes intermittent brownouts.
4. Measure the voltage at the encoder terminals under load during motion. Voltage drop on long cable runs (above 25 m) can cause encoder electronics to drop below the 4.75 V minimum required by RS-422 receivers. Use 24 V encoder supply on long runs.

3.2 Reference Point Switch Bounce or Loss

If "Reference point approach with reference point switch" is selected as the synchronization method, the FM 351 polls the digital input assigned as the reference point switch on the FM 351 front connector. The switch must close at the correct position with repeatable timing. Causes of Reference Lost from this path:

• Switch contact bounce causing the FM 351 to interpret multiple edges during the reference point approach.
• Mechanical wear causing the switch to miss the cam (especially on die casting machines with vibration).
• Wiring break in the reference point switch cable causing the input to float.
• Debounce time in the parameterization tool set too low for the actual switch characteristics.

3.3 Configuration Mismatch After Module Replacement

If the original module was swapped for a replacement, the replacement may have been loaded with the previous module's parameter set, but the module's internal non-volatile memory may still hold a partial state. Force a full re-parameterization of the channel as described in Section 6.

4. Diagnostic Procedure

Execute the following procedure in the order given. Each step produces a binary (pass/fail) result that localizes the fault.

Step 1 — Capture the Live Status Word

Open the FM 351 parameterization tool (FM351 Configuration Package) online. With the program running and the axis in motion, switch to "Test/Commissioning" mode and observe:

• CHAN_ERR word — bit 2 ("Reference lost") set?
• CHAN_ERR word — bit 0 ("Encoder error") set?
• Position actual value — frozen or jumping?
• Encoder counter overflow/underflow flags.

If bit 0 (encoder error) is set immediately before bit 2 (reference lost), the root cause is encoder signal integrity. If bit 0 is not set, the root cause is the reference point switch or configuration.

Step 2 — Verify Encoder Signal with Oscilloscope

Probe each differential pair (A, B, N) at the FM 351 front connector during the failing axis motion. Required measurements:

Step 3 — Verify Reference Point Switch Signal

With the axis manually jogged, simulate a reference point approach and monitor the digital input assigned to the reference point switch. The signal must:

1. Transition from low to high at the mechanical reference position.
2. Remain high long enough for the FM 351 to count the index pulse N (if used).
3. Not exhibit contact bounce on either edge.

Step 4 — Check the CPU Diagnostic Buffer

Open STEP 7 → PLC → Diagnostic/Setting → Diagnostic Buffer. Filter on FM 351 events. Although the user reports no diagnostic events, a recent module restart or distributed I/O fault may have generated a transient entry that the user missed. Look specifically for OB 82 (diagnostic interrupt) calls on the FM 351 logical address.

5. Configuration Parameters That Affect Reference Loss

The following parameters are accessed in the FM 351 parameterization tool under "Incremental Encoder", "Reference Point Approach", and "Encoder Error Reaction". Misconfiguration of any of these can produce the symptom in question.

5.1 Encoder Error Reaction

If the application can tolerate a brief loss of positioning commands during an encoder glitch, switching to "Output clear, retain reference" prevents the entire axis from going down on a single noise event. This is the single most useful change for the field condition described.

5.2 Reference Point Approach Mode

5.3 Reference Point Switch Debounce Time

Default is typically 2 ms. For die casting machinery with vibration, increase to 5–10 ms. For high-speed axes with short cam contact, decrease to 1 ms. Trade-off: long debounce time rejects vibration, but may miss the switch event on fast traverses.

5.4 Reference Point Approach Direction

Set the direction (positive/negative) of the reference point approach to match the physical cam orientation. If the cam is approached from the wrong direction, the FM 351 will not recognize the reference point event and will time out, leaving the axis in a not-synchronized state and aborting the run.

5.5 Reference Point Offset

The reference point offset is the distance between the reference point switch and the mechanical zero of the axis. A mis-set offset does not cause Reference Lost, but it causes the axis to dock at the wrong physical position after a successful reference point approach. Verify separately if post-repair position errors appear.

6. Step-by-Step Resolution

Apply the following procedure in the order given. Do not skip steps; each step eliminates a class of root causes.

1. Stop the machine and lock out the hydraulic / servo drives per site safety procedure. Verify zero energy state before opening any enclosure.
2. Open the FM 351 front connector and inspect the encoder cable shield termination. The shield must be clamped 360° at the FM 351 connector and at the encoder body. Replace any pigtail or "drain wire" terminations with full 360° clamps.
3. Measure the encoder supply voltage at the encoder body (not at the FM 351) under load. The reading must be within the encoder's specified tolerance (typically 5 V ± 5% or 24 V ± 10%).
4. With an oscilloscope, probe the A, B, and N differential pairs at the FM 351 connector during a manual jog of the axis at maximum production speed. Verify the four parameters from the table in Section 4 Step 2.
5. Open the FM 351 parameterization tool. Go to the failing channel → "Encoder Error Reaction". Change the setting from "De-synchronize" to "Output clear, retain reference". Download to the module.
6. Go to "Reference Point Approach" → set debounce time to a value appropriate for the machine. Start with 5 ms and adjust per Section 5.3.
7. Go to "Incremental Encoder" → confirm the encoder type matches the physical encoder (5 V or 24 V, with or without index N). A type mismatch (e.g., parameter set to "with N" but encoder has no N pulse) produces exactly this intermittent Reference Lost behavior.
8. Save the project. Download hardware configuration to the CPU.
9. Run a manual reference point approach at slow speed (10% of max) and verify the axis docks at the correct position.
10. Run a manual reference point approach at production speed. Verify the axis docks correctly.
11. Run the production cycle for at least 30 minutes with the diagnostic buffer open and the FM 351 status word visible. Verify no Reference Lost events occur.

7. Hardware Verification Checklist

8. Software and Firmware Considerations

The FM 351 module exists in two hardware revisions: 6ES7351-1AH01-0AE0 and 6ES7351-1AH02-0AE0. The latter has higher maximum encoder frequency and improved EMC immunity. If the installed module is the -1AH01- version, an upgrade to -1AH02- is a one-for-one hardware swap that can be performed while the machine is down for other maintenance, with no STEP 7 project change required.

The FM 351 parameterization tool versions 3.x and 4.x are backward-compatible with both hardware revisions. Do not use a parameterization tool earlier than V3.0 with the -1AH02- module, as it will not recognize the new encoder frequency parameters.

For STEP 7 V5.x, ensure the FM 351 HSP (Hardware Support Package) is installed. Without the HSP, the module may be added to the project as a generic FM 351, which loses access to the full parameter set and can cause the configuration mismatch root cause described in Section 3.3.

9. Common Pitfalls on Die Casting Machines

Senator-group die casting machines and similar high-tonnage hydraulic presses present specific challenges for the FM 351 reference point system:

• High vibration on clamp and shot ends. Encoder mounts must use vibration isolators if the encoder cannot be relocated to a less-stressed part of the mechanism.
• Hydraulic fluid contamination on the encoder cable. Use oil-resistant PUR or TPE cable jacket, not standard PVC.
• Temperature cycling from cold start to full production. Encoder thermal expansion can shift the reference point switch alignment by fractions of a millimeter. Verify reference point approach at full operating temperature, not cold.
• EMC from VFDs driving the hydraulic pump. If the encoder cable runs in the same cable tray as the VFD output cables for more than 2 meters, separate them with a grounded metal divider or reroute the encoder cable in a separate conduit.
• Hydraulic pressure spikes causing the axis to overshoot. Check the FM 351's "Trailing Error" monitoring setting; an axis that overshoots the target position will be flagged as a position error, which the FM 351 can map to Reference Lost depending on the error reaction setting.

10. Verification Procedure After Repair

1. With the machine at full operating temperature, execute a manual reference point approach at maximum axis speed. Record the docking position.
2. Repeat 20 times. All docking positions must agree within the encoder's resolution (e.g., ± 0.01 mm for a 5 µm encoder).
3. Run 5 full production cycles. Open the diagnostic buffer at the end and verify zero OB 82 entries from the FM 351.
4. Open the application log and verify zero "Reference Lost" events in the FM 351 status word history.
5. Run the machine for an 8-hour production shift. At the end, re-check the diagnostic buffer and application log.

If the fault recurs within the 8-hour shift, the root cause is most likely an encoder cable intermittent (replace cable), encoder output stage failure under vibration (replace encoder), or a noise event tied to a specific machine cycle (use an oscilloscope trigger on the cycle event and capture the encoder signals).

11. Quick Reference: FM 351 Status Word Bits Relevant to Reference Lost

12. Spare Parts and Document References

• FM 351 module (current revision): 6ES7351-1AH02-0AE0
• FM 351 front connector set: 6ES7392-1BJ00-0AA0 (screw-type) or 6ES7392-1BM00-0AA0 (spring-type)
• FM 351 parameterization tool: included on the SIMATIC Documentation CD or downloadable from Siemens Support (search "FM 351 configuration package")
• Reference documents: SIMATIC FM 351 Installation Manual (V4), S7-300 FM 351 Operating Instructions, FM 351 First Steps in Commissioning