Siwarex 7MH4950-2AA01 SF Fault and ADC Overload on 7MH5103-3PD00 Load Cell

A Siemens SIMATIC S7-1200 application running a roller-conveyor weighing platform reports SF (Sensor Fault) on the Siwarex 7MH4950-2AA01 module, with SIWATOOL reporting ADC overload while the 7MH5103-3PD00 share-beam load cell delivers 225 mV on Sig+/Sig- for a 150 kg applied load. The same channel is confirmed healthy by swapping in a known-good cell, so the diagnostic path must separate cell damage, wiring error, mechanical over-load, and SIWATOOL configuration.

This reference documents the field-recovery procedure, evaluates the proposed CZL 632 replacement cell against the 7MH4950-2AA01 input specification, and quantifies the mechanical platform-size constraint that becomes dominant above 600 x 600 mm.

1. System Topology and Components

The weighing subsystem consists of three interacting elements:

1. Siwarex 7MH4950-2AA01 — Single-channel weighing module for SIMATIC S7-1200. Provides bridge excitation, instrumentation-amplifier front end, 24-bit sigma-delta ADC, and digital filtering. The "-2AA01" variant ships calibrated for full-bridge strain-gauge cells with 1 mV/V, 2 mV/V, and 4 mV/V nominal characteristic values.
2. 7MH5103-3PD00 share-beam load cell — Pre-configured Siemens cell matched to the Siwarex family. Force is applied through bending beams; rated capacities vary per part number suffix.
3. SIWATOOL V7 — Siemens PC service software used for parameterization, calibration, and live diagnostic of the Siwarex module over the RS-232 service interface.

2. Expected Signal vs. Measured Signal

The 7MH4950-2AA01 accepts a full-bridge cell whose differential output, scaled to the Siwarex-rated excitation voltage, must fall inside the ADC input range. For Siemens Siwarex modules the valid differential span at the SIG+/SIG- input is approximately 0 to +20 mV DC; the same module family is documented as accepting characteristic values up to 4 mV/V with up to 10 V excitation.

The 225 mV reading is more than ten times the maximum expected differential. It is not possible for an intact 7MH5103 cell to produce this output within its elastic range. Two failure modes are consistent with the reading:

• Load cell strain-gauge damage: one or more active gauges have open-circuited, forcing the bridge into an unbalanced state driven by leakage paths that can saturate the differential amplifier.
• Wiring cross or short: an EX+/EX- conductor is shorting to SIG+ or SIG-, applying the full ~10 V bridge excitation to the ADC input through the bridge resistors. A 10 V applied across a 350 Ω bridge with sense leads shorted to the excitation produces a differential of several hundred millivolts.

3. 7MH5103-3PD00 Share-Beam Load Cell Specification

The 7MH5103-3PD00 is a share-beam type cell from the Siemens Siwarex-compatible family. The capacity is encoded in the suffix; the "-3PD00" variant must be cross-checked against the order documentation. The following parameters apply to the share-beam family and are critical for selection:

4. CZL 632 Load Cell Specification (from manufacturer datasheet)

The proposed replacement is a CZL 632 parallel-beam load cell. The complete specification was supplied with the inquiry:

5. Compatibility Analysis — CZL 632 with 7MH4950-2AA01

From the electrical standpoint the CZL 632 is fully compatible with the 7MH4950-2AA01 input stage:

Siemens confirmed compatibility: "Yes, you can use the load cell CZL 632 with the 7MH4950-2AA01 module."

5.1 Critical mechanical caveat — platform size

The 7MH5103 share-beam cell is dimensioned for a maximum platform footprint of 600 x 600 mm. The actual installation uses a 1200 x 1200 mm plate, which is four times the area. The CZL 632 is a parallel-beam type, whose geometric footprint and stiffness differ from the share-beam; the manufacturer's datasheet does not state a published maximum platform size, so it cannot be unconditionally confirmed suitable for a 1.2 m square plate.

For a single-point load cell mounted at the centre of a plate, the rule of thumb is that the platform area must not exceed the cell's rated footprint or the corner-load error rises above the cell's combined error specification. A 1200 x 1200 mm plate supported at a single point almost always requires a multi-cell platform (three or four corners) rather than a single centre-mounted cell.

6. Root Cause Analysis — 225 mV / SF / ADC Overload

Working hypothesis, in order of probability:

6.1 Strain gauge damage (most likely)

Operating for two months without fault and then suddenly showing a 10x over-range reading strongly suggests mechanical damage. The actual system load (conveyor weight + applied load) totals ~450 kg on a 500 kg cell, which is 90% of rated capacity — only 10% headroom to safe overload (150% FS). A static load this close to rated is permissible, but any shock, side-load, or transient spike (a dropped package, a roller seizure, an object striking the conveyor) can push the gauges past their elastic limit and crack the bonded strain gauges. Once one gauge opens, the bridge becomes unbalanced and the differential amplifier saturates.

6.2 Wiring cross or short

If the cell had been wired correctly the first time and operated for two months, a wiring fault now would imply mechanical disturbance of the cable. Verify:

• Cable gland strain relief on the cell head
• Cable routing through the conveyor frame — any chance of pinch, abrasion, or contact with VFD output cables?
• Shield termination at the Siwarex module end only (single-point ground)

6.3 Capacity margin

The 500 kg cell on a 450 kg steady-state load leaves almost no margin. For an application that includes the conveyor dead-weight, a guideline is to size the cell so that dead-weight + maximum live load does not exceed 70–80% of rated capacity. The original cell selection appears marginal.

7. Diagnostic Procedure

1. De-energize the Siwarex module and disconnect the load cell connector at the module end.
2. Measure resistances at the cable-end connector with a calibrated ohmmeter:
   • EX+ to EX-: 400 ± 10 Ω
   • SIG+ to SIG-: 350 ± 3 Ω
   • EX+ to SIG+: > 5000 MΩ (must be open)
   • EX- to SIG-: > 5000 MΩ (must be open)
   • Any pin to cell body: > 5000 MΩ
3. If any of the resistance checks fail, the cell is damaged and must be replaced.
4. Reconnect the cell, energize the Siwarex module, and read the differential millivolt signal at the cell connector using a precision DMM (set to mV DC) with the cell unloaded. Expected: 0 ± 0.05 mV for a properly zeroed cell.
5. Apply a known calibration weight and verify the differential scales linearly. Expected slope: characteristic value (mV/V) × excitation voltage (V). For a 2 mV/V cell at 10 V excitation: 20 mV at full rated load, e.g. 4 mV at 100 kg on a 500 kg cell.
6. Connect SIWATOOL V7 via the RS-232 service port and capture the Siwatool parameter file. Inspect DR-Cycle, A/D limits, calibration digits, low-pass filter setting.
7. Cross-check the wiring diagram against the cell's data sheet. Siemens cells use a 6-wire cable with colour coding; verify against the Siwarex manual pin map.

8. SIWATOOL Configuration Parameters

Below are the key Siwatool parameters relevant to this fault. Values shown are typical for a 2 mV/V, 500 kg application; calibrate in SIWATOOL per the actual cell and platform.

9. Replacement & Verification Procedure (CZL 632)

9.1 Mechanical

1. Remove the damaged 7MH5103-3PD00 cell, retain the mounting hardware.
2. Verify that the CZL 632 mounting hole pattern matches the existing fixture. If not, machine an adapter plate.
3. Install CZL 632, torque mounting bolts to manufacturer-specified value (typical: grade 8.8 M10 at 35 N·m for share-beam type; consult CZL 632 datasheet for exact figure).
4. Confirm the platform sits on the cell with no bind, no side load, and full perpendicular force transmission. Use a bubble level on the platform.
5. For the 1200 x 1200 mm platform, evaluate whether a single CZL 632 will meet corner-load error. If not, fit two or four cells in a summing box.

9.2 Electrical

1. Pull a 6-wire shielded cable from the cell to the Siwarex 7MH4950-2AA01. Minimum conductor size 0.25 mm² (24 AWG).
2. Connect per the Siwarex manual: EX+, EX-, SENSE+, SENSE-, SIG+, SIG-. Some 4-wire cells omit the sense leads; in that case link SENSE+ to EX+ and SENSE- to EX- at the cell connector.
3. Ground the cable shield at the Siwarex module end only, through the EMC cable gland.
4. Keep the cell cable at least 200 mm away from VFD motor cables. Cross at 90° if unavoidable.

9.3 SIWATOOL Commissioning

1. Connect SIWATOOL V7 to the 7MH4950-2AA01 service port (RS-232, 19200 8N1).
2. Open the device, read the existing parameter set, and save it as a backup file.
3. Update Rated load of cell to 2000 kg.
4. Update Cell characteristic value to 2.000 mV/V.
5. Reset Calibration weight parameter to the value of your test weight.
6. With the platform empty, execute Adjustment — Zero (CAL0 / 0% adjustment) in SIWATOOL.
7. Place the calibration weight on the platform and execute Adjustment — Span (CAL100% / 100% adjustment).
8. Verify linearity: 0%, 25%, 50%, 75%, 100% of rated load. Maximum deviation should be within the CZL 632 combined error of ± 0.02% FS.
9. Save the parameter set and write it back to the PLC project.

9.4 PLC Program Verification

In the S7-1200 program, verify that the scale weight (e.g., DB10.DBD0 as REAL) reads the expected value when the calibration weight is applied. Add a fault-handling block that:

• Reads the Siwarex status word (e.g., bits for "ADC overflow", "Sense line error", "Calibration in progress").
• Generates a process alarm if SF is set.
• Prevents batching from advancing if the weight reading is in "invalid" state.

An example ST snippet:

// Read Siwarex status
#siwarex_status := "Siwarex_DB".Status_Word;

// Detect ADC overflow (bit pattern depends on FW version)
IF (#siwarex_status AND 16#0004) <> 0 THEN
    #adc_overflow := TRUE;
    #scale_weight_valid := FALSE;
ELSE
    #adc_overflow := FALSE;
    #scale_weight_valid := TRUE;
END_IF;

// Gate downstream process
IF #scale_weight_valid THEN
    "Process_DB".BatchingPermit := TRUE;
ELSE
    "Process_DB".BatchingPermit := FALSE;
    "HMI_DB".AlarmText := 'Siwarex ADC overload - check load cell';
END_IF;

10. Field-Proven Caveats

1. Don't trust "it worked for two months" as a baseline. Mechanical wear, cable fatigue, and creep all progress; a stable operating period is not predictive.
2. Measure the cell, not the module. Disconnecting the cell and verifying bridge resistances is faster and more diagnostic than toggling SIWATOOL parameters.
3. Don't paper over an ADC overload by increasing the rated-load parameter. Doing so only shifts the calibration gain; the underlying fault remains and will eventually corrupt the batch weight.
4. Single-cell vs multi-cell: if your platform area is more than 4x the cell's recommended footprint, plan a multi-cell scale from the start. The mechanical retrofit is far cheaper than a recalibration cycle on a marginal single-cell installation.
5. Keep the SIWATOOL parameter file under version control. After every calibration, archive the file with a timestamp and the calibration weight used.

11. Recommended Spare Strategy

12. Quick Reference — Fault Matrix