Resetting SIWAREX MS: Factory Defaults and Zeroing Recovery

Overview

The SIWAREX MS weighing module (order number 7MH4930-0AA01) integrates a 4-wire load cell interface into a SIMATIC S7-200 (and selected S7-22x) PLC backplane. In the field, a recurring failure mode is a "stuck" gross weight reading (for example, 320 kg displayed indefinitely after the mechanical load has been removed) that prevents the operator from issuing a zero command from the HMI. The HMI sends the zero request, the SIWAREX rejects it, and the only recovery many technicians discover is to physically reseat the module or break the S7-200 bus connector.

This article documents the correct recovery procedure for that failure mode: how to identify the SF-LED state, how to verify the load cell signal directly with a multimeter, how to use SIWATOOL MS to confirm the in-module weight value, how to enlarge the zeroing range (DR2) to clear the existing reading, and how to execute a factory reset via command code 8 when the calibration data must be rebuilt from scratch. The reference for command code semantics is the SIWAREX MS operating instructions and the SIWAREX command list documentation maintained on the Siemens TIA documentation cloud.

SIWAREX MS Identification and SF-LED Diagnostics

Before any reset action, classify the module's state using the front-panel LEDs. The SIWAREX MS exposes a group-fault LED that carries significant diagnostic weight:

A steady SF-LED combined with a frozen gross weight value that does not respond to mechanical disturbance (shaking the scale hopper) is the canonical signature of an out-of-range gross weight that the module has flagged as invalid. The scale is mechanically empty, but the in-module "current gross" exceeds the configured DR2 - Zeroing range % parameter. The zeroing command is therefore rejected by design, not by communication fault.

Root Cause: Out-of-Range Gross Weight and Zeroing Rejection

SIWAREX MS enforces zeroing only when the current gross weight lies within the window defined by parameter DR2. The default DR2 value is typically 1.0 % of the load cell's nominal range, configurable in 0.1 % steps up to a maximum defined by the firmware. If a transient overload (a worker placing a load exceeding DR2, then removing it) drives the in-module gross momentarily out of range, the firmware latches a "scale out of zeroing range" condition. Subsequent zeroing commands return an error and have no effect on the displayed weight.

The field-recoverable triggers for this condition are:

1. Mechanical overshoot of DR2 during a process transient (load above zeroing window).
2. Load cell failure causing a saturated output signal (>5 V at SIG+/SIG-, or a mV/V output that exceeds the configured calibration).
3. Wiring fault: open or shorted sense lead (SEN+/SEN-) causing the excitation regulator to drift, or shielded cable break introducing drift on SIG+/SIG-.
4. Module internal parameter corruption after an incomplete firmware upgrade or a PLC STOP-RUN transition with active write operations to DR parameters.

Three separate measurements resolve the ambiguity:

1. SF-LED state - distinguishes parameter/calibration faults from communication faults.
2. Mechanical disturbance test - shaking an empty hopper and confirming whether the displayed value tracks the disturbance. If the value is flat, the gross reading is latched, not live.
3. SIWATOOL MS gross reading - the same value displayed on the HMI should appear in SIWATOOL under the gross weight field. Agreement confirms the module is the source of truth; disagreement confirms an HMI/PLC mapping fault.

Load Cell Signal Verification at the Module

The four-wire load cell interface on SIWAREX MS uses these terminals:

With an S-type load cell rated for 50 kg and a nominal sensitivity of 2.000 mV/V at 10 V excitation, the expected full-scale differential output (SIG+ to SIG-) is approximately 20 mV DC. A measurement of 520 mV DC at SIG+/SIG- on an unloaded S-type cell is roughly 26x the full-scale output - this is impossible for a healthy 50 kg cell. It indicates one of the following:

• The cell has been mechanically overloaded and the strain gauges have been permanently deformed (output no longer tracks load).
• The cell is not actually S-type, or has been replaced with a higher-capacity cell without recalibration.
• Wiring is crossed (SIG+ connected to SEN+, or shield grounded to SIG-).
• A cell in a multi-cell load assembly has failed mechanically and is dragging the parallel sum.

Disconnecting the load cell wiring at the SIWAREX MS terminal block and measuring again should produce 0 mV DC if the module is healthy (no signal source). If the reading persists at a high mV value with no load cell connected, the SIWAREX MS input stage is damaged and the module must be replaced.

Recovery Procedure: Increase DR2 First, Factory Reset Last

The correct sequence when the scale is mechanically empty but the gross reading is latched and DR2 rejects zeroing is:

1. Confirm mechanical state. Ensure the hopper is empty, no process material is contacting the load cell, and no piping or auxiliary equipment is loading the structure.
2. Confirm SF-LED is on. A lit SF-LED confirms a module-side condition rather than an HMI mapping issue.
3. Read the gross value through SIWATOOL MS. Connect SIWATOOL MS over the RS-232 service port (the SIWAREX MS front-panel service interface). Verify the gross reading matches the HMI display. Note the value (in this case, 320 kg).
4. Increase DR2 temporarily to a value larger than (gross / nominal) x 100 %. For a 50 kg nominal cell with a 320 kg frozen reading, DR2 must be set above 640 %. Use the upper bound supported by firmware, typically 1000 % or the platform-specific maximum. Save the parameter through SIWATOOL MS.
5. Issue a zero command from SIWATOOL MS (command code 1 - zeroing, depending on firmware revision) or from the HMI once DR2 is widened. The module will accept the zero and the gross value will drop to 0.000 kg.
6. Restore DR2 to its operational value (typically 1.0 % to 5.0 %). This re-imposes the safety window that prevents accidental zeroing of a partially loaded scale.
7. If the gross reading still does not track mechanical disturbance after zeroing, suspect a load cell fault. The procedure then requires replacement of the failed cell, recalibration with calibration weights, and verification under load.

Command Code 8: Factory Reset for SIWAREX MS

When the parameter memory is corrupted, when an unknown parameter set is preventing zeroing, or when a full rebuild from defaults is required, command code 8 on the SIWAREX MS restores the as-shipped parameter set. This is the equivalent of a "recovery point" for the MS family.

SIWAREX Family Comparison: MS vs WP231 vs WP321

The source material notes that the SIWAREX WP231 (S7-1200 platform) supports a "recovery point" feature, while the SIWAREX MS (S7-200 platform) does not. The factory-reset command codes also differ between families:

For an engineer planning a migration from S7-200 + SIWAREX MS to S7-1200 + SIWAREX WP231, the recovery-point feature on the WP231 should be confirmed before commissioning; it allows the operator to revert to a known-good parameter snapshot without recalibration.

SIWATOOL MS: Direct Module Access

SIWATOOL MS is the dedicated service tool for the SIWAREX MS. It bypasses the S7-200 PLC entirely and reads parameter memory and live process values directly through the RS-232 service port on the front of the module. Use it whenever:

• The HMI reports a frozen weight but the PLC program appears correct.
• The SF-LED is on and the cause must be isolated to the weighing channel.
• DR parameters must be inspected or modified without affecting the PLC scan.
• A factory reset (command 8) must be executed.

Connection parameters:

Field-Proven Diagnostic Matrix

Verification After Recovery

1. With the scale empty, issue a zero command from the HMI. The gross value must settle to 0.000 kg within 2 seconds.
2. Apply a calibration weight of known mass (typically 50 % to 100 % of nominal). Verify the displayed gross matches the applied mass within the configured resolution step (DR5).
3. Remove the weight. Verify the gross returns to 0.000 kg within the drift tolerance defined by the application.
4. Cycle power to the S7-200 backplane. After restart, verify the gross reads 0.000 kg and the SF-LED is off without operator intervention. This confirms that the zero and the parameters were committed to non-volatile memory.
5. Apply a process load equal to the worst-case production load. Verify the gross tracks the load in real time, the SF-LED remains off, and the HMI zero command is accepted when the scale returns to empty.
6. Record the calibrated DR parameters (DR1 zero, DR2 zeroing range, DR3 nominal, DR4 unit, DR5 resolution, DR6 filter) and the calibration date in the plant asset record. The next technician who services this scale needs these to verify any future complaint.

Safety and Operational Notes

FAQ