SIWAREX U vs WP231 WP251: Weighing Module Comparison Guide

Overview of the SIWAREX Weighing Module Family

The Siemens SIWAREX family comprises dedicated electronic weighing modules engineered for seamless integration into the SIMATIC automation environment. Two principal platform generations are fielded in production: the legacy SIWAREX U for the SIMATIC S7-300, and the current SIWAREX WP231 / WP251 for the SIMATIC S7-1200. Both module families deliver the same nominal error limit of 0.05% of the measurement range end value (per Chapter 13 of the respective device manuals), but they diverge substantially in feature set, connectivity, approval scope, and integration architecture.

This reference consolidates the differentiating parameters that are typically not exposed in marketing collateral but are decisive for sizing, retrofit, and obsolescence decisions. Where a parameter is platform-ambiguous in vendor literature, the ambiguity is flagged and a decision path is documented.

Module Identification, Catalog Numbers, and Mechanical Form Factor

The SIWAREX U is hosted on the classic S7-300 single-width (40 mm) backplane footprint, occupying one slot on the SIMATIC S7-300 rack and consuming a single logical address in the I/O image. The SIWAREX WP231 and WP251 are designed for the SIMATIC S7-1200 and use the compact 70 mm-wide front connector form factor that matches the S7-1200 signal module family. The catalog (MLFB) identifier for the SIWAREX WP231 is 7MH4960-2AA01, documented on the Siemens Industry Online Support portal at SIWAREX WP231 Weighing module - 7MH4960-2AA01 - Support.

Accuracy, Error Limit, and Resolution Specifications

All three modules advertise a 0.05% error limit from measurement range end value, verified in the calibration chapter (Chapter 13) of the device manual. The metric is expressed as a percentage of the configured full-scale weight, not as a percentage of the analog-to-digital converter's least significant bit. For a 1,000 kg nominal cell and a 0.05% error limit, the absolute maximum permissible error band is ±0.5 kg after zero and span calibration.

The three modules differ in their legal-for-trade scope and in the resolutions they can be calibrated to:

The internal conversion rate must be selected based on the dynamic behaviour of the scale. A static silo at 10 Hz minimises mains-induced noise. A checkweigher or filling station at 100 Hz allows fine-grained dosing control at the cost of a higher process noise envelope. The WP251 further applies digital filtering on top of the conversion rate to stabilise the dosing fine-signal as the weight approaches the setpoint.

Performance, Dosing, and Application Coverage

The SIWAREX U is a pure weight transmitter: it forwards the current weight to the S7-300 CPU for application-level processing such as batching, totalisation, or threshold monitoring. All dosing, averaging, and alarm logic must be programmed in STEP 7 (or the legacy PCS 7 weighting blocks).

The SIWAREX WP231 retains the transmitter role but off-loads several control functions into the module firmware:

• Standalone mode without a SIMATIC CPU.
• Direct connection of a Siemens HMI panel via the onboard Modbus TCP server, eliminating the S7-1200 from the HMI path.
• Onboard limit switch logic on the four digital outputs (e.g., gross above / below / zero band / net setpoint).
• Configurable 4–20 mA analog output driven by gross, net, or a derived variable.

The SIWAREX WP251 extends the WP231 firmware with full dosing functionality including a coarse and a fine flow cutoff. The module executes the dosing sequence in firmware, switching the coarse and fine digital outputs at the appropriate setpoints and tolerating in-flight material via a configurable lead time. The PLC is notified at start, end, and on tolerance breach, but the time-critical cutoff decisions are made inside the module to avoid CPU scan-time jitter.

EMC, Interference Immunity, and Environmental Compliance

The SIWAREX U is qualified against EN 61000-6-4 (Emission standard for industrial environments) and EN 61000-6-2 (Immunity standard for industrial environments). These are the generic emission/immunity standards referenced in the 2000s-era SIMATIC documentation and they map to the CE Machinery Directive presumption of conformity for industrial automation cabinets.

The SIWAREX WP231 and WP251 are qualified against IEC 61010-1 (Safety requirements for electrical equipment for measurement, control, and laboratory use) and IEC 61131-2 (Programmable controllers - Part 2: Equipment requirements and tests). IEC 61131-2 supersedes the older EN 61000-6-2 generic immunity standard by specifying the exact test levels, cable lengths, and coupling methods for programmable logic controllers and their peripheral modules.

Field installation practice is identical for both families: the load cell cable must be shielded, the shield bonded to the cabinet ground bar at the module end with a low-impedance connection (≤ 0.1 Ω to PE), and the cable routed at least 200 mm away from VFD output cables or other high-di/dt conductors. When these practices are observed, the difference between the two standard families is not field-visible.

Integration Architecture: S7-300 vs S7-1200 Backplane

The SIWAREX U is configured as an SM (Signal Module) on the S7-300 backplane. The module occupies 16 input bytes and 16 output bytes in the I/O image and is addressed via the standard P/PI access path in STEP 7 HW Config. Weight, status, and command registers are mapped to fixed offsets within that 16-byte window. The CPU-side function block (FB SIWA) handles calibration, tare, and readout.

The SIWAREX WP231 and WP251 are configured as SMs on the S7-1200 backplane in TIA Portal. The module occupies 16 input bytes and 16 output bytes; the input area carries the filtered weight, the status word, and the diagnostic word. The output area carries commands (zero, tare, calibration trigger). The Modbus TCP server is a parallel interface and does not require the CPU to be online for the HMI to display weight.

Standalone Operation and HMI Connectivity

A defining capability of the WP231 and WP251 is that the module can operate without a SIMATIC CPU online. The module boots, reads the load cell, exposes the weight on the analog output and the digital outputs, and serves the weight value on Modbus TCP. A Siemens HMI panel (e.g., a SIMATIC Comfort or a third-party panel speaking Modbus TCP) can display and tare the scale directly. This is advantageous in retrofit cabinets where an existing controller is being retained, and in low-cost installations where the weighing function is logically separate from the batch automation.

The SIWAREX U does not provide this standalone mode. It is functionally dependent on a configured CPU to drive the calibration and to interpret the weight value. The S7-300 + SIWAREX U combination is therefore best viewed as a tightly coupled transmitter, not an autonomous weighing device.

Digital and Analog I/O Integration

The four onboard digital inputs on the WP231 and WP251 are typically used for:

1. External zero command (e.g., a panel-mounted pushbutton).
2. External tare command.
3. Two configurable function inputs (e.g., start dosing, abort dosing, select recipe).

The four onboard digital outputs are software-configurable as:

1. Gross above setpoint 1 / 2 / 3 / 4.
2. Net above setpoint 1 / 2 / 3 / 4.
3. Zero band.
4. Stable weight.
5. (WP251 only) Coarse flow valve, fine flow valve, tolerance breach, dosing complete.

The onboard analog output is a 4–20 mA current loop, sourcing or sinking depending on the wiring scheme. The output is assigned in the module's web-based or TIA Portal configuration to gross, net, or a derived variable (e.g., fill level as a percentage of full scale). The analog output updates at the configured conversion rate (10/50/100 Hz).

The SIWAREX U has no onboard DIs, DQs, or AO. All interfacing is backplane-based, requiring the S7-300 CPU to issue commands to a digital output SM and to read a digital input SM. This is acceptable in high-channel-count panels but adds two slot positions and an analog output SM to the bill of materials compared to a WP231 installation.

Legal-for-Trade and OIML R76 Compliance

The SIWAREX WP231 and WP251 are approved for legal-for-trade use to 3000d, OIML R76 class III. This means that for a 1,000 kg nominal scale, the smallest legal division is 0.33 kg, and the maximum permissible error is governed by the OIML R76 table for class III instruments. The calibration software and module firmware enforce the configuration limits (number of divisions, multi-range/multi-interval, minimum load cell verification interval) required by the approval.

The SIWAREX U is not approved for legal-for-trade. If the application requires a NTEP- or OIML-certified scale, the SIWAREX U cannot be the legal weighing instrument. The NTEP Certificate of Conformance number must be quoted on the project datasheet and verified against the Siemens approval database before procurement.

Reliability Considerations and Field-Proven Caveats

The SIWAREX U is built on the S7-300 technology platform, which has been in continuous production since 1994 and has a documented mean time between failures in the order of 30+ years at typical cabinet temperatures. The mechanical connector, the housing, and the firmware update mechanism are mature. The platform is, however, classified as phase-out for new installations in many regions as Siemens has consolidated the SIMATIC portfolio around the S7-1500 and S7-1200 families.

The SIWAREX WP231 and WP251 are built on the S7-1200 technology platform, which is in active production. The hardware revision indicator is on the module's side label (e.g., ES 02 or FS: 01). Modules with the early ES 01 engineering status have known diagnostic-interrupt issues that are resolved in firmware ≥ V2.0.0. Verify the firmware level on delivery and during commissioning using the TIA Portal online diagnostics view.

Common field issues, regardless of platform, are:

1. Load cell cable shield not bonded at the module end. The shield must be terminated in the module's shield clamp, not in the cabinet ground bar. Floating shields cause millivolt-level noise that biases the weight reading by tens of kilograms on a 1,000 kg scale.
2. Multiple load cells in parallel without a junction box. Parallel connection of two or more load cells without a summing junction box causes corner-load errors that exceed the 0.05% error limit. Use a SIWAREX JB junction box, with one corner-load trim resistor per cell, when the scale is supported on more than one load cell.
3. Mechanical vibration coupled into the weigh platform. Even with a 10 Hz conversion rate and digital filtering, mechanical vibration at line frequency (50/60 Hz) can inject noise. Add rubber isolation pads or a vibration-damping mounting kit if the scale is on a vibrating structure.
4. Stale HMI connection in standalone mode. When the WP231/WP251 is in standalone mode, a stale Modbus TCP connection from the HMI may hold a stale weight. Configure the HMI to drop the connection after three failed polls and to reissue the read command set.

Sizing and Selection Procedure

To select between SIWAREX U, WP231, and WP251 for a new project, follow this decision path:

1. Is the application legal-for-trade? If yes, eliminate the SIWAREX U. Choose WP231 for static scales and WP251 for filling/dosing applications.
2. Is the host PLC S7-300 (brownfield) or S7-1200 (greenfield)? S7-300 brownfield with no CPU replacement in scope can stay on SIWAREX U. Any project with a new S7-1200 or S7-1500 in scope should use the WP2x1 family. The S7-1500 connects to the WP2x1 family via PROFINET from a distributed I/O perspective; the S7-1500 does not host the S7-300-era SIWAREX U directly.
3. Is dosing required? Coarse/fine dosing or high-precision filling requires the WP251. The WP231 does not execute firmware-level dosing; the CPU-side program must implement the dosing sequence at the appropriate scan rate, and time-critical cutoff must be triggered from a hardware interrupt or a fast output card.
4. Is the weighing function logically separate from the line automation? A standalone mode (WP231/WP251 with direct HMI) is appropriate when the scale is a stand-alone inspection station. The SIWAREX U requires a configured S7-300 CPU online for any display output.
5. Is the load cell count greater than 1? Use a SIWAREX JB junction box with corner-load trimming. The module's load cell interface is a single full-bridge input regardless of the platform.

Load Cell Interface and Cabling Specification

All three modules use a 6-wire load cell interface:

• 2 wires for excitation (typically 10 V DC, regulated to within ±0.05% per OIML R76).
• 2 wires for sense (feed back to the regulator to compensate for cable drop).
• 2 wires for the differential mV signal.

The maximum cable length from the module to the load cell, with 6-wire connection and standard SIWAREX cable, is approximately 1,000 m for a 350 Ω cell. The 4-wire variant, often used as a fallback when the load cell is integrated into the platform without a remote junction box, is limited to approximately 100 m before cable-drop compensation becomes dominant.

The cable specification is LiYCY-CY (overall foil + braid shielded) with a minimum conductor cross-section of 0.25 mm². The shield must be bonded to the SIWAREX shield clamp at the module end and to the JB housing at the cell end, with no intermediate ground points that would create a ground loop.

Commissioning Procedure

Follow this sequence when commissioning any of the three modules:

1. Verify the module's catalog number (MLFB) and firmware version against the project specification.
2. Insert the module in the slot specified by HW Config / TIA Portal device configuration.
3. Wire the load cell, observing the shield clamp termination.
4. Wire any digital inputs/outputs and the analog output (WP231/WP251 only).
5. Connect the engineering tool to the CPU and download the hardware configuration.
6. Perform a theoretical calibration using the load cell's mV/V and the mechanical lever ratio (for hopper scales).
7. Perform a dead-weight calibration with at least three known weights covering 0%, 50%, and 100% of the nominal range.
8. Verify the corner load by placing the test weight at each corner of the platform. The maximum deviation between any two corners must be within the 0.05% error limit (e.g., ±0.5 kg on a 1,000 kg scale).
9. For WP231/WP251, verify the Modbus TCP connection from the HMI by polling the weight register (function code 03, register 0x0000 for gross, 0x0002 for net, depending on the firmware version).
10. For WP251 dosing applications, perform a five-shot dosing test with the configured recipe and record the actual fill weight, the in-flight correction, and the tolerance window. Adjust the coarse and fine lead times until the ±0.1% dosing tolerance is met.
11. Lock the module's configuration against unintended writes by setting the protection bit in the calibration menu (requires a password by default, configurable in the project specification).

Diagnostic Word and Fault Code Mapping

The module's status/diagnostic word exposes the operating state. The most common states for the WP2x1 family (bit positions are module-specific; refer to the device manual for the current generation):

Obsolescence and Lifecycle Status

The SIWAREX U is in the SIMATIC S7-300 product line, which is in the maintenance phase of its lifecycle as of the current product portfolio notice. Spare parts availability is supported for the next several years, but new installations in S7-300 should be justified on the basis of existing cabinets and existing program reuse. The S7-1200 is the active platform, and the WP231/WP251 are the recommended weighing modules for new installations.

When planning a brownfield migration from S7-300 + SIWAREX U to S7-1200 + WP2x1, account for the following reusable artefacts:

• Load cell, junction box, and field cabling (no rework required).
• Load cell mV/V certificate (re-enter into the WP2x1 calibration menu).
• Application logic for batching, totalisation, and recipe management (rewrite in TIA Portal; the STEP 7 FBs are not directly portable).
• HMI screens (rebuild in TIA Portal; the WP2x1 exposes the same Modbus TCP weight registers, so the HMI tag database can be re-mapped rather than re-engineered).

Verification Checklist for Field Acceptance Test

1. Module boots, no diagnostic interrupt in the CPU diagnostic buffer.
2. Load cell input reads 0.000 kg with the platform empty and stable.
3. Test weight at 50% of nominal reads within ±0.05% of the applied mass.
4. Test weight at 100% of nominal reads within ±0.05% of the applied mass.
5. Corner-load test: deviation between any two corners within ±0.05%.
6. Repeatability: five consecutive readings of the same static weight, standard deviation below the smallest displayed division.
7. (WP231/WP251) Modbus TCP poll from the HMI returns the current weight within 200 ms of the platform change.
8. (WP251) Five-shot dosing test: each shot within the configured ±0.1% tolerance, with the in-flight correction consistent across the five shots.
9. (Legal-for-trade) Calibration seal applied and witnessed.

Summary and Recommendation Matrix

FAQ