SIWAREX U Module Configuration for S7-300 Weighing Systems

The SIWAREX U function module is a SIMATIC S7-300 weighing electronics module designed for high-speed, high-precision force and mass measurement in industrial automation. This reference covers module selection, S7-300 rack integration (including expansion racks coupled via CP 360/CP 361), load cell compatibility, EMC-compliant cable routing up to 500 m, configuration package ordering, and the mechanical limits of single-point load cells such as the Huntleigh Tedea 240 when used in multi-cell platforms.

1. SIWAREX U Module Family Overview

SIWAREX U is a function module (FM) in the SIMATIC S7-300 automation system. It can be installed in any S7-300 slot intended for function modules, including the central rack (CR) and expansion racks (ER) connected to the central rack via IM 360/IM 361 or via fibre-optic CP 360/CP 361 link modules. The module provides direct connection of strain-gauge load cells (typically 4-wire or 6-wire) and offers digital signal processing, tare, zeroing, and weight-value scaling in the SIMATIC backplane without additional external hardware.

The device manual (SIWAREX U Device Manual, Edition 38) describes all wiring diagrams, the operation of the diagnostic LEDs, the assignment of the 20-pin front connector, and the FB/FC blocks for STEP 7 integration.

Key characteristics:

• 1 or 2 independent weighing channels per module.
• Internal resolution 16 bit, accuracy class III to OIML R76.
• Measurement rate up to 50 Hz (adjustable).
• Communication to the S7-300 CPU via the backplane bus (no PROFIBUS required).
• Configuration via SIWATOOL U (RS-232) and integration into STEP 7 HW-Config.

2. Ordering Information and Part Numbers

The current SIWAREX U ordering structure (post-successor designation) is as follows:

Configuration software SIWATOOL U is supplied as a Windows executable and is required to set the load cell characteristic (mV/V), scale range, filter constants, and limit values. The configuration is downloaded to the module via the RS-232 service port on the front of the module using the SIWAREX serial cable (6ES7 972-0CA00-0XA0 or equivalent). The latest SIWATOOL U version is backward compatible with all 7MH4601 and 7MH4950 hardware revisions.

3. S7-300 Hardware Integration and Rack Placement

3.1 Slot and Backplane Rules

SIWAREX U occupies one standard S7-300 slot. It uses the same 40-pin backplane connector as digital and analog I/O modules and can be installed in any slot in any S7-300 rack where a function module is permitted. This includes:

• Central rack (CR) slots 4 to 11 (slots 0-3 are reserved for CPU, PS, IM).
• Expansion rack (ER1, ER2) with IM 360/IM 361 interface pair.
• Fibre-optic expansion rack via CP 360 (in CR) and CP 361 (in ER).

3.2 Placement in CP 360/CP 361 Coupled Rack

When the S7-300 is distributed across multiple racks using CP 360 (central side) and CP 361 (decentralised side), the SIWAREX U can be placed in the decentralised rack without restrictions. The CP 360/CP 361 pair provides transparent backplane communication up to a maximum of 90 m (multi-mode fibre) or 1000 m (single-mode fibre with appropriate SFP). The function module address space is mapped to the CPU process image exactly as if it were installed in the central rack.

STEP 7 hardware configuration must reflect the actual physical slot:

1. Open the SIMATIC 300 Station in HW-Config.
2. Insert the CP 360 in the central rack (rack 0).
3. Insert the CP 361 in the expansion rack (rack 1).
4. Insert SIWAREX U in the required slot of rack 1, slot 4 onward.
5. Save and download the hardware configuration.

Diagnostic interrupts and hardware faults generated by the SIWAREX U in the expansion rack are reported back to the CPU over the CP link and appear in the CPU diagnostic buffer in the same format as for central-rack modules.

3.3 Power and Grounding

SIWAREX U draws 24 V DC from the S7-300 backplane. No external power supply is required. Shield continuity for the load cell cable is established through the front connector shield clamp, which must be tightened to the specified torque (typically 0.6 Nm) to provide 360° shield contact.

4. Load Cell Selection: Single-Point vs. Multi-Cell Applications

4.1 Huntleigh Tedea 240 Cell Characteristics

The Huntleigh Tedea 240 (Vishay part family 240) is a single-point aluminium load cell with the following key parameters (verify against the actual datasheet revision):

The datasheet is published at Vishay Precision Group - Model 240 Datasheet.

4.2 Mechanical Constraint: Single-Cell Use Only

For an application with three weighing cells supporting a common platform:

• Each cell must be a true multi-cell (compression or tension) load cell with matched nominal capacity and output (mV/V).
• The three cells must be connected electrically in parallel (signal lines tied together) and the SIWAREX U supplies the excitation voltage to the combined load cell network.
• Corner-load trimming is performed by adding parallel calibration resistors at each cell, or electronically via SIWATOOL U corner-load adjustment parameters.

4.3 Cannot Re-Rate a Load Cell Mechanically

A load cell cannot be "physically re-calibrated" to a different nominal capacity than the value for which it was manufactured. The strain-gauge geometry, the proof-load rating, the creep performance, and the temperature coefficient are all fixed by the mechanical design. Reducing the rated capacity from 7 kg to 3 kg by mechanical means (drilling, machining, preload adjustment) is not supported by the manufacturer and invalidates the calibration certificate.

To change scale capacity, either:

1. Replace the cell with a different model of the correct nominal capacity, or
2. Use the existing cell but configure SIWAREX U for the actual capacity used (the SIWAREX supports any capacity up to the cell's rated load, scaled in SIWATOOL U), provided the cell is never loaded above its rated nominal load and proof load.

5. Signal Cable Routing and EMC Compliance

5.1 Maximum Cable Length

Siemens specifies a maximum cable length of 500 m for SIWAREX U load cell connections, provided the cable meets the required cross-section and EMC specifications. The limit is driven by:

• Excitation voltage drop in the cable (cells require 5 to 12 V DC excitation; total loop resistance must not exceed the value given in the cell datasheet).
• Capacitive loading on the differential sense lines (causes gain error at the SIWAREX input).
• Common-mode noise pickup on unshielded or improperly terminated runs.

For a 10 m run (the typical case in a control panel or short machine harness), cable losses are negligible and standard LiYCY or similar shielded twisted-pair cable is sufficient.

5.2 EMC Installation Rules

For longer runs, follow Siemens EMC installation guidelines (Siemens Industry Online Support: SIMATIC S7-300 EMC Installation Guidelines):

1. Use a shielded, twisted-pair cable with a dedicated drain wire. Six-wire connection (sense + excitation) is recommended over 4-wire for any run over 10 m.
2. Ground the cable shield at the SIWAREX end only, using the front-connector shield clamp. Do not ground the shield at the load cell end unless the cell manufacturer explicitly requires it.
3. Maintain a minimum 200 mm separation from VFD power cables, motor cables, and any cable carrying > 24 V at > 1 A.
4. Cross power cables at 90° if they must intersect.
5. Route the load cell cable in a dedicated cable tray, not mixed with switched-mode power supply wiring.

5.3 Cable Specifications

6. Configuration with SIWATOOL U

6.1 Parameter Set Overview

SIWATOOL U exposes the following primary parameters for each channel:

6.2 STEP 7 Integration

After SIWATOOL U has been used to download the scale parameters, the STEP 7 user program reads the weight through the function block FB SIWA (or the package-supplied FBs). The read value is in the configured engineering units. The block provides a status word with bits for tare, zero, stable value, and limit-violation flags.

7. Backward Compatibility and Migration

The new configuration package 7MH4950-1AK01 contains:

• SIWATOOL U (latest revision)
• Updated HW-Config type file (GSD-style integration into STEP 7 HW-Catalog)
• User manual PDF

This package supports the new 7MH4950-1AA01 and 7MH4950-2AA01 modules. It can also be used to configure the older 7MH4601-1BA01 modules, because the firmware register layout and the SIWATOOL U parameter tree are identical between the two designations. The only difference between using the old vs. new configuration package for an old module is the catalog integration in HW-Config (the new package includes a more recent type file that may be required by STEP 7 V5.7 or TIA Portal migration paths).

8. Step-by-Step Commissioning Procedure

8.1 Prerequisites

• STEP 7 V5.5 or later, with HW-Catalog support for SIWAREX U.
• SIWATOOL U installed on the engineering PC.
• RS-232 serial cable (Siemens 6ES7 972-0CA00-0XA0) or USB-to-RS232 adapter.
• Load cell datasheet and traceability certificate (mV/V, capacity, calibration data).
• Reference weight (calibration mass) at 50% to 100% of the planned scale capacity.

8.2 Commissioning Steps

1. Mount the load cell(s) on the platform. Verify mechanical alignment (level, no twist, no side load). For a multi-cell platform, perform a preliminary equal-height adjustment.
2. Wire the cell to the SIWAREX U front connector using the 4-wire or 6-wire schematic. Tighten the shield clamp to 0.6 Nm.
3. Insert the SIWAREX U into the planned slot. Power up the rack.
4. Connect the RS-232 cable from PC to the SIWAREX U service port.
5. Start SIWATOOL U, select the correct COM port, and read the current module parameters (read = upload from module).
6. Enter the cell rated output (mV/V) from the cell calibration certificate.
7. Enter the nominal load, dead load, decimal point, measurement rate, and filter time.
8. Download the parameters to the module (write = download to module).
9. With no load on the platform, send the "Adjust zero" command from SIWATOOL U.
10. Apply the calibration mass to the centre of the platform. Adjust the scale span until the displayed value matches the calibration mass.
11. Perform a linearity check at 0%, 25%, 50%, 75%, 100% of nominal load.
12. For multi-cell platforms, perform corner-load testing. If a corner reads high or low, adjust with the cell's pre-load shim or with the corner-adjust parameter in SIWATOOL U.
13. Save the parameter set as a backup file (.par) and archive it with the cell calibration certificate.

8.3 Verification

• Read the weight value via the STEP 7 FB and verify stable value flags.
• Trigger a hardware interrupt (OB40) by exceeding a limit, and verify the CPU interrupt handler executes.
• Cycle the S7-300 power and verify that the SIWAREX U re-initialises with the stored parameters.
• Check the CPU diagnostic buffer for any SIWAREX U fault entries (e.g., load cell signal out of range, sense line break).

9. Troubleshooting Matrix

10. Application Constraint Summary

Before specifying a SIWAREX U solution with three weighing cells, verify:

1. All three cells are true multi-cell platform-grade load cells, matched in nominal capacity and output, and approved by the manufacturer for parallel mechanical mounting on a single weighing platform.
2. The platform is constrained horizontally (typically with check rods or flexures) to prevent side loads on the cells.
3. The total platform weight (dead load) does not exceed the combined safe load of the cells, and the maximum live load leaves adequate margin below the combined nominal load.
4. The signal cable length is within 500 m, and EMC routing rules are followed.
5. The SIWAREX U is placed in an S7-300 rack that the CPU can address via the backplane (central or CP 360/361 expansion).

If the original specification is to use three Tedea 240 single-point cells, the application must be re-evaluated. Single-point cells can be used one-per-scale, but the typical "balance" application (one platform, three support points) requires three multi-cell platform load cells in parallel, not three single-point cells.

11. References to Official Documentation

Engineers commissioning a SIWAREX U should consult the device manual directly:

• SIWAREX U Device Manual (Edition 38, Siemens Industry Online Support)
• Siemens Industry Online Support: S7-300 EMC Installation Guidelines
• Vishay Precision Group - Model 240 Load Cell Datasheet

FAQ