SIWAREX WP251 Replacement and Configuration with S7-1200 PLCs

Overview of the SIWAREX WP251 in S7-1200 Architectures

The SIWAREX WP251 is a multi-functional, freely-configurable weighing module for the SIMATIC S7-1200 automation system. It integrates a legal-for-trade weighing function directly into the S7-1200 PLC environment, eliminating the need for a stand-alone weighing transmitter. Each WP251 occupies one slot on the S7-1200 signal-module rack and exchanges both process data (weight, status, commands) and parameter data (calibration, filter, limits) with the CPU through the backplane bus.

The module is part of the broader SIWAREX electronic weighing family used in S7-1200 systems, which also includes the WP231 (belt-scale) and WP241 (process weighing) variants. All three modules share the same form factor, terminal layout, and TIA Portal device description. This commonality is critical for spare-parts handling, because a slot configured for any of the three can host any of the others after a parameter change in the Device Configuration.

Per the Siemens SIMATIC S7-1200 manual collection, the SIWAREX modules "use all the features of a modern automation system," including direct integration into TIA Portal, parameter backup in the CPU, and HMI visibility through standard tags.

SIWAREX WP251 and SM 1278 IO-Link Master Compatibility

The SIWAREX WP251 weighing module and the SM 1278 IO-Link master are spare-part-compatible in the sense that each can be ordered and replaced independently of the other. They are not, however, functionally paired — the WP251 is a backplane-mounted weighing module, while the SM 1278 is an IO-Link master that communicates over the S7-1200 backplane but is used to connect downstream IO-Link devices such as smart sensors.

The two modules are never stacked, daisy-chained, or electrically dependent. Replacing one does not force a re-configuration of the other, and a failure on the IO-Link master does not interrupt the WP251's weighing data path.

Spare-Part Procurement Rules

The WP251 is supplied as a standard catalog product and is not classified as a "spare-part-only" article. Customers can order replacement units through normal commercial channels (Siemens DIT/IA catalog, regional distributors) without invoking a special spare-parts agreement. Each unit ships factory-calibrated but un-parameterized; the load-cell calibration, scale name, and limit values are project-specific and must be re-applied at commissioning through SIWATOOL V7 or the TIA Portal device view.

Address Assignment: Fixed by PLC, Not by Module

The I/O address of a SIWAREX WP251 — both input (process data from load cell) and output (commands, zero/calibrate triggers) — is fixed by the S7-1200 CPU's Device Configuration, not by any DIP switch or hardware strap on the module itself. The WP251 module has no rotary address selector; all address information lives in the CPU's project.

Consequences for replacement:

1. The CPU assigns the same start address to every module that occupies the same physical slot position as the one declared in the project.
2. Removing the WP251 and inserting a new one in the identical slot with the power off yields the same logical address as the original unit.
3. If the new module is inserted while the CPU is in RUN, S7-1200 hot-swap behavior treats the slot as a module replacement and re-initializes the input image on the next bus cycle (typically within 100 ms).
4. If the new module is inserted in a different slot, the CPU either raises an I/O access error (if the original slot is now empty) or leaves the replacement un-configured (if the new slot has no project entry).

Required Hardware Configuration in TIA Portal

Hardware configuration is mandatory, even for a like-for-like swap. A bare insertion does not load any parameter set. The steps in TIA Portal are:

1. Open the project and double-click Devices > Device Configuration.
2. Locate the WP251 in the catalog tree under SIMATIC S7-1200 > Signal modules > Weighing > SIWAREX WP251.
3. Drag the module to the slot matching its physical position (counting left of the CPU from slot 101).
4. Open the module properties and assign a unique start address (input and output, default I-Address 0 / Q-Address 0 — change this if multiple WP251 are present).
5. Download the device configuration to the CPU.

After download, the CPU holds the configuration in retentive memory. A subsequent module swap on the same slot inherits this configuration without any further download, provided the replacement module's firmware is compatible (see Firmware Compatibility below).

SIWATOOL V7 Commissioning Software

SIWATOOL V7 is the Windows-based service tool used for fine calibration, parameter adjustment, scale diagnostics, and trace recording. It connects to the WP251 over the RS-232 service interface (point-to-point, 57.6 kbaud, 8N1) on the bottom of the module.

The connection sequence:

1. Connect a PC running SIWATOOL V7 to the WP251's service port using a 1:1 serial cable (Siemens 6ES7901-3DB30-0XA0 or equivalent).
2. Launch SIWATOOL V7 and select the COM port.
3. Power on the WP251 (or restart the S7-1200, which powers the backplane).
4. Click Connect. The tool reads the live parameters and current weight.
5. Open Scale > Calibration and run the calibration wizard: zero, span, optional linearity.
6. Save the parameter set back to the module (Module > Save to module).

SIWATOOL V7 is mandatory when the user does not have the S7 project file. Even if the slot address is preserved by the CPU, the load-cell-specific calibration constants (DR1, DR2, DR3, zero calibration) are stored in the WP251's non-volatile memory and must be either re-applied manually or restored from a backup file.

Operating Multiple WP251 Modules on a Single S7-1200

Although Siemens' documentation typically shows one or two WP251 per CPU, the S7-1200 backplane accepts up to eight signal modules per CPU (subject to the specific CPU variant's limit). In practice, 18 WP251 modules on one S7-1200 are unusual but technically possible if distributed across multiple CPU racks linked by PROFIBUS or PROFINET.

For multi-module deployments, each WP251 must have a unique I/O address range. A typical address plan for 18 WP251 over an S7-1200 cluster uses an offset of 16 bytes per module:

WP251 #01:  IB  0..15 / QB  0..15
WP251 #02:  IB 16..31 / QB 16..31
...
WP251 #18:  IB272..287 / QB272..287

The same plan must be reflected in the WP251 function block (FB) instance DB so that each call of SiwarexWP251 reads its own address range. The library ships with the SIWAREX WP251 TIA Portal HSP / function block library, normally delivered with the module.

Function Block Interface for Programmatic Access

The S7-1200 user program accesses the WP251 through the supplied function block SiwarexWP251 (FB). The block exposes the following standard I/O:

The function block is called in OB1 (or OB35 if a fixed cycle is desired for filtering). Each instance DB must be assigned to a unique background DB to avoid address overlap. Example call:

// Call in OB1 (SCL)
SiwarexWP251_DB(  // WP251 #03 instance
    iHWAddress := 32,              // start I/O address
    iType      := 1,               // single-range scale
    xZeroCmd   := HMI_Btn_Zero_3,
    xTareCmd   := HMI_Btn_Tare_3,
    iwWeight   => Weight_3,
    wStatus    => Status_3,
    wError     => Error_3
);

HMI Standalone Operation Without S7-1200

If the WP251 is used without an S7-1200 CPU — for example, in a retrofit with an HMI as the sole controller — the module can be operated in stand-alone mode. In this configuration:

• The WP251 retains its calibration internally and reads the load cell directly.
• An HMI panel (Siemens KTP or TP series, or a third-party panel with Modbus TCP/RTU master) reads the process data through the WP251's service port (Modbus RTU) or through the optional Ethernet port on the WP251 module.
• SIWATOOL V7 is used to configure the Modbus register map before commissioning.

Stand-alone operation eliminates the need for TIA Portal and the function block library, but the user accepts the loss of CPU-side alarm handling, recipe management, and cross-module interlocking.

Firmware Compatibility and Module Version Notes

A module from a newer firmware family can usually be inserted in a slot configured for an older family; the CPU writes the project parameters and may downgrade certain advanced features but does not raise a fault. A module from an older firmware inserted into a newer project will raise a configuration mismatch on the diagnostic buffer.

Replacement Procedure — Step-by-Step Verification

1. Open TIA Portal, connect online, and force the module's process image to a known state. Note the existing I/O address.
2. Power down the S7-1200 (PSU OFF). Do not hot-swap a weighing module while the load cell is under load.
3. Remove the defective WP251 by unlocking the front connector and pulling the module straight out.
4. Insert the replacement module in the same slot until the bus connector seats fully.
5. Restore the front connector wiring, paying attention to the load-cell shield terminal on the bottom of the module.
6. Power up the CPU. Confirm the module's channel-status LED is solid green (RUN).
7. Connect SIWATOOL V7 and re-run the calibration wizard with the load cell in the empty state.
8. Compare the zero reading to the previously archived value; deviation should be < 0.05 % of full scale for legal-for-trade acceptance.
9. Force the PLC into RUN. Verify the weight value appears in the FB instance DB and on any connected HMI.

Verification checklist:

• Channel-status LED: solid green.
• SF (group fault) LED: off.
• Weight register (FB output): non-zero with known test load.
• HMI live tag: refresh rate > 4 Hz.
• Diagnostic buffer: no entry "Module replacement detected" or "Configuration error".

Troubleshooting Matrix

Documentation and References for the Field Engineer

Engineers commissioning or replacing a WP251 should keep the following Siemens deliverables on hand:

• SIMATIC S7-1200 Manual Collection, chapter "SIWAREX electronic weighing systems" — SIWAREX electronic weighing systems
• SIWAREX WP251 device manual (function description, calibration, Modbus map)
• SIWATOOL V7 online help and the connection-cable ordering note
• TIA Portal HSP (Hardware Support Package) for WP251, distributed through the Siemens Industry Online Support

FAQ