Configuring SIWAREX WP321 with S7-1200 and ET200SP

Overview

SIWAREX is Siemens' family of PLC-integrated weighing electronics. Modules convert the low-level millivolt output of strain-gauge load cells into engineering units (kg, t, lb, N) and expose the gross, net, tare, and status values to the SIMATIC controller through PROFINET, PROFIBUS, or Modbus TCP. The modules are application-tuned: WP231 for static weighing, WP241 for belt scales, WP251 for dosing/filling, and WP321 for static weighing in distributed I/O. The WP321 (7MH4138-6AA00-0BA0) plugs into an ET200SP station; the WP231/WP241/WP251 mount directly beside an S7-1200 CPU as distributed I/O on PROFINET.

At the time of the latest published catalog state, no SIWAREX module is offered for the ET200MP I/O family. A user who needs to integrate a load cell with an S7-1214C therefore has two viable paths: add an ET200SP station to host the WP321, or use a WP231/WP241/WP251 beside the S7-1200. Both paths are documented here with part numbers, baseUnit selection, configuration package content, and commissioning checks.

SIWAREX Module Family Comparison

The four WP modules currently relevant for S7-1200 users are summarized below. Choose the variant whose application profile matches the use case rather than retrofitting firmware.

The WP321 is the correct static-weighing module for a single-channel load-cell application where the S7-1214C is on PROFINET with an ET200SP head station. The WP231 supports Modbus TCP and can be operated headlessly without an S7-1200 in the loop, which is useful for retrofit or panel-only installations.

WP321 Hardware Specifications

The WP321 occupies one slot in an ET200SP station and requires a Type A0 baseUnit. The Type A0 designation identifies the spring-loaded terminal variant with no auxiliary terminals (the equivalent Type A1 has auxiliary terminals for an internal voltage bus). Verify your baseUnit ordering code: 6ES7193-6BP00-0DA0 is the typical Type A0 (light) variant; the dark variant 6ES7193-6BP00-0BA0 provides a different terminal color for segregation of hazardous and non-hazardous wiring.

Load Cell Compatibility and Wiring

SIWAREX modules accept standard four-wire and six-wire load cells. Six-wire connection compensates for cable length-induced voltage drop on the excitation and sense lines, which matters on weigh hoppers where the cable run from the junction box to the electronics exceeds 10 m. Four-wire connection is acceptable for bench scales and short cable runs.

Connect the shield on one end only — typically at the panel earth bar near the ET200SP — to prevent ground-loop currents from injecting noise into the microvolt-level signal. If the load cell is mounted on a separate structure, bond the structure to the same equipotential bonding bar to keep shield current < 100 mA.

Cable length and gauge

The maximum cable length between load cell and WP321 is a function of bridge resistance and cable capacitance. Practical limits:

• Four-wire with 350 Ω cell: 100 m with 0.5 mm² shielded cable.
• Six-wire with 350 Ω cell: 500 m with 0.75 mm² shielded cable.
• Multiple cells in parallel: total parallel resistance must exceed 40 Ω. With four 350 Ω cells the parallel resistance is 87.5 Ω.

Junction box

For multi-cell installations, use a SIWAREX JB junction box (e.g., 7MH5001-... family) with individual corner-load trim resistors. Without corner-load trimming, off-center loads can produce errors exceeding 0.1 % of full scale. Verify the trimmed mV/V output of the paralleled cell cluster falls within the 1 to 4 mV/V input window of the WP321.

SITRANS Load Cells

Siemens offers a matched load cell family for use with the WP modules. The SITRANS load cell portfolio covers nominal loads from 0.3 kg (compression) to 500 t (rocker-column and shear-beam designs). When selecting a cell, match the nominal load to the live load (weighed material + vessel + structure) with a target utilization of 50 % to 80 % of nominal. Lower utilization wastes resolution; higher utilization shortens fatigue life and increases creep.

Refer to the Siemens SITRANS Load Cells product page for the current catalog of compression, tension, shear-beam, and bending-ring load cells with their respective OIML accuracy classes and hazardous-area approvals.

BaseUnit and ET200SP Rack Requirements

An ET200SP station consists of an interface module (IM) and a rail of baseUnits. The WP321 inserts into a Type A0 baseUnit only. The complete bill of materials for a single-channel static-weighing station:

1. ET200SP interface module, e.g., IM 155-6 PN HF (6ES7155-6AU00-0CN0) for PROFINET connection to S7-1214C.
2. BaseUnit Type A0, light variant (6ES7193-6BP00-0DA0).
3. SIWAREX WP321 module (7MH4138-6AA00-0BA0).
4. Server module to terminate the backplane (6ES7193-6PA00-0AA0).
5. Configuration package for TIA Portal (7MH4138-1AK01) — supplies the GSDML, the application example program, and the SIWATOOL parameterization tool.

TIA Portal Configuration with Configuration Package 7MH4138-1AK01

The SIWAREX WP321 Configuration Package for TIA Portal (7MH4138-1AK01) supplies a ready-to-use application program plus the SIWATOOL PC configuration software. The configuration sequence:

1. Install the HSP (Hardware Support Package): In TIA Portal, navigate to Options → Manage General Station Description Files (GSD) and install the SIWAREX WP321 GSDML. Restart TIA Portal so the module appears in the Hardware Catalog under Other field devices → PROFINET IO → Sensors → Siemens AG → SIWAREX.
2. Insert the IM 155-6 PN: Add the ET200SP interface module to the project. The S7-1214C must be configured as the PROFINET IO controller with the IM 155-6 PN assigned to the same subnet.
3. Insert the WP321: Drag the WP321 onto slot 1 of the ET200SP station. TIA Portal will validate the baseUnit type; only Type A0 will accept the module.
4. Assign device name and IP address: Right-click the WP321 → Assign device name. The default PROFINET device name is siwarex-wp321 and the default IP is 192.168.0.x. Match the subnet to the controller.
5. Configure I/O data: The WP321 exposes a fixed I/O image. Slot 0 is the weight/status input word; slot 1 is the gross/net/tare output words. Map these to a global DB in the S7-1200 user program.
6. Import the example FB: From the configuration package DVD, copy the SIWAREX_WP321 function block into the S7-1200 program. The FB handles command/datum handshake, tare, zero, and calibration commands.
7. Download and verify: Compile, download HW Config to the ET200SP IM, and download the S7-1200 program to the CPU. The WP321 will report Run on its front-panel LEDs within 5 seconds of PROFINET cyclic data exchange.

Program structure on S7-1214C

The example FB provides the following command interface:

// SIWAREX WP321 command interface (example)
"WP321_DB".Command := 16#0001;        // 0x0001 = tare, 0x0002 = zero, 0x0003 = calibrate
"WP321_DB".Command_Trigger := TRUE;   // one-shot rising edge
"WP321_DB".Gross_Weight_kg := "WP321_DB".Weight_input_word * 0.001;
"WP321_DB".Status := "WP321_DB".Status_input_word;

Reading the gross weight: the WP321 outputs a 32-bit signed integer representing grams by default. Divide by 1000 in the PLC to obtain kg. The unit, decimal place, and sign are configured in SIWATOOL under DR3 → Scale parameters.

SIWATOOL V7 Service Parameterization

SIWATOOL is a Windows service tool that connects to the WP321 over RS485 using a SIWAREX USB service interface (order number 7MH4603-1AA01). It is the only way to set calibration parameters, scale data records (DR0…DR39), and limits; the TIA Portal configuration program only exposes the process-side I/O.

1. Connect the USB service cable to the RS485 port on the WP321 front panel.
2. Launch SIWATOOL V7 → File → Connect → select the COM port assigned to the USB-RS485 adapter.
3. Click Read DR to load the current data record set from the module.
4. Edit DR3 (scale parameters) for unit, decimal place, and step size.
5. Edit DR7 (calibration weights) and DR9 (zero/span) for the actual dead load and test load values.
6. Click Write DR to push the changes back to the WP321 non-volatile memory.

WP231 Standalone Modbus TCP Option

If the project requires a load-cell read-out independent of the S7-1200 program (for example, a panel meter on the floor displaying weight), the WP231 supports Modbus TCP in standalone mode. Wire a PROFINET cable from the WP321 to the panel PC or use an IM 155-6 PN as the gateway. The default port is 502; the holding register map mirrors the I/O word map. A SCADA or HMI can poll weight and status without the S7-1200 in the loop.

The WP231 also supports the same RS485 service port for SIWATOOL; the standalone mode simply means the PROFINET cyclic data exchange is optional rather than required.

RS485 Remote Display

The WP321's RS485 port supports connection to a remote SIWAREX display (e.g., 7MH4601-1AA01 family) using the SIWAREX protocol at 9600 bps, 8N1. Wiring is two-wire half-duplex on terminals A and B of the WP321 front-panel RS485 connector. The remote display shows gross weight and a status LED; tare/zero commands can be issued from the display's keypad if enabled in DR15.

Commissioning Procedure

Use this sequence on a fresh install to verify the WP321 end-to-end before connecting to the S7-1200 program logic:

1. Visual check: Confirm the WP321 is fully seated in the baseUnit with the locking tab engaged; confirm shield drain wire is bonded to the panel earth bar.
2. Power up: Apply 24 V to the ET200SP power infeed. The WP321 RDY LED should transition from red to green within 5 s; a flashing red indicates a hardware fault.
3. SIWATOOL health check: Connect via USB-RS485. Verify FW version (DR0) matches the release notes for the configuration package. Verify DR3 unit/step matches the load cell datasheet.
4. Zero calibration: With the vessel empty, issue Command 0x0002 (zero) from SIWATOOL or the TIA Portal FB. Confirm the gross weight reads 0 ± 1 digit.
5. Span calibration: Apply a known test load (typically 50 % to 100 % of full scale). Issue Command 0x0003 (calibrate) with the actual load value entered in DR9. Confirm the displayed weight matches the applied load within ± 0.05 %.
6. Linearity check: Apply 25 %, 50 %, 75 %, 100 % of full scale in turn; record the deviation. Maximum permissible deviation depends on the OIML class — OIML R76 C3 is ± 0.025 % of full scale at the verification points.
7. PROFINET cyclic exchange: With TIA Portal online, monitor the I/O words from the WP321. Confirm Status_word bit 0 (Data valid) is set within 10 s of Going Online.

Diagnostics and Fault Codes

The WP321 reports faults in the I/O status word and in SIWATOOL under DR0. The most commonly encountered fault codes during integration are:

The status word returned to the S7-1200 user program exposes a bit-field summary: bit 0 = Data valid, bit 1 = Above max, bit 2 = Below zero, bit 3 = Calibration in progress, bit 4 = Limit 1 exceeded, bit 5 = Limit 2 exceeded. Use these bits for interlocks in the PLC program rather than polling the numeric fault code.

Troubleshooting Matrix

Field-Proven Caveats

• Legal-for-trade applications: If the scale is used for trade (custody transfer, invoicing by weight), the WP321 must be sealed by a Weights and Measures inspector after calibration. The configuration package provides the sealing software; do not break the seal after commissioning without re-certification.
• ATEX zone 2: The WP321 is approved for zone 2 mounting. The load cell itself must have its own ATEX rating for the zone in which it is installed. Verify the load cell datasheet explicitly states IECEx/ATEX marking, not just "industrial".
• Replacement module: When swapping a defective WP321 with a new one, you must transfer DR3, DR7, DR9, and DR15 from the old module (or from your backup) to the new module via SIWATOOL. Without this, the scale reads zero or arbitrary values.
• Multiple load cells: Four 350 Ω cells in parallel = 87.5 Ω, well above the 40 Ω minimum. If you mix cell capacities (e.g., three 1000 kg and one 500 kg), corner-load trimming in the junction box is mandatory; otherwise off-center loads produce errors larger than the rated accuracy.
• Mechanical isolation: The load cell must be mechanically isolated from the supporting structure except at its load-introduction point. Otherwise side loads, thermal expansion of the mounting frame, and piping reaction forces will bias the reading and accelerate fatigue.

FAQ