SIWAREX WP321 Calibration Errors on CPU 1215C: Multi-Module Setup

SIWAREX WP321 Calibration Errors on CPU 1215C: Multi-Module Setup

When three SIWAREX WP321 weighing modules (Siemens order number 7MH4960-2AA01) are operated on a SIMATIC S7-1200 CPU 1215C through three independent instance data blocks, the automatic calibration path driven from an HMI such as the KTP1000 Basic frequently stalls: the module accepts the command number, but the calibration weight value is rejected or the command never reaches a terminal state. The failure is almost always a command/data-record routing issue, not a load-cell issue. This reference documents the underlying data record protocol, the role of DR 3 (calibration weight 0), the proper WRREC/RDREC parameterization for multiple HW_IDs, and the verification sequence that restores a working three-module installation.

1. System Architecture Overview

The reference configuration is:

• 1 × SIMATIC S7-1200 CPU 1215C (firmware ≥ V4.2 for full WP321 support; V4.4 recommended).
• 3 × SIWAREX WP321 modules, order number 7MH4960-2AA01, mounted either in the CPU's signal board / left connector or in an ET 200SP station on PROFINET.
• 3 × load cells per WP321 (full-bridge, 1–4 mV/V), terminated on the module's front connector.
• 1 × KTP1000 Basic HMI on the same PROFINET subnet, communicating with the S7-1200 via S7 communication.
• 1 × TIA Portal project with one instance FB (WP321_PE / WP321_WP) and three independent DBs (e.g., DB_WP321_SCALE_1, DB_WP321_SCALE_2, DB_WP321_SCALE_3).

Each WP321 occupies a slot in the device configuration and is assigned a unique PROFINET device name and a unique hardware identifier (HW_ID) under System constants in TIA Portal. The HW_ID is what the WRREC and RDREC system blocks use to route a data record to a specific physical module; it is not the same as the module's diagnostic address or input address.

2. How the WP321 Receives Calibration Commands

The SIWAREX WP321 exposes its parameter, command, and process interface exclusively through PROFIBUS/PROFINET data records. There is no direct I/O byte for "start calibration" — the HMI or user program must:

1. Write the calibration parameters to DR 3 (calibration weight 0 / nominal weight) and to DR 4 (calibration weight 1) as required.
2. Write a command code to DR 2 to trigger the calibration step.
3. Poll the command state by reading DR 2 and the module status by reading DR 0 and DR 1.

Per the WP321 manual (SIWAREX WP321 function manual, A5E33715669-02):

• The command area of DR 2 uses a fixed coding: command code 2000 + 3 = 2003 addresses the data record number 3 of the module. The pattern is 2000 + n for read-side addressing, and 4000 + n for write-side addressing of the same DR n.
• Writing to a data record: 4000 + n with the relevant payload.
• Reading a data record: 2000 + n returns the current contents of DR n.

3. Why the Standard Example Breaks with Three Modules

The Siemens sample project (entry ID 66825585) instantiates a single WP321 FB with a single instance DB and a single HW_ID. To scale to three modules, engineers typically copy the instance DB three times and change two parameters at each call site:

• LADDR (or HWi_ID in newer library versions) — input address of the module.
• HW_ID — the symbolic constant from the device configuration that uniquely identifies the slot.

That is the correct top-level approach, but three errors are common in the field:

1. Symbolic HW_ID collision. The constant from System constants must be re-resolved after each module is added. If the engineer copy-pastes the first module's HW_ID variable into the second/third call, both calls route WRREC/RDREC to the same module.
2. Single instance DB used for command state. The example FB stores the last command number, command state machine, and result bytes in its instance DB. If the same DB is referenced for all three scales, the second and third calibrations overwrite the first's command state mid-execution, producing "command in progress" / "no command in progress" flicker on the HMI.
3. HMI tag binding not duplicated. The KTP1000 tag database still points to the original module's DR 3 symbolic tag, so two of the three scales never receive their calibration weight at all.

4. Diagnostic Procedure

Run the following checks in order. Each step ends with a pass/fail decision that points to the next action.

4.1 Verify module identity and firmware

1. Open the S7-1200's Online & Diagnostics view in TIA Portal.
2. Expand Distributed I/O and confirm each WP321 reports order number 7MH4960-2AA01 and firmware version compatible with TIA Portal V16+ (typical field firmware is V1.x for WP321; V1.3+ is recommended).
3. Capture the Module Information > Status screen. A WP321 reporting diagnostic interrupt "channel error" or "parameter assignment error" will reject all data-record writes until cleared.

Pass: three modules online, no diagnostic alarms. Fail: clear alarms first (usually a missing load-cell terminator, mismatched full-bridge impedance, or unassigned HW identifier) before continuing.

4.2 Confirm distinct HW_IDs

1. In the project tree, select PLC > System constants.
2. Filter the constant list for the substring WP321 or the module name.
3. There must be exactly three HW_ID constants, one per slot, and the symbolic name must follow the pattern <Head module name>~WP321~HW_ID (or the equivalent local constant name generated by TIA Portal).

Pass: three unique constants. Fail: the modules are likely mapped to the same PROFINET device or the project was not recompiled after adding modules 2 and 3.

4.3 Confirm three instance DBs are wired

1. Open the cyclic OB (typically OB1 or OB35).
2. Confirm three calls of the WP321 FB exist, each with its own instance DB and its own HW_ID input.
3. Watch the three DBs in the watch table simultaneously. Each call must toggle its Done, Busy, and Error bits independently.

Pass: independent state machines. Fail: the same DB is being reused; create three new instance DBs and re-wire the calls.

4.4 Read DR 0 / DR 1 from each module

From the watch table, force a one-shot RDREC against each module's HW_ID with RECORD = 0 and RECORD = 1. Verify:

• All three return non-zero, non-static data.
• No module returns all-zero payload (would indicate the read landed on a non-existent slot or wrong device).
• Weight bits change when a calibrated test weight is placed on the scale.

4.5 Read DR 2 to confirm command interface

1. From the HMI, start calibration on scale 2 only (leave scales 1 and 3 idle).
2. Watch DR 2 on scale 2 in the watch table. The command code should appear immediately, the busy bit should clear, and the result byte should report success.
3. Repeat for scale 1 and scale 3.

If the command code never appears in DR 2 of the selected module but does appear in the other module's DR 2, the HW_ID wiring is wrong. If the command code appears but Error is set with reason "invalid calibration weight", DR 3 was not written correctly.

5. Step-by-Step Resolution

5.1 Replicate the FB and DB correctly

In the project tree under Program blocks:

1. Duplicate the WP321 instance DB three times: DB_WP321_1, DB_WP321_2, DB_WP321_3.
2. Open each call of the WP321 FB in OB1 and assign the matching instance DB.
3. Set the HW_ID input on each call to the matching constant from System constants. Do not paste the same constant into all three calls.

5.2 Build the HMI command path

The HMI should not write directly to DR 2. Use an S7 tag that the FB exposes for "start calibration", and let the FB assemble the data-record write. In the KTP1000 tag list:

• Three start bits, one per scale: SCALE_n.START_CAL.
• Three reference-weight tags: SCALE_n.REF_WEIGHT_0, bound to a numeric input field that updates DR 3 on each entry.
• Three status fields: SCALE_n.CMD_STATE and SCALE_n.LAST_ERROR bound from the FB's outputs.

5.3 Drive DR 3 from the FB before each command

Extend the WP321 FB (or use the official "Siwarex" library block) so that the calibration sequence is:

1. Write the user-entered calibration weight to DR 3 using WRREC with HW_ID = the scale's identifier and RECORD = 3.
2. Wait for WRREC.Done = TRUE and WRREC.Error = FALSE.
3. Write command code 5 (calibration with weight 0) or 6 (calibration with weight 1) to DR 2.
4. Wait for command completion by polling DR 2 via RDREC; the result byte should reach the success state defined in the manual.
5. Update the FB's Done, Busy, Error, and CmdState outputs.

5.4 Sample SCL skeleton

The following Structured Control Language fragment shows the multi-module routing pattern. Adapt the type names to your installed library version.

// Scale 1
WRREC_DB.WP321[1](REQ := SCALE_1.START_CAL,
                  LADDR := SCALE_1.HW_ID,
                  RECNUM := 3,
                  RECORD := SCALE_1.REF_WEIGHT_0_BUF,
                  DONE => SCALE_1.DR3_DONE,
                  BUSY => SCALE_1.DR3_BUSY,
                  ERROR => SCALE_1.DR3_ERROR,
                  STATUS => SCALE_1.DR3_STATUS);

// Trigger the calibration command on DR 2 after DR 3 has latched
IF SCALE_1.DR3_DONE AND NOT SCALE_1.DR3_ERROR THEN
    WRREC_CMD_DB.WP321[1](REQ := TRUE,
                          LADDR := SCALE_1.HW_ID,
                          RECNUM := 2,
                          RECORD := CMD_CAL_W0,    // command code 5
                          DONE => SCALE_1.CMD_DONE,
                          BUSY => SCALE_1.CMD_BUSY,
                          ERROR => SCALE_1.CMD_ERROR,
                          STATUS => SCALE_1.CMD_STATUS);
END_IF;

Repeat the same pattern for SCALE_2 and SCALE_3, each with its own WRREC call and its own HW_ID constant. Do not share a multi-instance FB output across scales; the WP321 command state is per-module and must remain so.

6. Verification Checklist

After the corrections, run this end-to-end check before releasing the machine:

1. Power-cycle the S7-1200 and the ET 200SP station so the WP321 modules come up with a known-clean state.
2. From the KTP1000, enter a 100.00 kg reference weight on scale 1 and press Start Calibration. The status field should transition: Idle → Sending DR 3 → Sending command → Calibrating → Done.
3. Place a 100 kg test weight on scale 1. The process display should read 100.00 kg ± the module's specified accuracy class (typically ±0.05% for WP321).
4. Repeat for scale 2 and scale 3. Each must transition through its own state machine independently.
5. Read DR 0 from each module and confirm the weight signal is stable, non-zero, and tracks applied load.
6. Trigger a "taring" command on each scale; verify zero is set, then remove the test weight and confirm the display returns to 0.00 kg with no drift.
7. Power-cycle again. Re-read DR 3 on each module; the calibration weight value must persist (it is stored non-volatile in the module), proving the write reached the correct physical module.

7. Troubleshooting Matrix

8. Parameter Reference

9. WP231 vs WP321 Distinction

Field engineers frequently mix up the two modules. The WP231 is the older S7-1200-direct module (order 7MH4960-2AA01 in some catalogs, distinct from the WP321 7MH4960-2AA01), while the WP321 is the ET 200SP variant. The data-record command codes are similar but not identical:

• WP231: command interface in DR 2, weight 0 in DR 10, weight 1 in DR 11.
• WP321: weight 0 in DR 3, weight 1 in DR 4; command interface in DR 2 with the same 2000/4000 prefix convention.

If the project loads the WP231 example onto WP321 hardware (or vice versa), the command sequence is accepted but the calibration weight value is never written, because the DR numbers do not align. Always confirm the module variant by reading the order number from the device label, not from a generic catalog page.

10. Common Command Codes

When the field report says "no command in progress" yet the HMI shows calibration "started", it usually means DR 2 was written to the wrong module's HW_ID and the selected scale never received the command. The HMI is observing the local DB state ("command sent from PLC"), not the module's DR 2 echo.

11. Safety and Operational Notes

12. Related Documentation

• SIWAREX WP321 function manual (PDF, A5E33715669-02)
• ET 200SP manual collection, SIWAREX WP321 chapter
• SIWAREX WP321 sample project (entry ID 66825585)

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