SIWAREX WP241: Reading Data Records via CMD_A/CMD_B Commands

The Siemens SIWAREX WP241 is a single-channel, high-precision weighing electronics module designed for automatic scales, fillers, checkweighers, and totalizing scales in ET 200SP. All weighing parameters, calibration data, process values, totalizers, and status information are grouped into Data Records (DR) that are exchanged between the PLC and the module over the backplane bus. A clear understanding of the command architecture (CMD_A, CMD_B, CMD1, CMD2, CMD3) and the trigger/status flag behavior of the Siemens Function Block (FB) is essential to read DRs reliably without entering service mode and without corrupting the cyclic data exchange.

1. SIWAREX WP241 Module Overview and Command Architecture

The WP241 occupies one slot in an ET 200SP station and exchanges process data with the S7-1200 / S7-1500 IO controller. The official SIWAREX WP241 Equipment Manual (A5E33713528A-01) defines the parameter set as numbered data records. Each DR has a fixed length and a fixed command code used to trigger the read or write. The module supports three command levels, and the example project shipped by Siemens uses an additional two internal command codes (CMD_A, CMD_B) to drive CMD3 from a cyclic OB.

1.1 Command Levels (CMD1, CMD2, CMD3)

1.2 CMD_A and CMD_B – The Cyclic Toggle Mechanism

CMD_A and CMD_B are not independent command levels. They are simply two command-code variables that the FC30/OB example code alternately writes into the CMD3 command word. The behaviour is:

1. OB35 fires every 100 ms (configurable in the CPU properties under Cyclic Interrupts).
2. On the first call, FC30 reads s_CMD_A and copies it to s_CMD3 in DB241.
3. On the next OB35 call, FC30 reads s_CMD_B and copies it to s_CMD3.
4. On the third call, FC30 toggles back to s_CMD_A, and so on.

This means the user only has to write the desired command codes into CMD_A (DB241.DBW468) and CMD_B (DB241.DBW470) once. The module then sees command 2033 (Read DR 33) alternating with command 2030 (Read DR 30) at a 5-second composite cycle (each DR is read every 200 ms cycle pair, with 100 ms between alternations), giving you two parallel cyclic data streams from CMD3.

2. Reading a Data Record – Step-by-Step Procedure

2.1 Prerequisites

• SIWAREX WP241 (6AG1135-1AA01-1AA0 or current catalog number) installed in ET 200SP with HSP installed in TIA Portal.
• S7-1200 (CPU V2.2, V3, or V4 supported by the Siemens "Ready for use" example) or S7-1500 IO controller.
• Siemens WP241 Function Block imported into the project (downloadable from the Siemens Support site as part of the "Ready for use" project).
• DB241 instantiated (this is the standard instance DB the FB works against).
• Module configured in device view, base addresses noted (default inputs start at a slot-dependent offset).

2.2 Trigger Pulse Rules

The Siemens FB handles trigger discipline for you:

• Trigger input: bo_CMD_trigger (BOOL) at the FB input.
• Behaviour: Set the trigger for exactly one PLC cycle. The FB detects the rising edge, launches the command on the backplane, and automatically resets the trigger to FALSE internally.
• Consequence: You should never hold the trigger TRUE. A held trigger will be ignored after the first cycle and will not cause re-triggering.

2.3 Status Flag Semantics

The FB exposes three status BOOLs. Unlike the trigger, these are latched:

2.4 Reading DR 33 (Totalizers) – Concrete Example

DR 33 holds all totalizer registers and is the most commonly read DR for totalizing-scale applications. The command code is 2033 decimal.

2.4.1 One-shot read via CMD1

1. Write the constant 2033 to i_CMD_Code (INT) at the FB input.
2. Pulse bo_CMD_trigger TRUE for one cycle.
3. Monitor bo_CMD_FinishedOK. When it goes TRUE, the totalizers in DB241 are valid.
4. Read Totalizer 2 from DB241.DBD506 (DWORD, signed 32-bit, scaled in the unit configured in DR 30 / DR 31).

2.4.2 Cyclic read via CMD_A / CMD_B

1. Open DB241.
2. Set CMD_A (DBW468) to a different DR, e.g. 2030 for DR 30 (process values).
3. Set CMD_B (DBW470) to 2033 for DR 33 (totalizers).
4. Ensure OB35 is configured for 100 ms and FC30 is called inside it.
5. The module alternates 2030 / 2033 every 100 ms; the DB is always current within 200 ms.

3. Data Record Reference (DR numbers used in this article)

The full DR list is published in chapter 6 of the SIWAREX WP241 Equipment Manual and the SIWAREX WP241 FAQ / Siwatool Calibration document.

4. Totalizer Reset Commands

Totalizers are cleared with dedicated command codes. Service mode is not required for reset of totalizers — it is only required for adjustment commands (calibration). The relevant codes:

5. Cyclic OB Architecture and Timing

The "Ready for use" example project ships two cyclic OBs:

The OB cycle must be ≥ the module's command processing latency. 100 ms gives a comfortable margin for both DR 30 and DR 33 reads. If you read more than two DRs, insert an additional counter in FC30 to expand the rotation cycle — do not chain commands back-to-back without a gap.

5.1 Sample FC30 Toggle Logic (Structured Text, equivalent to the example FB)

// FC30 – called from OB35 every 100 ms
IF "Toggle" THEN
    "DB241".CMD3 := "DB241".CMD_A;     // first half-cycle
    "Toggle" := FALSE;
else
    "DB241".CMD3 := "DB241".CMD_B;     // second half-cycle
    "Toggle" := TRUE;
END_IF;

// CMD_A = 2030 (Read DR 30 process values)
// CMD_B = 2033 (Read DR 33 totalizers)

6. Reading the Hardware Clock (DR 8) and Process Status (DR 80)

The SIWAREX WP241 manual documents DR 8 as the gateway to the module's hardware clock. If you log batches with a legal-for-trade timestamp, the time on the WP241 — not the CPU clock — is the legally relevant value. Read it with command code 2008, write it with 4008.

DR 80 carries the SIWAREX DB process status, including error bits, maintenance events, and the operating-mode byte. Per the TIA Portal SIMATIC ET 200SP Manual Collection (DR 80 reference page), the function block automatically reads DR 80 when a status change is detected. Cyclic polling of DR 80 from user code is therefore redundant and wastes command slots.

7. Service Mode – When It Is Required

Service mode is a module-level operating state that protects calibration data. Activate it by writing a password / entering the level before issuing certain commands. The complete rules are:

• Service mode required: writing DR 3 to the module; starting zero calibration (701); starting span calibration (711); starting linearity test (721); writing calibration-relevant parameters.
• Service mode not required: reading any DR (including DR 3); resetting totalizers (660 / 670 / 680 / 690); writing non-calibration parameters such as date/time (DR 8).

A common production integration error is to wrap all WP241 commands in a service-mode prologue. This forces the operator to know the service password, blocks HMI operation during calibration, and slows down cyclic reads. Strip the service mode out of the read path and the totalizer-reset path; keep it only on the calibration path.

8. Verification Checklist

1. In TIA Portal, go online and open DB241 in monitor mode. Confirm CMD3 toggles between the two codes you set in CMD_A and CMD_B every 100 ms.
2. Watch bo_CMD_FinishedOK on a single-shot CMD1 trigger: it must go TRUE on the same cycle the WP241 acknowledges, and stay TRUE until the next trigger.
3. Read DB241.DBD506 after a known quantity has been weighed and confirm the value increments in the configured unit.
4. Force the totalizer to a known value with command 670 (Reset S2) and verify DBD506 returns to 0 within 200 ms.
5. Trigger a deliberate error (e.g. command code 9999) and confirm bo_CMD_FinishedError latches TRUE while bo_CMD_FinishedOK stays FALSE.
6. Check the WP241 diagnostic buffer in the device view — the error code from the module should match a documented code in the equipment manual.

9. Common Pitfalls and Field-Proven Caveats

10. Sizing and Throughput Considerations

Each OB35 cycle issues one CMD3. With 100 ms OB35 and two alternating codes, you get a 200 ms composite cycle for two DRs. To estimate command throughput:

Throughput [cmd/s] = 1000 ms / OB35_ms  /  (number of alternating commands)
Example: 1000 / 100 / 2 = 5 cmd/s per DR

If the application requires a tighter update of DR 33 (e.g. fast batching where the totalizer must reflect the last gram within 50 ms), reduce OB35 to 50 ms and limit the CMD_A/CMD_B alternation to a single DR. Note: WP241's backplane latency is typically 10–30 ms, so the 50 ms floor leaves no headroom and is not recommended for noisy industrial environments.

11. Integration with the ET 200SP and CPU Variants

The "Ready for use" example project is published in three CPU variants:

• CPU V2.2 (S7-1200 legacy)
• CPU V3 (S7-1200, TIA Portal V12 SP1 minimum)
• CPU V4 (S7-1200, current)

For S7-1500 IO controllers, re-import the FB into the TIA Portal project; the backplane interface and the DB241 layout are identical. Verify the HSP version matches the firmware of the WP241 in your station (read the firmware version in the device properties of the module in the TIA Portal device view).

12. Step-by-Step Commissioning Recipe

1. Install the SIWAREX HSP for TIA Portal matching your WP241 firmware.
2. Drag the WP241 from the catalog to the ET 200SP slot.
3. Import the "Ready for use" project (or the FB + DB241 + FC30 + OB35 bundle) into your project.
4. Wire the FB instance in OB1 (or in a dedicated user FB) — call it once per cycle.
5. Configure DB241: CMD_A = 2030, CMD_B = 2033.
6. Configure OB35 to 100 ms and confirm FC30 is called inside it.
7. Download to the PLC; go online; monitor CMD3 — it must alternate 2030 / 2033 every 100 ms.
8. Place a test weight on the scale; confirm the gross / net value in DR 30.
9. Run a totalization cycle; confirm DBD506 increments.
10. Trigger command 670 to reset S2; confirm DBD506 returns to 0.
11. Trigger an intentional error (e.g. command 9999) and verify FinishedError latches.