SIWAREX U Adjustment Command Not Executed: Distance Error Fix

Problem Overview

The SIWAREX U weighing module (Siemens part number 7MH4601-1AA01) rejects the Adjustment weight valid command with the message "Adjustment command could not be executed as distance between adjustment points is too small." The error appears in the SIWATOOL U service tool during the span (gain) calibration step, after the zero point has been saved successfully. The command is sent by the user, but the firmware refuses to copy the current ADC value into Data Record 3 (DR3, Adjustment digits 1) because the digit delta from DR2 (Adjustment digits 0) does not meet the 3000-digit minimum threshold mandated by the module's internal plausibility check.

Affected Hardware and Firmware

Root Cause Analysis

SIWAREX U performs a self-check on every command received. The plausibility test compares the live ADC value (read from DR31, byte offset 0-3, signed 32-bit) against the stored zero-point digits (DR2, Adj.digits 0). For the span calibration, the firmware requires:

Actual digit value adc (DR31) ≥ Adj.digits 0 (DR2) + 3000

The 3000-digit floor exists to guarantee that the resulting scale characteristic (DR3 = span digits, divided by the calibration weight in the configured engineering unit) yields a specific load cell sensitivity greater than 0.5 mV/V. If the digit delta is smaller, the slope becomes mathematically unstable and the resulting weight display would be highly sensitive to noise, drift, and temperature. The module firmware therefore refuses to commit the command and returns the diagnostic.

Required Preconditions for Successful Adjustment

1. The load cells must be wired in parallel per the manual wiring diagram, with sense lines connected where supplied (4-wire or 6-wire load cells).
2. Excitation voltage at the load cell junction must be between 4.85 V and 5.04 V (measure at DR30 byte 0 if the module reports it, or with a DMM at the cell).
3. Mechanical installation must be pre-loaded so that the resting weight (vessel, hopper, mounting hardware) produces a positive, stable ADC value.
4. DR2 (Adj.digits 0) must already be set to the live zero point using the Adjustment zero valid command.
5. The live DR31 value under the calibration load must exceed DR2 by at least 3000 digits.
6. Calibration weight must be applied to the platform without shock, with the scale settled for at least 3 seconds before the command is issued.

Diagnostic Procedure

1. Connect the SIWAREX U service cable (Siemens 7MH4601-1AA or equivalent 1:1 DB9) between the module's front RS-232 port and the PC.
2. Launch SIWATOOL U and open the online connection. The default parameters: COM port, 9600 baud, even parity, 8 data bits, 1 stop bit.
3. Read Data Record 2 (DR2). Note the value of Adj.digits 0. Example: 4895.
4. Read Data Record 31 (DR31). Note the value of Actual digit value adc under the resting (pre-load) condition.
5. Apply the calibration weight to the platform. Wait for mechanical settling (5-10 seconds for a hopper; 3-5 seconds for a platform scale).
6. Read DR31 again. This is the span digit candidate.
7. Compute the delta: DR31 (loaded) - DR2. If the result is below 3000, the adjustment command will be rejected.

The formula is:

Required DR31 ≥ Adj.digits 0 + 3000

Worked example using the source field case:

Step-by-Step Resolution

1. Confirm DR2 is correct. If the zero point has drifted, repeat the zero adjustment sequence: unload the scale, read DR31, send the Adjustment zero valid command, re-read DR2.
2. Increase the calibration weight. Use at least 10% of the total connected load cell capacity to guarantee the 3000-digit minimum. For a 3500 kg aggregate cell capacity, target 350 kg or more of calibrated test weights.
3. Alternatively, increase the digit sensitivity by reconfiguring DR7 (Measurement range) and DR8 (Integration time) to a higher gain setting. Each step doubles the digit output per mV/V input.
4. Verify that the platform and load cell wiring are mechanically sound. A loose mounting bolt introduces micro-vibration that can suppress the live ADC value.
5. With the larger calibration weight applied, re-read DR31. Verify that DR31 ≥ DR2 + 3000.
6. In SIWATOOL U, navigate to the Adjustment tab. Enter the calibration weight value in the configured engineering unit (kg, lb, t).
7. Click Adjustment weight valid. The module will copy the current DR31 value into DR3 (Adjustment digits 1) and report the command status in the result panel.
8. If the command succeeds, the result field shows the computed characteristic: slope = (DR3 - DR2) / weight. A typical SIWAREX U calibration should yield between 0.8 and 2.0 digits per kg for a 3500 kg system.

Verification and Validation

1. Read DR3 and confirm it is non-zero and within the expected range (typically 4000-25000 digits for a correctly installed module).
2. Compute the characteristic: k = (DR3 - DR2) / Calibration_weight. Compare against the load cell data sheet value of mV/V, expected to be 2.000 ± 0.002 mV/V for most industrial cells.
3. Remove the calibration weight, then re-apply at 25%, 50%, 75%, and 100% of the calibration value. Confirm the displayed weight tracks within ±0.05% of applied load.
4. Trigger a hardware restart (power cycle or DR0 command "Restart") and re-verify that DR2 and DR3 are retained in non-volatile memory.
5. Save the parameter set in SIWATOOL U using File > Save parameter set as .... Archive the .uw file with the calibration certificate.

Related Diagnostic and Operating Errors

The SIWAREX family shares a structured error code set. When the Adjustment command not executed message appears, inspect DR1 (Operating state) and DR0 (Command / status) for the following codes:

For the ET 200SP variant, the diagnostic block returns the same code family via the channel diagnostics; the data-and-command-errors reference describes the structure of the additional information field that holds the failing subcode.

SIWATOOL U Configuration Reference

The following data records are involved in the adjustment sequence. Access via the SIWATOOL U Service menu or from the S7-200 user program using the SIWAREX_U_READ / SIWAREX_U_WRITE subroutines from the example library.

Load Cell Sizing and Calibration Weight Guidelines

The 3000-digit rule interacts with the load cell sensitivity and the total installed capacity. Use the following field-proven guidance to avoid the error on the first attempt:

• Target a calibration weight of at least 10% of the total load cell capacity (sum of all cells in parallel). Below this value, the digit delta is rarely sufficient even at maximum gain.
• For multi-cell systems, ensure the weight is distributed across all cells proportionally; an off-center load can produce misleading ADC values in bending-link or single-point cells.
• If test weight inventory is limited, increase DR7 gain to 4 mV/V. This quadruples the digit output per applied mV/V and reduces the required physical weight.
• Verify load cell sensitivity from the calibration certificate. Cells rated below 1.0 mV/V (some aluminum or composite cells) will require more weight or higher gain to meet the 3000-digit minimum.
• For very small scales (under 50 kg total capacity), consider SIWAREX FTC or SIWAREX WP321, which have a 1000-digit minimum and are better suited to low-capacity applications.

Edge Cases and Field Notes

• Mechanically damped installations (e.g. suspended hoppers) can suppress the ADC value if the scale has not settled. Always wait at least 5 seconds after applying weight, and read DR31 multiple times to confirm stability within ±2 digits.
• Excitation voltage drop: long cable runs between the module and the load cell junction reduce the excitation at the cell. The ADC output scales linearly with excitation, so a 10% drop in excitation reduces the digit distance by 10%.
• Temperature drift: load cell output changes with temperature. If zero is calibrated cold and span is calibrated warm, the digit distance may shrink. Calibrate both points at the same temperature for best accuracy.
• Tare vs zero: The Adjustment zero valid command overwrites DR2, but does not change the displayed process weight. Use the tare function (DR0 bit pattern for Tare valid) to zero the displayed weight after a successful calibration.
• Multi-interval scales: SIWAREX U supports a single interval range. For multi-range or multi-interval scales per OIML R76, use SIWAREX FTC or SIWAREX WP351 HF.
• Communication timeouts: A slow PC COM port or USB-to-serial adapter can introduce read latency. If DR31 reads stale, SIWATOOL U will report an "out of range" rejection even when the digit distance is sufficient. Reconnect at 9600 baud and disable any power-saving mode on the COM port.
• Non-volatile retention: DR2 and DR3 are stored in EEPROM on every successful adjustment. A power cycle does not affect the calibration.

Integration with SIMATIC S7-200 / PCS 7

When SIWAREX U is operated as a distributed module via the ET 200S or directly on the S7-200 bus, the adjustment command is issued through the command bit pattern in DR0. The PCS 7 channel block (CFC) provides a connection block that toggles the bit pattern with a positive edge; the manual describes the bit layout:

• Bit 0: Adjustment zero valid
• Bit 1: Adjustment weight valid (span)
• Bit 2: Tare
• Bit 3: Reset tare
• Bit 4: Restart

The connection block is configured in PCS 7 with the SF_CNT counter block feeding the command edge. See the SIWAREX U PCS 7 manual for the full CFC block diagram and the ModPreCon library for the S7-200 side.

Documentation References

• SIWAREX U Device Manual (Edition 38) - Chapter on "Adjustment of the scale" defines the 3000-digit minimum and lists every error code returned by the firmware.
• TIA Portal: Data and Command Errors (TM SIWAREX WP351 HF) - Describes the 9500 generic command-not-executed code and the additional information structure for sub-code inspection.
• SIWAREX U Project Planning in SIMATIC PCS 7 - Connection block and command edge handling for PCS 7 integration.
• TIA Portal: Data and Operating Errors Message List (SIWAREX WP321) - The 5101 operating-state error code reference shared across the WP family.

Frequently Asked Questions