Resolving SIWAREX U SIWATOOL V4 Channel 2 Error 2 on S7-300

Resolving SIWAREX U SIWATOOL V4 Channel 2 Error 2 on S7-300

This field reference documents the complete resolution of the "non existent or inactive channel – Error 2" condition encountered in SIWATOOL V4 when commissioning a SIWAREX U (7MH4950-1AA01) weighing module on a SIMATIC S7-300 rack, plus the secondary "Adjustment weight valid" spanner-icon condition that appears after the first error is fixed. The guidance is written for engineers performing bench-top commissioning with a four-load-cell test rig before deployment on a production scale.

1. Problem Overview

During SIWATOOL V4 commissioning, an operator can read parameters from the SIWAREX U (Receive operations succeed), but every Send (write) operation returns:

Non existent or inactive channel – Error 2

The error blocks all parameter writes, including zero adjustment, calibration, and limit-value setting. Once the channel configuration is repaired, a second problem commonly surfaces: the SIWATOOL status panel displays a small wrench/spanner icon on the Adjustment weight valid line even though Adjustment zero valid and Zeroing clear normally. The combination of both symptoms is typical of an SIWAREX U variant that has Channel 2 disabled in hardware but still occupies the related Data Record (DR) in the firmware map.

2. Affected Hardware and System Configuration

The bench rig used in the originating case is documented below. Use this as a reference topology when validating your own rack layout.

Load cells: 4 × BB-50Kg-C3 compression / bending-beam type, 2 mV/V nominal output, 350 Ω bridge impedance, connected in parallel at the SIWAREX U sense lead input to form a 50 kg × 4 = 200 kg platform scale (or a single summed 200 kg channel, depending on mechanical mounting).

3. Root Cause: Channel 2 Enable/Disable in DR4

The SIWAREX U firmware exposes Channel 1 and Channel 2 as independent calibrations. Data Record 3 (DR3) carries the calibration parameters for Channel 1, and Data Record 4 (DR4) carries the calibration parameters for Channel 2. Both records are always present in the module's parameter memory, regardless of whether the hardware is the single-channel (-1AA01) or two-channel (-2AA01) variant.

When Channel 2 is enabled in DR4 but the hardware is the single-channel variant (or Channel 2 is intentionally inactive on a two-channel variant), SIWATOOL V4's write routines attempt to write to a non-existent channel and return Error 2: non existent or inactive channel. Receive (read) operations are unaffected because the read path simply returns the data record even when the channel is disabled.

This is why the symptoms are so consistent: "I can read everything, I cannot write anything." The repair is to align the firmware state with the hardware by explicitly disabling Channel 2 in DR4.

4. Step-by-Step Resolution: Disable Channel 2 in DR4

1. Launch SIWATOOL V4 and open the offline configuration for the connected SIWAREX U module.
2. Connect SIWATOOL to the module using the serial / MPI / PROFIBUS path normally used. Confirm the green Online indicator is active.
3. From the left-hand navigation, choose "Calibration channel 2 (DR4)".
4. Open the sub-page "Adjustment digits".
5. Set the Channel 2 enable bit to disabled (or 0). On a single-channel -1AA01 module this is the factory-correct state; on a two-channel module, leave it enabled if Channel 2 hardware is fitted and active.
6. Click Send to write the modified DR4 to the module.
7. Watch the SIWATOOL status bar: the "non existent or inactive channel – Error 2" message should no longer appear on subsequent Send operations against any other DR.
8. Cycle the SIWATOOL Receive all data command to verify the change has been accepted: Communication → Receive all data and store the file for the as-commissioned record.

5. Secondary Issue: "Adjustment Weight Valid" Spanner Icon

After the DR4 repair, SIWATOOL V4 may still display a small wrench / spanner icon next to the Adjustment weight valid status line, while Adjustment zero valid and Zeroing show clear. This means:

• Adjustment zero valid – the dead-load zero has been successfully captured and stored in DR3 (good).
• Adjustment weight valid – the span / calibration weight has not been successfully applied (spanner icon remains).
• Zeroing – the current scale zero is stable (good).

The spanner icon is a calibration-state indicator, not a hardware fault. Two common field conditions trigger it:

1. The span calibration was performed using theoretical adjustment (calculated from load-cell mV/V and scale geometry) without a real test weight. Theoretical adjustment populates the calculation registers but leaves the Adjustment weight valid flag in DR3 set to not valid. This is the expected, healthy state for a theoretical-only calibration.
2. A real test weight was applied, but DR4 Channel 2 was still incorrectly enabled at the time of the span write, so the Channel 1 span write did not commit. After the DR4 fix in Section 4, repeat the Adjustment weight procedure with a calibrated test weight and the flag will clear.

If the bench rig is intended for a production scale where traceability of span calibration is required, always perform the adjustment with a calibrated test weight of at least 50% of the scale's nominal load. The Adjustment weight valid spanner icon should clear once the weight is removed and the parameter is sent.

6. DR (Data Record) Parameter Reference for SIWAREX U

The SIWAREX U parameter map is organised as 16-bit data records. The most-commonly-modified records during bench commissioning are summarised below.

7. Load Cell Wiring for 4 × BB-50Kg-C3

Four BB-50Kg-C3 load cells in parallel at the SIWAREX U sense terminal give a single summed channel. The recommended wiring is:

• Excitation + (EX+) – bridge-excitation positive, common to all four cells.
• Excitation – (EX–) – bridge-excitation negative, common to all four cells.
• Sense + (SN+) – sense lead, top of bridge.
• Sense – (SN–) – sense lead, bottom of bridge.
• Signal + (SIG+) – output signal, top of bridge.
• Signal – (SIG–) – output signal, bottom of bridge.

For a parallel connection, tie the EX+, EX–, SIG+, and SIG– of all four cells together at the SIWAREX U terminals. The sense leads are not strictly required for short cable runs under 10 m, but should be connected on a production rig to compensate for cable resistance. Match the load cell cable shield to the SIWAREX U shield terminal at one end only (the module end).

The theoretical mV/V total for four parallel 2 mV/V cells remains approximately 2 mV/V (parallel cells sum conductance, not output). Nominal capacity becomes 4 × 50 kg = 200 kg per scale. Maximum excitation is 12 V DC; the SIWAREX U provides 10 V DC regulated, well within cell rating.

8. SIWATOOL V4 Communication Setup

SIWATOOL V4 is a Windows commissioning tool. It can communicate with the SIWAREX U over:

• Serial RS-232 directly to the SIWAREX U front service port (point-to-point, no PLC required for bench work).
• MPI / PROFIBUS through the S7-300 backplane using a PC adapter (e.g. PC Adapter USB A2, 6GK1571-1BA00).
• Ethernet using a PROFIBUS-to-Ethernet gateway (e.g. IE/PB Link, 6GK1411-5AB00) – not native on the SIWAREX U.

For a bench rig, prefer the direct serial connection – it removes the S7-300 CPU as a variable and isolates SIWATOOL problems from PLC program problems. The default SIWATOOL V4 communication parameters are 9600 baud, 8 data bits, no parity, 1 stop bit.

9. Commissioning Verification Procedure

1. With the bench rig unloaded, Receive all data from SIWATOOL and store the file. Confirm DR4 Channel 2 enable = 0.
2. Perform Adjustment zero: navigate to Calibration channel 1 (DR3) → Adjustment digits → Zero and Send. The Adjustment zero valid line should clear its spanner icon.
3. Apply a calibrated test weight of known mass (e.g. 100 kg on a 200 kg scale, or 50% of nominal).
4. Enter the test weight value in Calibration channel 1 (DR3) → Adjustment weight and Send. Allow the displayed weight to stabilise, then trigger Adjust from SIWATOOL.
5. Remove the test weight. Confirm the displayed gross weight returns to 0.000 kg (or the small dead-load value captured during Adjustment zero).
6. Confirm the Adjustment weight valid spanner icon has cleared. If it is still present, repeat the adjustment; if it persists, capture a Receive all data snapshot and re-check DR4 channel enable state.
7. Cycle power to the S7-300 rack to confirm the calibration persists in non-volatile memory.
8. Store the final Receive all data Siwatool file as the as-commissioned baseline.

10. Troubleshooting Matrix

11. Field-Proven Notes

• The "non existent or inactive channel – Error 2" wording in SIWATOOL V4 is the canonical English translation. German versions may show "Nicht vorhandener oder inaktiver Kanal – Fehler 2". Both refer to the same condition.
• On a -1AA01 single-channel module, the factory default for DR4 Channel 2 enable is 0; the bit should only ever become 1 if a previous engineer edited it. A simple Receive all data will reveal which state your module shipped in.
• If SIWATOOL V4 still refuses to write after the DR4 fix, check the active PLC connection: when the CPU is online and the SIWAREX U FB is running, some DRs are write-locked from the SIWATOOL side. Stop the CPU or remove the FB call temporarily to write those DRs.
• The SIWAREX U stores calibration in non-volatile memory; a full power cycle does not wipe it. A factory reset is only available through SIWATOOL V4 with the special factory-reset password (see the SIWAREX U manual, entry ID 109769624).
• Always perform the bench calibration with the same wiring topology (cable lengths, junction boxes) that will be used in production – mV/V drift with cable resistance is a common source of mystery span errors after the rig is reinstalled on the line.

12. FAQ