Siwarex FTC Siwatool V4: Resolving Module Access Error

Problem Overview

Engineers commissioning a SIWAREX FTC belt-scale module frequently encounter the Siwatool V4 popup:

"The connected module cannot be accessed by this application."

This message is intentionally ambiguous. Siwatool cannot distinguish, at the point of the dialog, between three physical situations: a USB-to-serial adapter that the Siwatool service driver cannot enumerate, a Siwarex FTC module whose on-board firmware pre-dates the minimum compatibility baseline of the Siwatool V4 release in use, and a successful RS232 link to a module variant that is not represented in the Siwatool V4 device database for the firmware family that the module reports in its boot header. The dialog is the same in all three cases; the corrective action is not.

The fault is normally first observed during bench commissioning of a weigh feeder or belt scale before deployment to site. The PLC side of the system is healthy: the IM 153-4 PN Profinet interface module reports the FTC as a configured I/O slot, the SIWAREX_FTC_B_DB data block in TIA Portal V19 is updating with live weight, speed, and flow values, and the CPU 1515-2 PN shows the FTC in the device list with no diagnostic alarms. The fault is therefore isolated to the RS232 service port on the front of the FTC module, not to the Profinet path used by the PLC.

System Context

The configuration in which the fault was reproduced uses the following Siemens components:

The 7MH4900-3AA01 is the standard SIWAREX FTC for non-trade applications. The closely related FTC_L (Legal for Trade) module is parameterised with the same Siwatool V4 tool and follows an almost identical connection sequence; the Siwatool V4.3.3 procedure documentation is published for the FTC_L variant and is applicable to the FTC for the RS232 service port steps documented here. See the SIWAREX FTC_L Quick Guide for Siwatool V4.3.3 for the manufacturer-published step sequence.

Root Cause Analysis

The "module cannot be accessed" dialog resolves to one of two physical-layer or firmware-mismatch conditions, both of which the Siwatool V4 user interface reports with the same error string.

Cause A — RS232 service-port electrical incompatibility. The FTC service port is a true RS232 level (±12 V) interface, not a 5 V TTL UART. Many low-cost USB-to-serial adapters (in particular unbranded Prolific PL2303 clones and several FTDI-based units in the lowest price tier) do not drive the FTC bootloader's input threshold cleanly under all USB bus conditions, and Siwatool V4 is not tolerant of marginal signalling during the initial handshake. The Windows-side driver stack may report a perfectly functioning COM port (loopback test passes), but the FTC boot sequence still fails to complete.

Cause B — Siwatool V4 / FTC firmware version skew. Siwatool V4.x maintains a device database that is keyed on the firmware family reported by the FTC during its boot handshake. If the on-board firmware is older than the lowest entry in the Siwatool V4 device list for the FTC family, Siwatool V4 displays the access error rather than opening the parameter view. Siwatool V2 has a wider firmware compatibility window and is the recommended first-pass tool to confirm basic RS232 connectivity before attempting a firmware upgrade.

Diagnostic Procedure

Run the following checks in order before applying any corrective action.

1. Confirm PLC-side health. Open TIA Portal V19, go online to the CPU 1515-2 PN, and verify that the FTC slot under the IM 153-4 PN is online and that SIWAREX_FTC_B_DB is updating. If the PLC side is faulted, fix that first — Siwatool cannot reach a module the PLC has rejected.
2. Inspect the FTC LED pattern. On power-up, the FTC briefly cycles the SF, BF, and DC24V indicators before settling. A solid SF LED with no Profinet traffic indicates the module has failed self-test; an unlit service-status indicator (where present) during Siwatool attempts indicates no serial handshake is completing.
3. Verify the service cable. The cable from the PC COM port to the FTC service connector must be a null-modem configuration (RXD/TXD crossed, common ground) with handshaking loops where required. Do not use a straight-through cable.
4. Establish a Siwatool V2 baseline. Connect with Siwatool V2 first. If V2 establishes a session and reports the module's firmware revision, the RS232 physical layer is healthy and the cause is a Siwatool V4 / firmware version mismatch. If V2 also fails, the cause is the USB-to-serial adapter or the cable.
5. Read the COM-port driver. Open Windows Device Manager and confirm the adapter enumerates as an enhanced COM port (COMx, no yellow warning). Drivers older than the Windows 10 / 11 inbox driver should be updated from the adapter manufacturer's site.

Solution 1 — USB/RS232 Adapter Selection

Siemens has published a list of tested USB/RS232 converters for the Siwarex service ports. The officially recommended unit for the SIWAREX FTC service port is the ATEN UC-232A.

Reference: Siemens Support Entry 28391503 — USB/RS232 converter ATEN UC-232A for Siwarex.

Procedure:

1. Disconnect any previously installed USB-to-serial adapter.
2. Plug the ATEN UC-232A into a USB port directly on the service PC (avoid unpowered USB hubs; the FTC boot sequence is sensitive to brown-out conditions on the host side).
3. Install the ATEN / Prolific vendor driver supplied with the unit. Do not rely on the Windows inbox driver if Windows offers it as the default for the device.
4. Verify in Device Manager that the unit enumerates as ATEN UC-232A USB Serial Port (COMx) with no warning icon.
5. Connect a null-modem RS232 cable between the UC-232A DB-9 and the FTC service port.
6. Launch Siwatool V4, open the COM-port selection, and select the new COMx entry. The baud rate, data bits, parity, and stop bits are fixed by the FTC firmware and do not need to be set on the PC side beyond selecting the correct COM port.

Solution 2 — Siwatool Version Selection

If the ATEN UC-232A does not resolve the access error, the cause is firmware-version skew between the FTC module and the Siwatool V4 device database. The corrective path is two-step: validate with Siwatool V2, then upgrade the module firmware.

Siwatool V2 is the legacy configuration tool that supports a wider range of historical FTC firmware revisions. It is still distributed by Siemens Siwarex Support for the express purpose of validating older or pre-shipment modules. Request the current Siwatool V2 build from [email protected]. Do not download V2 from third-party sites; the version distributed by Siemens is signed and matches the parameter file format that V4 can import.

Procedure:

1. With the ATEN UC-232A installed and the null-modem cable connected, launch Siwatool V2.
2. Select the COM port corresponding to the UC-232A.
3. Siwatool V2 will report the on-board firmware revision in the module information dialog. Record the firmware string exactly — it is required for the V4 device selection and for any V2-to-V4 parameter migration.
4. If V2 opens a parameter session and V4 does not, the V4 device database does not yet contain an entry for the reported firmware family. The two paths forward are: upgrade the module firmware to a V4-supported family (covered below), or perform all parameterisation under V2 and export the parameter set for the V4 commissioning tool to import on a later visit.

Device Variant Selection in Siwatool V4

Once the RS232 link is established and Siwatool V4 completes the handshake, the tool prompts the engineer to select the device type. The 7MH4900-3AA01 SIWAREX FTC appears in the Siwatool V4 device database under the following entries:

For the 7MH4900-3AA01 module, Siemens Siwarex Support directs the engineer to select 5.3.10s. The "s" suffix designates the variant of the firmware family that maps to the 7MH4900-3AA01 hardware revision; selecting the base 5.3.10 entry on a 7MH4900-3AA01 module results in an out-of-range parameter fault on the first download attempt. If the engineer is unsure which entry to select, the Siwatool V2 firmware readout can be cross-referenced against the Siemens Siwarex firmware compatibility matrix supplied with the V4 release notes.

Firmware Update Procedure

The standard FTC firmware file for V5.3.11 is shipped as SWFT3_V5-3-11.bin. The file is loaded via Siwatool V4's firmware-load function, not via Siwatool V2.

1. Place the .bin file in a known local directory on the service PC. Do not load the firmware from a network share — the load is CRC-checked and a network glitch mid-load can corrupt the module's boot sector.
2. In Siwatool V4, connect to the module on the COM port, complete the handshake, and select the appropriate device entry (5.3.10s for 7MH4900-3AA01).
3. Open the Firmware / Load firmware function (the menu label is release-dependent; in Siwatool V4.0.11 it sits under the module menu).
4. Select SWFT3_V5-3-11.bin and start the load. The module reboots during the load — this is normal. Do not power-cycle the module or disconnect the RS232 cable while the load is in progress.
5. When the load completes, Siwatool V4 reconnects automatically. Confirm the firmware revision reported in the module information dialog matches the loaded version (5.3.11).

Siwatool V2 Alternative Workflow

Where the engineering schedule does not permit a firmware upgrade (for example, the module is to be left at its current revision for repeatability with an existing V2 parameter set on site), Siwatool V2 is a complete configuration tool in its own right. The V2 workflow is:

1. Connect with the ATEN UC-232A and null-modem cable.
2. Open the parameter view, edit belt scale geometry, calibration weights, and filter constants, and download to the module.
3. Export the parameter set to disk. The export file is in the legacy *.sww format.
4. On a later visit, Siwatool V4 can import the legacy parameter set and bring it forward to the V4 schema.

This is the recommended path for sites that have a long-standing V2 parameter file in their documentation set and do not want to re-validate the scale after a firmware upgrade.

Verification

After applying the corrective action, confirm the following in order:

1. Siwatool V4 opens the parameter view without the access-error dialog.
2. The module information dialog reports the expected firmware revision (5.3.11) and the expected device class (5.3.10s).
3. Live process values (belt load, belt speed, flow rate, totaliser) appear in Siwatool V4's monitoring view and agree with the values reported in TIA Portal V19's SIWAREX_FTC_B_DB.
4. A parameter download to the module completes with a CRC-OK indication, not a CRC-error indication.
5. The IM 153-4 PN Profinet interface in TIA Portal V19 continues to report the FTC as online with no new diagnostic entries.

If any of the above fails, return to the diagnostic procedure and re-validate the RS232 link with Siwatool V2 before changing the configuration further.

Field Commissioning Considerations

Three field conditions are worth checking before the bench commissioning result is treated as conclusive.

Cable routing on site. The RS232 service cable on a working belt scale is typically run in a separate conduit from the load-cell cable. The load-cell signal is in the microvolt range and the service cable carries RS232 levels that are not, on their own, a noise threat, but shared conduit with VFD output cables will produce intermittent handshake failures. A bench result that works in the office can fail intermittently on site for this reason.

Service PC power. The ATEN UC-232A draws its power from the USB bus. On a site with poor power conditioning, an under-voltage on the USB bus during the FTC boot sequence can drop the RS232 driver into an undefined state. Plug the service PC into the same conditioned power source as the PLC and FTC, or use a laptop on battery for the parameterisation session.

Module revision vs spare-part stock. The 7MH4900-3AA01 module is the current MLFB for the SIWAREX FTC. Earlier module revisions (different -3AAxx suffix) require a different Siwatool V4 device entry. Confirm the exact MLFB on the module's nameplate before selecting the device entry; the device entry is keyed on the MLFB, not on the firmware version.

Comparison: Siwatool V2 vs Siwatool V4

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