Siwarex FTA Profibus DP Integration with Third-Party PLC

Overview of the Integration Challenge

The Siwarex FTA (Function Module Technology, Automatic weighing, catalog number 7MH4900-2AA01) is a Siemens weighing electronics module designed for dosing and filling applications. It is mechanically and electrically a SIMATIC S7-300 family module, which means it is normally inserted in an S7-300 rack next to a CPU, or inside an ET 200M distributed I/O station under an IM 153-x Profibus head. When the host PLC is a Siemens S7-300 or S7-400, configuration is fully supported by STEP 7 / TIA Portal hardware catalogs.

When a non-Siemens controller (a third-party PLC from vendors such as Allen-Bradley, Schneider, ABB, Beckhoff, or Wago) acts as the Profibus DP master, the Siwarex FTA cannot be added to the master configuration. The reason is structural: the FTA is a function module (FM) and is not represented as a slot module in the public GSD files for the ET 200M. Without a GSD entry, the third-party configurator (e.g., Sycon, ProSoft, Studio 5000 Logix Designer with a Profibus option, EcoStruxure Control Expert) cannot enumerate the module, and DP-V0 cyclic I/O exchange cannot be established for that slot.

Siemens publishes an FAQ 10182429 that documents how the Siwarex U, FTA, and FTC families integrate in an S7-400 station using an IM 153-2 head. The FAQ confirms that the function modules require the S7-300 / S7-400 engineering environment and are not standalone Profibus DP slaves from the perspective of a non-Siemens master.

Why the Siwarex FTA Is Not in the ET 200M GSD

The PROFIBUS GSD files for the ET 200M station (for example the legacy GSD si01801d.gse or the more recent GSD revision for the IM 153-2) describe the head module and the standard signal modules (SM 321/322/323/331/332/334, FM 350/351/352/353/354, and certain position-decoder modules). The Siwarex family of weighing modules is treated as application-specific firmware objects and is intentionally excluded from the generic GSD because:

1. Parameter length exceeds DP-V0 GSD limits. The FTA exposes dozens of application-specific parameters (calibration, dosing recipes, filter coefficients, A/D conversion, setpoint switching) that cannot be represented within the standard 244-byte DP-V0 parameter field.
2. Driver integration is required. The FTA relies on the Siwatool FTA commissioning tool and the Siwarex function block library (FW x; FB FB Siwa standard block set) running on the CPU. The slave side does not self-describe the data interface to a foreign master.
3. Module-specific GSD rules. Siemens publishes a dedicated GSD for the Siwarex U as a slot module within the ET 200M catalog, but the FTA is published only inside the STEP 7 hardware catalog, not as a generic DP-V0 GSD.

The Siemens entry 113498 lists the official GSD downloads for the ET 200M family and confirms the Siwarex FTA is not selectable from the third-party master side even with the latest GSD revision.

Siwarex FTA Technical Specifications (7MH4900-2AA01)

Siwarex U Alternative (7MH4950-1AA01)

The Siwarex U (catalog number 7MH4950-1AA01) is a simpler weighing module that is represented in the ET 200M public GSD catalog and therefore can be configured on a third-party Profibus DP master. The GSD entry provides cyclic I/O (e.g., gross weight, net weight, status, command word) but with substantially reduced capability compared to the FTA.

Siwarex FTA versus Siwarex U – Decision Matrix

Workaround 1 – S7-300 PLC as Profibus DP Slave Gateway

This is the recommended path for applications that need the full Siwarex FTA feature set. The architecture is:

1. Insert the Siwarex FTA (7MH4900-2AA01) in an S7-300 rack (or an ET 200M station under an IM 153-2 if a separate S7-300 CPU is present).
2. Add a CP 342-5 (Profibus DP master/slave) or CP 343-5 (Profibus) to the S7-300 backplane, or use a CPU 315-2 DP / CPU 317-2 with integrated DP port.
3. Configure the S7-300 as a DP slave on the third-party master's network. The third-party master sees the S7-300 as a normal DP-V0 slave.
4. Use the Siwarex function block library on the S7-300 to read/write the FTA's process data, calibration, and dosing setpoints.
5. Map the FTA data into the S7-300's DP slave I/O image (e.g., 16 bytes gross, 16 bytes net, 8 bytes command, 8 bytes status) using the CP's transfer area configuration.

Key configuration parameters on the S7-300 side (STEP 7 V5.x or TIA Portal):

• CP 342-5 operating mode: DP slave
• Transfer area 1 (PKW / parameter): 8 bytes in / 8 bytes out – used for calibration and recipe data exchange
• Transfer area 2 (PZD / process data): 8 to 32 bytes in / out – used for live weight, status, command bits
• DP slave diagnostics enabled – enables diagnostic interrupts back to the third-party master

On the third-party master side, the S7-300 is configured as a single slave with a fixed I/O length of 16 / 16 bytes (or 32 / 32 bytes for a richer interface). The master software simply reads the pre-mapped weight values – it has no awareness of the Siwarex FTA at all.

Hardware required for Workaround 1

Workaround 2 – Substitute the Siwarex FTA with a Siwarex U

If the application only needs the current weight value and the dosing can be implemented in the third-party PLC, a direct drop-in is possible:

1. Verify the Siwarex U 7MH4950-1AA01 is the installed GSD slot entry (the legacy si01801d.gse and the newer si02801d.gsd revisions both include it).
2. Pull the FTA from the ET 200M rack, insert the Siwarex U in the same slot.
3. In the third-party master's Profibus configurator, the Siwarex U appears as a selectable module. Add it to slot 4 (typical) and assign the cyclic I/O addresses.
4. Re-calibrate the load cell with Siwatool U. The Siwarex U exposes 4-wire / 6-wire calibration through the DP parameter channel.
5. Implement dosing logic (coarse / fine / tolerance) inside the third-party PLC, using the Siwarex U's status bits (stable, under-range, over-range, zero, tare) and the weight value as process data.

ET 200M GSD File Selection for IM 153-2

Multiple revisions of the IM 153-2 exist and each has its own GSD file. Choose the file that matches the head module's firmware (printed on the front label). The Siemens entry 113498 lists all GSD files. Common variants:

Procedure to install the GSD in a third-party configurator (example for Sycon by Hilscher):

1. Download the GSD archive from the Siemens entry 113498.
2. Extract the .gsd, .gse, and bitmap files to a known directory.
3. In Sycon, open Device Catalog → right-click the Profibus DP master → Import GSD File. Point to the extracted files.
4. The IM 153-2 appears in the catalog tree. Drag it onto the network.
5. Open the slave's slot list. Slot 1 must be the IM 153-2 itself. Add signal modules to slots 4–11 (slot 3 is reserved).
6. If using the Siwarex U, select it from the function module group; the Siwarex FTA is intentionally absent.

HMI / Display Devices over Profibus DP Slave

When an operator panel needs to display the Siwarex weight over Profibus, a DP slave interface is typically used. Devices such as the Pro-face GP-Pro EX operator interfaces expose a PROFIBUS DP Slave Driver. In this case the Siwarex data is read into the third-party PLC, then mirrored to a DP slave panel using the master's output area; the panel displays gross / net / setpoint values without ever needing a direct GSD entry for the FTA.

Typical memory map for the panel (DP slave) side:

Diagnostic and Verification Procedures

After commissioning, verify the data flow end-to-end:

1. Force a known weight on the load cell (e.g., test weight of 50 % of the calibrated range).
2. On the S7-300, read the Siwarex FTA's process data (FB Siwa output ProcessValue). It must match the applied weight within the configured resolution.
3. On the third-party master, read the mapped I/O word that contains the gross weight. It must match the S7-300 reading exactly.
4. Toggle a command bit (e.g., tare) from the third-party master. Verify the S7-300 receives the bit, forwards it to the FTA, and the net weight goes to zero.
5. Disconnect the load cell connector. The FTA should raise a diagnostic interrupt that propagates through the S7-300 to the third-party master as a slave diagnostic message.

Troubleshooting Matrix

Performance and Timing Considerations

For a dosing system with 200 ms target cycle time, the latency stack is approximately:

• Siwarex FTA measurement rate: 50 Hz (20 ms) or 100 Hz (10 ms), selectable
• Backplane S7-300 ↔ FTA: 1 to 2 OB1 cycles (~10 ms at 100 ms OB1)
• CP 342-5 PZD update: 1 to 2 DP cycles (~2 ms at 1.5 Mbit/s, 16-byte payload)
• Third-party master polling: depends on master's OB1 / task, typically 5–20 ms
• Total end-to-end: roughly 30 to 60 ms, well below the 200 ms dosing budget

When using the Siwarex U directly, the cycle is shorter (typically 10 to 30 ms) because there is no gateway hop, but the resolution and accuracy are lower.

Field-Proven Commissioning Caveats

1. Always calibrate with the actual load cell installed and the system in its final mechanical state. Calibration is highly sensitive to mechanical preload, mounting torque, and load cell alignment.
2. If the system is required to be legal-for-trade, use the Siwarex FW (legal-for-trade) calibration mode and set the sealing switch on the FTA module.
3. When implementing the S7-300 gateway, use the Siemens standard FB Siwa function blocks from the Siwarex library. Do not attempt to write custom raw I/O drivers – the parameter channel protocol is proprietary.
4. On the third-party master, never assume the Siwarex data is refreshed every scan. Always verify the status bit "data valid" before acting on the weight value.
5. If the load cell cable runs near VFD motor cables, use shielded, twisted-pair (STP) cable with shield grounded only at the FTA end to avoid ground loops.

FAQ