Siwarex FTA: Resolving 'Command Transfer Not Permitted' Error

Problem Statement

On a SIMATIC S7-300 or ET 200M station fitted with a Siwarex FTA weighing module (e.g. 6SE7090-0XX84, 7MH4900-2AA01, 7MH4900-3AA01), operator-triggered commands such as TARE, ZERO, or write operations to the module's data records are rejected. The Siwatool FTA configuration software and the PLC user program both surface the same error message:

"Data record transfer or command transfer to weighing cycle not permitted"

The TARE bit in the Siwarex DB (typically DB_FTA.CommandTare or bit 0 of command word) is set, but the module does not execute the command. The bit is reset by the module after one cycle, and the CommandTare pulse is never acknowledged in the status word. In Siwatool, the manual Tare / Zero / Calibration buttons return the same text and no parameter is written to the device.

This symptom is independent of the field wiring, the load cell health, and the load cell connection path (SIWATOOL reports a healthy load cell signal), which makes the error misleading: a healthy weighing signal does not imply the module is ready to accept a command.

Affected Hardware and Firmware

The error is generated by the firmware itself, not by the Siwatool PC application, so the same fault string appears whether the operator issues the command from the panel of the weighing terminal, from Siwatool online mode, or from the S7 user program via WR_REC / WR_DREC SFCs.

Root Cause Analysis

Siwarex FTA implements an internal dosing state machine. Whenever the module is not in manual / gross / net mode, but is executing a recipe-driven filling cycle, the firmware refuses any command that would alter the weighing state. This is a safety-integrity feature: a TARE or ZERO while material is being dosed would corrupt the in-cycle net total and could leave the process in a non-reproducible state.

The error is therefore not a wiring or calibration defect. It is a state-machine lockout. The triggering conditions are:

1. A dosing recipe is configured in Siwatool (DR30 → Recipe) and the module is currently executing one of the seven weighing steps.
2. The last recipe did not terminate cleanly (e.g. the operator pulled the field bus before the "End" step, or the PLC was stopped in the middle of a cycle).
3. The START bit (command code 100) was set but no STOP (code 105) or ABORT (code 108) followed, leaving the module parked in weighing step 1..7.

Field experience: a Siwarex FTA that has been powered down and back up will not automatically clear an orphaned dosing step. The internal "active step" register is non-volatile for as long as backup voltage is present on the backplane, and only the explicit Stop / Abort sequence brings it back to Weighing step 0.

Siwarex FTA Dosing State Machine

Siwarex FTA knows the following weighing steps, visible in Siwatool under DR30 → Status → 3rd column:

Step 0 is the only state in which the firmware will accept a TARE command or any other state-modifying write. If you read DR30 from the module while in any of steps 1..7, the step number is contained in bytes 4..5 of the status data record and the module will reject the next write with the error text given above.

Solution: Clear the Dosing Step with Commands 105 and 108

Two firmware commands are available to bring the dosing state machine back to step 0 without a power cycle:

Issuing the Commands from Siwatool (manual recovery)

1. Connect to the Siwarex FTA in Siwatool (Online → Start connection).
2. Open Service → Commands (or in older Siwatool versions: Commands in the toolbar).
3. Enter command number 105 and press Execute. Wait for the module to acknowledge ("OK").
4. Enter command number 108 and press Execute. Wait for the module to acknowledge ("OK").
5. Re-open DR30 → Status. The third column should now read Weighing step 0.
6. Issue the TARE command (code 1) — it will now be accepted.

Issuing the Commands from the S7 User Program (automatic recovery)

The Siwarex FTA function block FB_Siw_FTA (from the Siwarex FTA library) provides a "Command" input. The recommended sequence in Structured Text:

iCommand := 105;
iCommandPar := 0;
bExecute := TRUE;
// wait for bDone or bError
iCommand := 0;
bExecute := FALSE;

// 300 ms wait
TON_Pulse(IN := TRUE, PT := T#300ms); // wait 300 ms

iCommand := 108;
bExecute := TRUE;
// wait for bDone or bError
iCommand := 0;
bExecute := FALSE;

// now TARE
iCommand := 1; // TARE
bExecute := TRUE;
// wait for bDone

The same sequence can be done from ladder using MOVE / SET / RESET instructions, with a 300 ms timer between the two command writes.

If the FB bError output comes back TRUE, read the error code from wError and refer to the Siwarex FTA manual, chapter "Command error codes". A typical value is 0x0010 (busy), which means the second command was sent before the first was acknowledged — increase the inter-command delay.

Solution: Clean Reinstall of Siwatool (Secondary Cause)

If the module's DR30 status already reads Weighing step 0 and the command is still rejected, the error is generated by the Siwatool PC application, not by the Siwarex firmware. Siwatool maintains its own command state and parameter cache. A corrupted cache or registry entry can keep sending the command with the wrong locking flags. The fix is a complete Siwatool reinstall:

1. Close all Siwatool instances. Stop the Siwatool FTA Service from services.msc if present.
2. Uninstall Siwatool FTA from Control Panel → Programs and Features.
3. Delete the following folders if they still exist:
   • C:\Program Files (x86)\Siemens\Siwatool FTA (or C:\Program Files\Siemens\Siwatool FTA on 32-bit systems)
   • C:\ProgramData\Siemens\Siwatool
   • %APPDATA%\Siwatool
   • %LOCALAPPDATA%\Siwatool
4. Reinstall Siwatool FTA from the original Siemens DVD or from the Siemens Industry Online Support portal.
5. Reboot the engineering station.
6. Re-open the project file (.sft) and reconnect to the module.

Verification Procedure

1. State check: Read DR30 from the Siwarex FTA via Siwatool (Service → Read DR30) or via the S7 program. The third status field must be Weighing step 0 before any TARE / ZERO is attempted.
2. Siwatool command test: In Siwatool, click Service → Tare. The response in the log window must be Command executed within 1 s. If the log shows Command rejected — Data record transfer or command transfer to weighing cycle not permitted, the dosing step is still active; repeat the 105 / 108 sequence.
3. PLC command test: Set the TARE bit in the Siwarex DB from the S7 program. Within 2 s the bTareDone output of the FB must pulse TRUE. The status word in DR3 / DR30 must show the gross weight jump to the calibrated zero.
4. Recipe test (if dosing is in use): Start a recipe, observe the step counter in DR30 increment from 1 to 7, let the recipe terminate normally, and verify that DR30 returns to step 0 without operator intervention. If the step counter stays > 0 after the recipe has finished, the ABORT code (108) needs to be sent automatically by the recipe-end logic in the PLC.

Troubleshooting Matrix

Siwatool DR30 Status Layout (reference)

Data record 30 ("Status") is 16 bytes long on Siwarex FTA. Layout (Siemens manual A5E00328928 chapter "DR30"):

To clear an orphaned dosing step programmatically, polling bytes 4..5 until they return 0 is the most reliable feedback path. Do not rely on the command-acknowledge bit alone — the bit toggles for every command, success or fail.

Preventive Measures

1. Always close recipes cleanly. In the S7 user program, the "End of recipe" transition must unconditionally write command 108 (ABORT) to the Siwarex. Treat ABORT as the only safe way to leave a dosing state.
2. Add OB100 startup logic that issues 105 + 108 on every warm restart of the S7 CPU. This guarantees that a power loss during a cycle does not leave the Siwarex locked.
3. Watch the weighing step in HMI. Surface the value of DR30 bytes 4..5 on the operator panel. A visible "Step" tag gives the operator the chance to clear a stuck cycle before calling maintenance.
4. Lock command inputs while a recipe runs. In the S7 program, gate the TARE button with (iRecipeState = 0). A TARE pressed during a recipe is a process error, not a useful operator action.
5. Document the command matrix. Keep a printed table of the Siwarex FTA command codes (1 = TARE, 2 = ZERO, 100 = START, 105 = STOP, 108 = ABORT, 200 = CAL, …) next to the cabinet. It saves 30 minutes of head-scratching the next time the error appears.

Related Documentation

• Siemens Industry Online Support — main portal for the Siwarex FTA manual and the Siwatool FTA configuration manual.
• Siwarex FTA Function Manual — chapter "Command interface", "Data records DR3 / DR30", "Dosing operation".
• Siwatool FTA Online Help — section "Service → Commands".

FAQ