Troubleshooting SIWAREX FTC and SIWAREX U Service Functionality and EN_CO Freezes in SIMATIC PCS 7

In SIMATIC PCS 7, when communicating with SIWAREX weighing electronics (such as SIWAREX FTC for continuous loss-in-weight feeding or SIWAREX U for gravimetric level measurement) where process values (PROC_VAL1) update normally in the AS program but service commands (zeroing, calibration, taring) fail from the WinCC faceplate with parameter EN_CO frozen, the root cause is a mismatch between the number of SIWAREX/counter modules configured in HW Config and the module instances instantiated in the CFC charts, or unfulfilled service command authorization conditions. The SIWAREX PCS 7 driver library manages communication queues via a centralized coordinating routine (CoNo - Coordinator Number). If a configured SIWAREX module in HW Config lacks a corresponding CFC driver call, or if the faceplate command handshake EN_CO (Enable Command) is blocked by user authorization or missing diagnostic links, all service functions freeze.

Architecture of the SIWAREX PCS 7 Driver Library and CoNo Coordination

SIWAREX weighing processors (e.g., SIWAREX U 7MH4950-..., SIWAREX FTC 7MH4900-...) are specialized ET 200M function modules that digitize load cell strain gauges with legal-for-trade accuracy. The SIWAREX PCS 7 library utilizes a dual-path communication model:

Cyclic Path: High-speed weight measurements (PROC_VAL1, PROC_VAL2) and basic operational status bits are read cyclically through standard peripheral I/O words (PIW).
Acyclic Command Path: Calibration commands, zeroing, taring, and service parameter downloads are transferred acyclically using system data records (SFB 52 RDREC / SFB 53 WRREC).

To prevent simultaneous data record collisions on the backplane bus, the SIWAREX driver package deploys a centralized communication coordinator. Every module instance is assigned a unique Coordinator Number (CoNo).

Root Cause Analysis: Why Service Functionality Freezes Despite Valid Process Values

The phenomenon where live weight updates cleanly but faceplate service buttons are completely dead stems from specific configuration flaws:

Module Count Discrepancy (HW Config vs CFC): The SIWAREX driver generator requires a strict 1-to-1 parity between the number of physical SIWAREX modules inserted in HW Config and the number of driver blocks placed in CFC. If an engineer configures 4 SIWAREX modules in HW Config but only instantiates 3 in CFC, the internal coordinating routine halts the acyclic command queue, waiting indefinitely for the missing instance.
Frozen EN_CO Handshake: The parameter EN_CO (Enable Command) must toggle from 0 to 1 to authorize the module to execute service commands. If EN_CO does not change, the faceplate script cannot initiate the data record write.
Missing Service Authorization: In WinCC, service functionality (such as calibration adjustments) is protected by SIMATIC Logon authorization levels (typically Level 6 or Level 11). If the logged-in operator lacks service rights, the faceplate silently suppresses the command dispatch.

The Role of EN_CO (Enable Command) and Faceplate Command Handshaking

The command execution pipeline operates via a strict four-stage handshake:

Faceplate Command Selection: The operator selects a service command (e.g., Command 1 - Zero Scale) on the WinCC faceplate.
Enable Assertion (EN_CO = 1): WinCC writes the command code to CMD_IN and asserts EN_CO = 1 on the AS block.
Acyclic Dispatch: The AS block validates that CoNo is free, dispatches the record to the SIWAREX module via WRREC, and waits for the hardware acknowledgment.
Handshake Reset: Once the SIWAREX module confirms execution, the block resets EN_CO = 0 and pulses CMD_OK = 1 (or CMD_ERR = 1 if rejected). If the command queue is frozen, EN_CO remains locked at 0 or 1, blocking all further actions.

Step-by-Step Procedure to Restore Service Command Execution

To resolve frozen service functionality and restore faceplate commands:

Audit HW Config vs CFC: Open HW Config. Count the exact number of SIWAREX and counter modules in the station. Open CFC; ensure that every configured module has a corresponding driver block instantiated with matching logical addresses (LADDR).
Verify Coordinator Parameters: Check parameter CoNo on each SIWAREX block. Ensure each instance has a unique sequential number (e.g., 1, 2, 3).
Compile Module Drivers: In SIMATIC Manager, execute Chart > Compile > As Program with Generate module drivers checked.
Reset Command Buffer: If a previous command hung in memory, pulse the reset parameter (CMD_RST = 1) in CFC test mode to flush the communication queue.
Verify WinCC User Rights: In WinCC User Administrator, verify that the active user account possesses authorization for 'Higher process controlling' (Authorization 6) and 'Service' (Authorization 11).

Comparison of SIWAREX Module Architectures in PCS 7 (SIWAREX U vs FTC)

The following table compares the operational capabilities and driver requirements of SIWAREX U and SIWAREX FTC modules:

| Weighing Parameter | SIWAREX U Module (7MH4950-...) | SIWAREX FTC Module (7MH4900-...) |
|---|---|---|---|
| Primary Use Case | Static scale, bin/hopper level, silo weighing | Loss-in-weight continuous dosing, belt weigher |
| Measurement Channels | 1 or 2 Independent Strain Gauge Channels | 1 High-Precision Multi-Function Channel |
| Internal Resolution | 65,000 divisions (16-bit) | Up to 16,000,000 divisions (24-bit) |
| PCS 7 Library Package | SIWAREX U PCS 7 APL Library | SIWAREX FTC PCS 7 APL Package |
| Service Functionality | Zero, Tare, Calibration Weight Adjust | Auto-tare, PID dosing tuning, flow rate calibration |
| Communication Protocol | Peripheral I/O + Data Records (DS 3 to 15) | Peripheral I/O + Extended Records (DS 1 to 128) |

Verification

Verify SIWAREX service command functionality:
1. In WinCC Runtime, open the SIWAREX FTC/U faceplate; navigate to the Service view.
2. Ensure the user is logged in with Service administrative credentials (check the key icon in the faceplate header).
3. Click the 'Zero Scale' button; observe that the faceplate command indicator shows active processing.
4. In CFC test mode, confirm that EN_CO briefly toggles to 1, followed by CMD_OK pulsing true and EN_CO returning to 0.
5. Verify that PROC_VAL1 on the faceplate immediately resets to 0.00 kg.

Limitations and Operational Precautions

Never execute scale calibration adjustments or zeroing commands while material is actively discharging or while pumps are running connected to the weighing vessel. Calibrating load cells requires certified test weights and stable environmental conditions (zero mechanical vibration and zero wind draft). Calibration data is stored inside non-volatile EEPROM on the SIWAREX module; avoid continuous automated writing of calibration parameters from cyclic SFCs to prevent premature flash memory wear.

Engineer FAQ

Why does live weight display correctly if the service functionality is frozen?

Live weight (PROC_VAL1) is transferred via cyclic peripheral I/O (PIW), which operates independently of data records. Service commands utilize acyclic data record transfers (WRREC), which are governed by the CoNo coordinator queue.

What does the parameter CoNo represent?

CoNo stands for 'Coordinator Number'. It is an internal scheduling index used by the SIWAREX driver library to serialize acyclic data record communications across multiple weighing modules on the same controller.

Where can I find official application examples for SIWAREX in PCS 7?

Siemens provides detailed application examples on SIOS: 'Gravimetric Level Measurement with SIWAREX U in PCS 7' (Entry ID: 98754978) and 'Continuous Dosing with SIWAREX FTC' (Entry ID: 86536965).