Resetting SIWAREX CS Load Cell Error Without PLC Restart

Problem Overview: Persistent Load Cell Error on SIWAREX CS

The SIWAREX CS weighing module (Siemens order number 7MH4910-0AA01) is a compact, single-channel weighing electronics module designed for use in the SIMATIC ET 200S distributed I/O system. It is widely deployed in static scale applications such as bin scales, weigh feeders, platform scales, level measurement silos, and force measurement stands where it functions as a tightly integrated weighing channel reporting net, gross, tare, and weight status to the host S7 CPU over PROFIBUS or PROFINET backplane.

A recurring field issue involves the following fault sequence:

1. An operator or technician disconnects the load cell cable from the SIWAREX CS front connector (for example, to swap a damaged cell, inspect a junction box, or replace the load cell cable).
2. The HMI weight display immediately changes to an ERROR indication.
3. After the load cell is reconnected (with proper wiring and bridge resistance verified), the error indication does not clear automatically.
4. A full PLC stop/run (or power cycle) restores normal operation.

This behaviour is intentional: the SIWAREX CS latches all critical hardware and process faults in its cyclic status word and in the user-allocated instance DB to prevent the application from trusting an unverified weight value. However, it also means that a momentary wiring event — even an authorized, planned event — leaves the system in a latched error state until the host controller resets the latch.

Affected Hardware and Module Identification

Confirm that the installation matches the components described in this article before applying the reset procedure.

Note that the exact DB number and flag address depend on the user's project configuration. The values DB100, DBX21.6, and M20.0 are taken directly from the official Siemens "Getting Started" sample project for SIWAREX CS. In a user project that was re-instantiated with a different DB number, locate the equivalent bit in the Error / Operating message word of the assigned instance DB.

Root Cause: Why the Error Does Not Self-Clear

When the load cell sense or supply lines are opened, the SIWAREX CS measures an out-of-range bridge excitation or detects a wire break. The module sets the corresponding bits in its internal status word and then propagates those bits to the user-allocated instance DB on the S7 CPU via the cyclic data interface. The relevant bit for the "load cell disconnected / wire break / bridge open" class of faults is bit 6 of byte 21 of DB100 — the bit the Siemens Getting Started project exposes as DB100.DBX21.6.

This bit is a latched status bit. The module deliberately retains the fault indication even after the physical cause has been removed because:

• The weight value during the fault window cannot be trusted for custody transfer, batching, or dosing.
• A subsequent loss of validity would create a misleading "everything fine" state without operator acknowledgement.
• Process safety standards (for example, weighing in explosive atmospheres, batching additives, overfill protection) require positive acknowledgement of any weighing error before the next production step is allowed.

Reconnecting the load cell restores the analog input to a valid range but does not reset the latched status bit. Only one of the following will reset the latch:

1. A CPU restart (STOP/RUN or power cycle) — the instance DB is re-initialised.
2. A explicit software command from the application program to the SIWAREX FB — the supported method, described below.
3. Re-parameterisation of the module via the Siwatool CS commissioning software (writes data record 0 / data record 1 and clears status flags).

Error and Operating Message Memory Layout

The SIWAREX CS exposes two categories of message words in the cyclic process image and instance DB:

• Hardware / process errors — for example, load cell wire break, sense lead open, ADC over-range, power supply out of tolerance. These populate the "Error" byte(s) and the bit DB100.DBX21.6.
• Operating / parameterisation errors — for example, calibration incomplete, data record invalid. On related Siemens SIWAREX platforms (e.g. WP321), these are reported in the 5000 to 8999 code range. See the SIWAREX WP321 ID 90893240 operating error message list for the canonical error-to-cause mapping that applies to the family.

The layout in DB100 (Getting Started default) is summarised below. Note that in a user-configured instance DB the offsets may differ by a constant.

Solution: Reset via the M20.0 Acknowledgement Flag

The supported, documented method for clearing the SIWAREX CS error latch without restarting the PLC is to set the acknowledgement flag M20.0 for one scan (a rising-edge trigger). In the Siemens Getting Started project this flag is wired directly into the SIWAREX FB as the "Acknowledge errors" / "Reset" input. A single positive edge on M20.0 instructs the FB to write the corresponding command record to the SIWAREX CS module, which clears all latched error bits and returns the status word to "operating OK".

Trigger sources commonly used in the field:

1. HMI button — an operator "Acknowledge" button on the SCADA/HMI screen that sets M20.0 TRUE for one scan. This is the configuration delivered with the Getting Started project.
2. Automatic retry after reconnect — if the application supervises the sense-lead voltage via the diagnostics word and detects that the load cell has been reconnected, the program can self-trigger M20.0.
3. Time-limited retry — if the wiring is part of a plug-and-socket maintenance procedure, a timer can trigger the reset after the operator confirms completion.

Implementation in STEP 7 ladder (FBD / LAD) for a one-shot reset:

// One-shot acknowledge — M20.0 pulses TRUE for one OB1 scan on rising edge
// of "Ack_Req" from HMI or auto-detect logic.
A     "Ack_Req"            // BOOL, request to acknowledge errors
FP                          // rising-edge detect
S     "M20.0"              // set reset command
// optional: clear M20.0 at end of OB1 so next cycle can detect new edge
A     "M20.0"
R     "M20.0"              // reset; pulse has done its job
// Alternatively, wire M20.0 directly into FB parameter "ACK_ERR" or "RESET".

Implementation in Structured Text (SCL) for TIA Portal:

IF "Ack_Req" AND NOT "Ack_Req_Old" THEN
    "M20.0" := TRUE;        // pulse acknowledge command
ELSE
    "M20.0" := FALSE;
END_IF;
"Ack_Req_Old" := "Ack_Req";

// "M20.0" is wired into the SIWAREX FB "Acknowledge errors" input.
// On the rising edge the FB issues the command record and the module
// clears DB100.DBX21.6 plus all latched operating error bits.

After the rising edge, verify that DB100.DBX21.6 returns to FALSE within one or two OB1 cycles (the command record is processed by the module asynchronously).

HMI / SCADA Integration: Operator Acknowledgement

The simplest deployment is an "Acknowledge weighing error" button on the HMI weight screen. Configure it with the following properties:

• Tag: M20.0 (BOOL) — or a project-specific equivalent.
• Event: Click → set TRUE for one cycle (use "Set bit while pressed" or "Invert bit" with edge evaluation).
• Visibility condition: visible only when DB100.DBX21.6 = TRUE (i.e. an error is currently latched), to prevent unintended resets.
• Optional confirmation dialog: "Weighing error will be acknowledged. Confirm?" with operator password if the scale is under custody transfer or 21 CFR Part 11 / OIML R76 regulated.

The Getting Started project already pre-wires this button so the operator can clear a transient error such as a load cell disconnect without involving the PLC programmer. If a different HMI system is used (WinCC flexible, WinCC, TIA Portal WinCC, third-party SCADA via OPC), the same tag wired to a momentary pushbutton delivers the same effect.

Safety Considerations: Hot Swapping of Load Cells

Connecting or disconnecting a load cell while the SIWAREX CS is powered should be avoided where possible:

• The sense and excitation lines are low-voltage analog signals referenced to the module ground, but the bridge excitation can deliver enough current at the open-circuit terminals to generate ESD-like transients when the connector is re-mated.
• An interrupted sense line during a calibration can leave the module reporting a frozen weight value that an operator may misinterpret as a true reading.
• If the application is part of a safety instrumented function (overfill prevention, SIL-rated batching end-switch), any wiring change must follow the site's Management of Change procedure.

Recommended practice for planned load cell replacement:

1. Place the affected scale in service / maintenance mode in the HMI to inhibit dosing, batching, or custody-transfer actions.
2. Stop the PLC or place the CPU in STOP if the application documentation requires it. For most non-SIL applications, leaving the PLC in RUN is acceptable provided that downstream consumers of the weight value are blocked.
3. De-energise the ET 200S station (or at minimum the SIWAREX module terminals) before disconnecting the load cell cable, where practical.
4. After reconnecting, verify the bridge resistance (typically 350 Ω or 700 Ω depending on the cell), then trigger M20.0 to clear the latch.
5. Perform a zero calibration (or restore the last good calibration via Siwatool CS) before returning the scale to production.

For an unplanned wire break in running production — the scenario the field report describes — there is no safe alternative to the software reset described above, since a CPU stop would interrupt the entire line. The M20.0 acknowledgement is the supported field remedy.

Operating Error Codes (Reference: SIWAREX Family)

The SIWAREX CS shares its operating-error philosophy with later SIWAREX modules such as the WP321. Operating errors are returned in the cyclic status word / operating-error byte. The canonical code list for the family is published in the SIWAREX WP321 entry (ID 90893240), with codes ranging from 5000 to 8999. A representative subset:

Always read the full operating-error word after a reset to confirm that the code has cleared; some codes indicate a persistent parameter problem that will reappear immediately on the next scan if not corrected.

Verification: Confirming the Reset

After pulsing M20.0, perform the following checks before returning the scale to production:

1. Monitor DB100.DBX21.6 in the STEP 7 status table or TIA Portal watch table. It must return to FALSE within one or two OB1 cycles.
2. Confirm the HMI weight display is no longer in the ERROR state and shows the expected live weight.
3. Read the operating-error byte (DB100.DBB22) and verify that no code is present (typically 0000 decimal).
4. Perform a zero check: with no material on the scale, the weight reading should be within the agreed zero tolerance (commonly ±0.05% of the calibrated span).
5. If a test weight is available, perform a span verification at one calibration point to confirm the analog chain is healthy.
6. Log the event in the audit trail if the application is regulated (custody transfer, pharmaceutical, food batching).

If DB100.DBX21.6 does not clear after the M20.0 pulse, the physical fault is still present (or a new one has appeared). Re-check the wiring, the bridge resistance, and the load cell health.

Troubleshooting Matrix

Frequently Asked Questions