Troubleshooting Milltronics BW500 Missing Load Cells C and D

Problem Description

The Milltronics BW500 is a belt scale integrator used in continuous weighing applications. The unit accepts up to four load cells (A, B, C, D) wired into its internal bridge. A common field failure mode presents as follows: parameter P940 (live load cell reading) shows only load cells A and B; load cells connected to terminals 6/7 and 8/9 do not appear, even though the physical cells measure correctly when checked with a voltmeter directly on the PCB.

Symptoms include:

• Intermittent totalizer errors (over- and under-estimation on the same conveyor).
• P940 displays two active load cell channels, two inactive or zero channels.
• External voltmeter test of load cell output at terminals 6/7 and 8/9 returns a normal mV/V signal.
• Parameter P002 cannot complete a stable E-Calibration because channels C and D are not enumerated.

Hardware identification for the affected unit in this case is Milltronics BW500, order code 7MH71522AA022BA, serial PBD/F9030071. The general product family, firmware register layout, and load cell wiring scheme are documented in the official Siemens operating manual A5E33482052 (BW500 / BW500/L operating instructions).

Affected Hardware and Firmware

The BW500 product family covers two variants: the standard BW500 and the BW500/L (leased-line modem variant). Both share the same load cell front-end. The article applies to the standard BW500 (7MH7152…). The firmware version is read from P900 on the keypad menu. Always record P900 before any reset operation:

If P900 reports a firmware that predates the load cell mapping fixes shipped with the manual A5E33482052-04 AD, contact Siemens support via SIOS for a firmware update evaluation before replacing hardware.

Root Cause Analysis

There are three primary root cause families for the "2 of 4 load cells visible" symptom. Always work through them in this order to avoid unnecessary hardware replacement.

1. Configuration / parameter corruption. The BW500 keeps calibration constants, linearization points, and the load cell enable map in non-volatile memory. A brown-out, a failed write during parameter entry, or accidental editing of P002 can disable the C and D channels. The P940 readout will then show only A and B regardless of the actual hardware.
2. Damaged load cell input stage. The PCB traces and protection devices associated with terminals 6/7 and 8/9 can fail if the conveyor suffered a lightning strike, a welding flashover, or if load cells were wired with the shield drain routed incorrectly. Volt-ohm-meter test of the cells at the terminals shows a valid signal because the cells themselves are intact; the integrator input is the fault.
3. Wiring / connection error. Reverse polarity, swapped sense/excitation leads, or a broken conductor inside the conduit. This is the simplest cause to rule out first.

The diagnostic flow below distinguishes these three cases deterministically.

Diagnostic Flow

Step 1: Read firmware and identify the unit

1. From the keypad, navigate to P900 and record the firmware string.
2. Record P901 (hardware revision).
3. Record the order code from the nameplate (here 7MH71522AA022BA) and the serial number (here PBD/F9030071). These are required for any SIOS Support Request.

Step 2: Inspect P940 load cell enumeration

P940 is the live load cell display. With all four cells connected and healthy it should show four values. With the reported fault it shows two. Toggle through the sub-pages and note which channels report a stable mV/V value and which report 0 or OL (open line).

Step 3: Manual voltmeter check on the PCB

1. Open the enclosure. Identify the load cell terminal block on the PCB.
2. With the integrator powered off, measure the resistance between the +Signal/-Signal pins for cells C and D (terminals 6/7 and 8/9). A healthy 350 Ω or 1000 Ω bridge returns the expected nominal value.
3. With the integrator powered on, measure the excitation voltage at the same terminals. It should match the rated excitation (typically 10 V DC) within ±2 %.
4. If the cells pass both checks when measured at the PCB terminals, the input stage is intact and the cause is configuration, not hardware. If the cells do not pass, the input stage or wiring is the fault and a service replacement is required.

Step 4: Save current parameters before any reset

Before executing P999, write down the live values of every operating parameter. The minimum set to preserve calibration is shown in the table below. These parameters control the entire integrator behavior and must be restored after a factory reset.

Factory Reset Procedure (P999)

1. Enter programming mode using the integrator keypad.
2. Navigate to P999.
3. Enter the reset code 9 and press Enter.
4. The display confirms the reset; all parameters revert to factory default.
5. Re-enter the saved values from P002 through P017, plus P367 and P368.

Electronic Calibration (E-Cal)

E-Cal allows the integrator to be calibrated without applying physical test weights. It relies on a stored reference for the load cell capacity and uses a calculated span constant. The procedure assumes the load cells are healthy, properly wired, and the belt is empty during the zero step.

1. Set P002 = 1 (E-Calibration mode).
2. Enter the rated capacity of one load cell in P694 (ECal Load Cell Capacity). For example, enter 50 if each cell is rated 50 kg. Use the manufacturer's nameplate value; do not estimate.
3. With the belt empty and the conveyor stopped, run the zero routine. The integrator stores the empty-belt reference in P367.
4. Apply a known test material and a known belt speed, then run the span routine. The integrator calculates the span constant and stores it in P368.
5. Validate against a physical test weight run; if E-Cal is not permitted by site QA, repeat using P002 = 2 (test weight calibration) with the actual weights on the test chain.

Electronic Calibration Without Test Weights

If physical test weights are not available but the load cells are healthy and enumerated, E-Cal is the only method that can establish a span constant. It is, however, less accurate than a physical-weight calibration and is generally not accepted for fiscal / trade weighing. The three-test-material cross-check listed in the BW500 manual is the field-accepted substitute:

1. Pass three different known quantities of material across the conveyor at design speed.
2. Compare the BW500 totalizer to the reference value (weighbridge, hopper scale, or batch records).
3. Adjust the E-Cal span (P368) until the totalizer matches the reference within site tolerance (typically ±0.5 % to ±1 % for non-trade, ±0.1 % for trade applications).

Verification After Reset and Calibration

1. Power-cycle the BW500 and confirm it returns to run mode with the saved parameter set.
2. Re-check P940: all four load cells should now appear with stable mV/V values within the expected operating range.
3. Run a no-load zero verification: with the belt empty and running at design speed, the integrator should report a stable zero load.
4. Run a material test of known quantity and confirm the totalizer reads within the site's specified tolerance.
5. Monitor for at least 30 minutes of continuous run; intermittent over/under-estimation usually reappears in the first 10-15 minutes if a load cell input is unstable.

Troubleshooting Matrix

When to Open a SIOS Support Request

Escalate to Siemens Industry Online Support when any of the following hold:

• The PCB voltmeter test fails — the input stage is damaged.
• Factory reset and parameter restore do not bring all four cells back into P940.
• The site requires a certified trade-accuracy calibration and E-Cal is not acceptable.
• Firmware in P900 is older than the latest AD update for the manual (the current AD is A5E33482052-04).

When opening the request, include the order code (7MH7152…), serial number, P900 firmware string, the values recorded in Step 4 above, and the belt scale model (7MH…) connected to the integrator. Siemens will lock the request and respond with an application questionnaire covering maximum feed rate, belt speed, conveyor incline, and the load cell specification.

FAQ