Connecting Multiple Load Cells to SIWAREX FTA Wiring

1. Overview: SIWAREX FTA Multi-Load-Cell Applications

The Siemens SIWAREX FTA (Function Technology Weighing) is a strain-gauge weighing module designed for integration into SIMATIC automation systems (S7-300, S7-400, ET 200S, ET 200SP via the appropriate adapter). Although the FTA module itself has only one analog load-cell input, large vessels, silos, hopper scales, and platform scales routinely use two, three, four, or more load cells mechanically supporting a single weighing structure. The electrical sum of these parallel load cells must be presented to the FTA as a single equivalent bridge.

This article explains:

• How to wire 2–4 (or more) load cells to a single SIWAREX FTA input.
• The role of the SIWAREX JB junction box.
• Bridge excitation / sense line compensation when 4-wire load cells are used with a 6-wire FTA cable.
• How the theoretical calibration value changes when multiple cells share the load.
• Step-by-step configuration with SIWATOOL FTA.
• Field verification, troubleshooting, and FAQ.

2. Prerequisites

3. Wiring Topology — One FTA, Multiple Load Cells

3.1 Why a Junction Box Is Mandatory

When a vessel rests on N mechanically parallel load supports, each cell sees only a fraction 1/N of the total weight (in the ideal equal-load-distribution case). The SIWAREX FTA can only read one bridge, so the individual bridges must be wired in parallel (input, output, and sense lines tied together) and routed through a junction box that also provides:

• A single multi-conductor output cable to the FTA.
• Optional trim resistors in each corner of each bridge, used to balance cell outputs so the combined bridge zero is near zero at no-load.
• A means to disconnect individual cells for diagnostics.

3.2 4-Wire vs. 6-Wire Load Cells

The 4-wire → 6-wire conversion is necessary because long cables between the JB and the FTA introduce a voltage drop on the EX lines that the FTA's differential amplifier would interpret as a weight error. By using the FTA's sense inputs to measure the excitation at the JB terminals, the excitation error is rejected.

3.3 Pinout Reference (FTA ↔ JB)

3.4 Schematic (Inline SVG Topology)

4. Theoretical Calibration With Multiple Load Cells

Two calibration paths exist. Both are documented in Siemens Support Entry 37734709 — SIWAREX FTA Basic Configuration & Calibration.

4.1 Calibration With Test Weights (Recommended)

1. Mechanically load the empty vessel. The FTA reports the dead load.
2. Place calibrated weights equal to ~50–100% of nominal capacity on the vessel.
3. Enter the value in SIWATOOL's Adjustment dialog: weight → 2,000.00 kg (example). The FTA scales its characteristic value automatically.

This method works identically for 1, 2, 4, or 6 cells.

4.2 Theoretical (Calculation-Based) Calibration

Use this only when no test weights are available. The total characteristic value Cv_total is the parallel sum of individual cell characteristic values:

Cv_total = Cv_1 + Cv_2 + … + Cv_N   [mV/V]

The characteristic value of each cell is given by the cell's data sheet:

Cv_i = (Sensitivity_i in mV/V) × (Capacity_i / Capacity_vessel_i)

Where Capacity_vessel_i is the load that cell i is expected to bear at full vessel load (ideal: nominal_load / N). With four identical cells of 2.0 mV/V and 5,000 kg capacity each supporting a 20,000 kg vessel equally:

• Cv_total = 2.0 + 2.0 + 2.0 + 2.0 = 8.0 mV/V

This value is entered in SIWATOOL under DR 14 / Characteristic value. The empty-vessel zero step is still required.

5. SIWATOOL FTA Configuration Procedure

1. Connect PC via USB SIWAREX IS service cable to the FTA's front X1 port.
2. Launch SIWATOOL FTA V4.x → choose Online → Connect. Read current parameters first (read-modify-write pattern).
3. Set the parameter set to a free DS (e.g. DS 0 = 'Standard').
4. Enter calibration values:
   • DR 1 — Calibration weight 0 (no-load)
   • DR 2 — Calibration weight 1 (e.g. 0 kg reference)
   • DR 3 — Calibration weight 2 (e.g. 2,000.00 kg)
   • DR 14 — Characteristic value (Cv_total from §4.2)
   • DR 30 — Zero setting range (% of nominal)
   • DR 31 — Nominal weight (kg)
   • DR 32 — Unit (kg / t / lb)
5. Trigger zero: with empty vessel, click Adjustment → Zero. The FTA stores the digital zero.
6. Trigger span: with calibrated weights on the vessel, click Adjustment → Span (only needed if you used a theoretical Cv).
7. Save to RAM and to the parameter set on the FTA.
8. Verify with intermediate load steps (25%, 50%, 75%, 100%).

6. Mechanical & Installation Best Practices

• All load cells must be the same capacity, same sensitivity class, same model. Mixing cells introduces thermal drift asymmetry.
• Use mounting kits with check-nodes or self-aligning load buttons to avoid side load.
• The vessel's center of gravity should fall inside the convex hull of the cell support points.
• Install arrestor rods / check rods so the vessel cannot tip during wind, vibration, or material impact.
• Route the 6-wire JB→FTA cable away from VFD output cables and welding leads. Maintain ≥30 cm separation; cross at 90° when needed.
• Ground the shield only at one end (FTA side recommended) to avoid ground loops.
• For outdoor scales, use SIWAREX JB with IP65 sealing and cable glands matched to cable OD.

7. Verification Procedure

1. Power cycle the FTA and confirm the Process value is stable for ≥5 minutes at no load.
2. Apply 0% / 25% / 50% / 75% / 100% / 0% known weights. Record FTA reading each step.
3. Compute error: e_i = (reading_i − applied_i) / applied_i × 100%. OIML R76 Class III typical acceptable limits: ≤ ±0.02% of span for non-automatic weighing, ≤ ±0.05% for industrial scales.
4. Repeatability: 3× 100% load cycles. Repeatability error should be < 1 internal count (≈ 0.01% of full scale at default resolution).
5. Check linearity: plot error vs. load; the curve must remain within the cell manufacturer's linearity band.
6. Corner-load test: with ¼ of full load placed directly above each individual cell. The reading must equal the total weight within tolerance. Differences reveal cell imbalance → adjust trim resistors in the JB or apply mechanical shimming.

8. Troubleshooting Matrix

9. SIWATOOL DR (Data Record) Reference Cheat-Sheet

10. PLC Integration Notes (STEP 7 / TIA Portal)

• Use the FB SIWA (TIA: in SIWAREX library under "SIWAREX FTA") to read FTA_DIAG, FTA_PROCESS_VALUE, and to write FTA_COMMAND.
• The process value is returned as a REAL in engineering units (kg) once calibration is complete; no scaling math is required in the PLC.
• Diagnostic word bit 5 = sense line error; bit 10 = calibration data invalid. Wire these to HMI alarms.
• For redundant weighing (two SIWAREX FTA on the same tank), use a voting block — never connect two FTA modules to the same cell junction box without isolation amplifiers.

11. Sample SCL / Structured Text Snippet

// Read SIWAREX FTA process value
IF FTA_DIAG.%X5 THEN  // Sense line error bit
    bAlarmSenseLine := TRUE;
END_IF;

IF FTA_DIAG.%X10 THEN  // Calibration invalid
    bAlarmCalInvalid := TRUE;
END_IF;

rGrossWeight := FTA_PROCESS_VALUE;          // REAL in kg
rNetWeight   := rGrossWeight - rTareWeight; // tare from HMI setpoint

// Tare command (rising edge on HMI button)
IF bTareRequest AND NOT bTareRequestOld THEN
    FTA_COMMAND := 16#0001;                 // zero command per FB SIWA spec
END_IF;
bTareRequestOld := bTareRequest;

12. Frequently Asked Questions