S7-300 High-Speed Weighing: SIWAREX and SM 335 Module Selection

Application Profile: 10-Channel Multi-Cell Weighing at 5–6 Hz

The reference application specifies a small-frame PLC (S7-300 / SLC 500 class) that must read ten strain-gage load cells simultaneously at 5–6 conversions per second per channel, deliver true weight values rather than threshold comparisons, and provide 10 V DC bridge excitation per cell. The aggregate throughput floor is therefore 10 channels × 6 Hz = 60 conversions / second. Because the design eliminates the external signal conditioner to minimize group delay, the analog front-end has to accept raw bridge output (typically 1–3 mV/V at full scale) and resolve to weighing accuracy without a discrete instrumentation amplifier in the path.

This profile is incompatible with general-purpose analog input cards because their resolution and channel scan architecture are tuned for process signals (4–20 mA, ±10 V), not microvolt bridge outputs. Three product families genuinely fit:

• Siemens SIWAREX weighing modules — purpose-built bridge front-ends that mount in the S7-300 rack.
• Siemens SM 335 (6ES7 335-7HG01-0AB0) — fast analog I/O module for closed-loop control.
• Generic analog input cards (e.g., 1746-NI4 on SLC 500, TSXISPY101 on Schneider TSX Premium) used with either external instrumentation amplifiers or with the load cell's internal amplifier.

Load Cell Signal Fundamentals

A 350 Ω, 2 mV/V load cell excited with 10 V DC produces a full-scale differential output of 20 mV. Resolving 0.05 % of full scale requires a least-significant bit (LSB) of 10 µV at the ADC input. The relationships below govern module selection:

Full-scale bridge output:

where S = sensitivity (V/V), V_EXC = excitation voltage.

Required ADC noise-free resolution:

For 20 mV / 10 µV: log₂(2000) ≈ 10.97 → 11 bits minimum for 0.05 %; 16 bits provides comfortable headroom for noise and tare.

The signal chain also must reject common-mode noise from motor VFDs, thermcouple-like ground loops, and 50/60 Hz mains pickup. Weighing electronics therefore combine a true instrumentation amplifier, a programmable gain stage, and either a sigma-delta or dual-slope ADC with internal digital filtering. Reference designs from converter vendors describe these topologies in detail in Texas Instruments SNAA099 — Ultra-High Speed ADCs Revolutionize Digital Receiver Design and Analog Devices AN-835 — Understanding High Speed ADC Testing and Evaluation.

Siemens S7-300 SIWAREX Weighing Module Family

SIWAREX modules are the dedicated weighing front-end for S7-300. They mount in the standard 40 mm slot of the S7-300 I/O rack and communicate to the CPU over the backplane as distributed I/O; programming is performed through a Siemens function block library (SIWATOOL) and standard STEP 7 ladder or SCL. The S7-300 platform is described in the Siemens SIMATIC S7-300 product documentation.

For ten channels of 5–6 Hz weighing on an S7-300, the typical configuration is one SIWAREX module per load cell pair (using the SIWAREX junction box to parallel up to four cells per module) or ten SIWAREX MS / U modules when channels must remain independent. Each module occupies a single slot and is addressed by the CPU as a standard analog input word. Update rate and excitation voltage are configured through SIWATOOL V7 (or the current Siemens weighing configuration utility), not through hardware jumpers.

Why 50 Hz is sufficient: 50 Hz corresponds to a 20 ms cycle. With a 5–6 Hz requirement the controller can average multiple reads (4–10 per decision) and still meet the update period, dramatically improving signal-to-noise ratio at the cost of one PLC scan of latency. Most weigh-in-motion and batching applications specify 10–20 readings averaged before the next control decision, which is well within the 60 Hz aggregate budget.

SM 335 High-Speed Analog I/O Module (6ES7 335-7HG01-0AB0)

The SM 335 (order number 6ES7 335-7HG01-0AB0) is a fast analog I/O module designed for closed-loop control, not dedicated weighing. Its relevant characteristics are summarized in the table below based on the Siemens SIMATIC S7-300 module data manual.

The SM 335 is the right answer when:

• the load cell already has an integrated amplifier producing 0–10 V or 4–20 mA,
• the application needs microsecond-class latency (e.g., weight-based force control inside a single servo cycle), or
• the same module is also used for fast actuator outputs.

It is the wrong answer when the load cell exposes a raw mV/V bridge, because the input voltage ranges (≥ ±5 V) are 250 times larger than a 20 mV bridge signal — the 14-bit ADC then offers no useful resolution below 5 mV, which corresponds to 25 % of a typical load cell range.

Allen-Bradley SLC 500 Alternative: 1746-NI Family

The Allen-Bradley SLC 500 platform offers analog input modules that physically fit the I/O rack but do not provide bridge excitation. They are usable only with pre-amplified load cells.

For ten raw bridge channels, a typical SLC 500 implementation would pair each 1746-NI4 input with a separate 4-channel load cell signal conditioner (e.g., RDP S7DC, Hardy HI 2151-30A, or similar). The total module count grows: 10 cells → 10 amplifier channels → 3 × 1746-NI4 slots minimum, plus 10 excitation supplies (or one shared precision 10 V reference with sufficient current budget).

Schneider TSX Premium TSXISPY101 ISP Plus

The Schneider TSX Premium platform hosts the TSXISPY101 "ISP Plus Integrated Weighing System" module, a dedicated weighing front-end for Modicon Premium / TSX Premium PLCs. The module:

• Supports 1 to 8 load cells per module through a junction box,
• Samples at 50 Hz (one conversion every 20 ms),
• Provides bridge excitation internally,
• Exposes measured weight, net weight, and zero/span calibration over the Premium backplane.

With one TSXISPY101 per eight cells, a ten-channel system needs two modules, leaving six of the rack slots free for additional Premium I/O. The TSX Premium processor accepts up to 32 ISP Plus modules in a single rack, so channel density is rarely the limit. Schneider's weighing block library handles filtering, tare, and flow-rate integration in PL7 or Unity Pro.

For greenfield installations where the integrator is not committed to Siemens, the TSXISPY101 is the closest direct competitor to SIWAREX U on the same architectural level (PLC-resident weighing front-end).

Aggregate Throughput Calculation

Validate the 5–6 Hz / 10-channel requirement with the following calculation:

All candidates satisfy the raw throughput. The discriminator becomes channel-count efficiency (how many cells per slot), excitation source (integral or external), and integration effort (vendor weighing library vs. custom math in STEP 7).

Wiring, Shielding, and Isolation

Low-level bridge signals are sensitive to ground loops, VFD common-mode noise, and capacitive coupling from parallel AC runs. The SIWAREX wiring manual specifies the following practices, which apply equally to any weighing front-end.

1. Use shielded twisted-pair (STP) cable with overall foil shield plus drain wire. Typical cable: Belden 8761 or Alpha 6330 for 4-wire bridge; 6-wire (sense line) cable if the load cell supports remote sensing to compensate lead resistance.
2. Ground the shield at the module end only. Multi-point grounding creates ground loops that couple 50/60 Hz directly into the mV signal.
3. Keep bridge wiring in a separate conduit at least 200 mm (8 in) from VFD output cables and unfiltered motor feeders. Cross at 90° if unavoidable.
4. Twist the four bridge conductors tightly (≥ 8 twists per metre) to cancel magnetic pickup.
5. Verify excitation sense lines at the load cell. 4-wire cells are tolerant of lead resistance but lose calibration with long cable runs; 6-wire cells compensate up to several hundred ohms of lead.
6. Use a precision 10.000 V reference at the module. Calibration drift in the excitation source appears directly as scale error.
7. Bond the load cell hardware to a single ground point if cells are mounted on a conductive frame; otherwise leave the cell body floating.

For SIWAREX, the module's "Load Cell Connection" diagram in the SIWAREX U manual (available on the Siemens Industry Online Support portal) shows the precise pin assignment for the 6ES7 standard front connector, including sense-line jumpers when used in 4-wire mode.

Configuration and STEP 7 / TIA Portal Integration

SIWAREX modules are configured through SIWATOOL V7 (or the current version) and integrated into STEP 7 / TIA Portal as standard analog input words. The following sketch illustrates a minimal ladder logic block that reads gross weight, applies a linearization, and publishes it to the process image for the S7-300 CPU.

// STEP 7 STL excerpt - read SIWAREX gross weight from PIW 288
      L     PIW 288         // raw 16-bit SIWAREX gross weight
      ITD                    // convert to double integer
      DTR                    // convert to REAL (engineering units)
      L     2.764810e+02    // calibration factor (counts per kg)
      /R                     // divide -> kg
      T     DB10.DBD0       // store in "ProcessWeight" DB

In TIA Portal on a newer S7-300 (or S7-1500 with SIWAREX WP321 on PROFINET), the Siemens "SIWAREX" library provides ready-made function blocks (FB250 "SIWA") that wrap the calibration, filtering, and zero-tracking logic. Engineers should not reimplement tare, span, and drift compensation from scratch — the library handles corner cases such as the digital filter settling time after a zero command.

For the SM 335 (6ES7 335-7HG01-0AB0), configuration is performed through HW Config (STEP 7) by inserting the module in the rack and assigning it to an analog input address range; no vendor library is required because the inputs are raw voltage / current values.

Calibration Procedure

Every SIWAREX and TSXISPY101 deployment must complete the following three-step calibration. Skipping any step invalidates the weighing accuracy certificate.

1. Zero calibration — with the load cell unloaded, execute the "Calibrate Zero" command through the vendor configuration tool. The module stores the offset in non-volatile memory.
2. Span calibration — apply a known reference weight (typically 80 % of the load cell nominal capacity, NIST-traceable), enter the reference mass, and execute "Calibrate Span." The module computes the gain factor.
3. Verification — apply 0 %, 25 %, 50 %, 75 %, 100 % of nominal mass and confirm each reading is within the specified accuracy class. Repeatability across three cycles should be within the load cell's combined error specification (typically ≤ 0.02 % C…O for precision cells).

For the SM 335 / 1746-NI4 path with an external amplifier, calibration is performed at the amplifier front end, not the PLC. The PLC sees a 0–10 V or 4–20 mA signal proportional to weight; the calibration factors live in the PLC scaling code rather than in a vendor-validated block.

Selection Decision Matrix

Common Pitfalls and Field-Engineering Notes

1. Confusing sampling rate with effective throughput. A 50 Hz SIWAREX module samples one cell 50 times per second. Ten cells in a single CPU do not yield 500 Hz per cell — they share the 50 Hz sample slot. The aggregate of 60 conversions / second in the application profile refers to one conversion per channel, which the 50 Hz architecture satisfies easily.
2. Forgetting the junction box current budget. SIWAREX U supplies up to 150 mA at 10 V. Four 350 Ω cells in parallel draw 4 × 28.6 mA = 114 mA — within spec. Six cells in parallel exceed the budget; reduce to four cells per SIWAREX U or move to SIWAREX WP321.
3. Using the SM 335 for raw bridge inputs. The 14-bit resolution across a ±10 V input gives 1.22 mV per LSB; a 20 mV full-scale bridge signal uses only 16 LSBs total. The result is unusable weighing accuracy.
4. Long cable runs without sense lines. A 100 m run of 24 AWG copper is approximately 8 Ω per conductor. In a 4-wire cell this introduces 16 Ω × 28.6 mA = 0.46 V drop on the excitation line, which directly becomes a 4.6 % scale error. Use 6-wire cells or shorten the cable.
5. Mixing isolation grounds. SIWAREX and SM 335 share a common backplane ground. Do not float the PLC chassis relative to the load cell structure ground — the resulting CM voltage will saturate the input amplifier.
6. Neglecting mechanical filter dynamics. A 5–6 Hz update is faster than many mechanical structures can respond. The reading will appear noisy because the structure itself is oscillating. Implement a digital low-pass filter with cutoff below the first structural mode.

Recommended Bill of Materials (Reference Application)

For ten channels of 5–6 Hz load cell weighing on an S7-300, the minimum BOM:

• 1 × S7-300 CPU (e.g., 6ES7 315-2EH14-0AB0) with STEP 7 V5.5 or TIA Portal V16+
• 10 × SIWAREX U (7MH4601-1AA01) — one per cell for full channel independence, OR
• 3 × SIWAREX WP321 (7MH4908-1AA01) on ET 200SP distributed I/O with PROFINET, using junction boxes to parallel up to 4 cells per module
• 3 × SIWAREX JB junction boxes (or compatible) for 6-wire bridge distribution
• 10 × 4-conductor shielded cable (Belden 8761 or equivalent), one per cell, terminated to the front connector
• 1 × SIWATOOL V7 configuration cable (7MH4601-1AA01 / USB or RS-232 depending on the variant)

Verification Checklist

After commissioning, complete the following checks before releasing the system to production:

1. Confirm SIWAREX "Operating Mode" LED is steady green; a flashing LED indicates parameterization or calibration error.
2. Apply a calibrated 80 % reference weight to each cell and verify reading is within ±0.05 % of nominal.
3. Capture 1 000 raw samples at idle; standard deviation should be < 0.01 % of full scale with the digital filter engaged.
4. Disconnect one excitation lead and verify the module flags a "Sense Lead Open" diagnostic bit in the process image — confirms fault detection is functional.
5. Repeat accuracy check after 24 hours of thermal soak to verify drift; expected < 0.02 % over 10 °C ambient swing.

FAQ