Siemens 7MH4900-3AA01 Siwarex Weighing Module

The 7MH4900-3AA01 is a weighing module from Siemens' Siwarex product line, designed for integration with S7-300 PLC systems. It interfaces with up to four load cells (strain gauge type, 350 Ω typical) to provide precise weight measurement for industrial scales, hoppers, and tanks. The module features 24-bit resolution and a sampling rate up to 1000 Hz, enabling accurate batching, filling, and check-weighing applications. It communicates with the PLC via the S7 backplane bus and can be configured using SIMATIC Step 7 or TIA Portal.

Technical Specifications

• Number of Load Cell Inputs: 1 (4-wire or 6-wire), with excitation voltage 5 VDC

• Measurement Range: ±20 mV (typical for 2 mV/V load cells)

• Resolution: 24-bit (approx. 0.03 µV per digit)

• Sampling Rate: 50 to 1000 Hz (selectable)

• Linearity Error:<0.01% of full scale

• Temperature Drift: ±1 ppm/K

• Internal Digital Filter: Low-pass, configurable from 0.1 to 50 Hz

Functions and Features

The module includes built-in functions for tare, zero, gross/net weighing, and material flow rate calculation. It also supports automatic drift compensation (on periodic intervals) and limit monitoring (high/low alarms). The module can perform a calibration using known weights or electronically using the load cell's sensitivity data. It has an integrated EEPROM for storing calibration parameters, retaining them even after power-off.

Integration with S7-300

The 7MH4900-3AA01 plugs into a standard S7-300 backplane slot and is addressed via the PLC's I/O area. The module provides weight values in engineering units (kg, lb, etc.) via process data exchange. Up to 8 weighing modules can be installed in a single S7-300 rack. The module supports diagnostics via the PLC's system status list and has an LED for power and communication status.

Physical and Environmental

Dimensions: 40 mm × 125 mm × 120 mm (W × H × D). Weight: 0.2 kg. Power consumption: 150 mA at 5 VDC from backplane, plus external 24 VDC (200 mA) for load cell excitation. Operating temperature: 0–60°C; storage: -20–80°C; humidity: 10–90% non-condensing. Protection class: IP20. The module has a screw terminal block for load cell connection (accepts 18–24 AWG).

Applications

Used in batch mixing plants, filling machines, checkweighers, and hopper level monitoring. The module is compatible with most strain gauge load cells (2–3 mV/V sensitivity). For multi-scale applications, multiple modules can be used. The 7MH4900-3AA01 is a direct replacement for older 7MH4900-3AA00 with improved accuracy and faster sampling.