Siemens 7MH4900-3AA01 Siwarex Weighing Module

The Siemens 7MH4900-3AA01, part of the Siwarex series, is a weighing module designed for the SIMATIC S7-300 PLC system. It connects directly to strain gauge load cells, providing high-precision weight measurement for industrial scales, batching systems, and filling machines. The module supports up to four load cells or one multi-cell scale per channel.

Measurement Specifications

The module has four channels, each capable of sampling at up to 800 Hz. Resolution is 16 bits (internal 24-bit ADC). Accuracy class: 0.05% of full scale. Input sensitivity: 0.5 µV per digit. The module can be calibrated using a known reference weight or via electronic calibration (using stored sensor data). It supports both 4-wire and 6-wire load cells, with remote sensing to compensate for cable resistance.

Excitation and Input Range

Excitation voltage: 10V DC (max 120 mA total). Bridge resistance range: 40 Ω to 4000 Ω. Signal input range: ±30 mV. The module includes a built-in anti-aliasing filter and selectable 50/60 Hz digital filter. Maximum cable length for load cells is 300 meters (6-wire configuration).

Interfaces and Integration

The 7MH4900-3AA01 communicates with the S7-300 via the backplane bus. It is configured using the Siwarex DMS software integrated into STEP 7. The module provides dedicated function blocks for tare, zero, gross/net weight, and limit monitoring. It also supports RS-232 serial communication for external printers or displays (via an optional adapter).

Diagnostic and Status

LED indicators per channel: green for valid weight, red for overload/underload, yellow for calibration error. The module detects wire break, short circuit, and exceeding of measuring range. A diagnostic buffer logs the last 10 events. The module also performs drift compensation based on internal temperature sensor.

Power and Environmental

Power consumption: 100 mA from 5V backplane, 120 mA from 24V field supply. Operating temperature: 0°C to 55°C; storage: -40°C to 70°C. Dimensions: 40 x 125 x 120 mm (W x H x D). The module is rated IP20 and mounts on a DIN rail. Terminals are screw-type, removable.

Applications

Used in hopper scales, belt weighers, checkweighers, and batch mixing processes. Typical industries include food and beverage, chemical, and cement production.

Installation Notes

Use shielded, twisted-pair cables for load cell wiring. Connect shield to the ground terminal on the module. Avoid parallel routing with power cables to prevent interference. Calibration should be performed after installation with the scale fully assembled.

Conclusion

The Siemens 7MH4900-3AA01 Siwarex weighing module brings four channels of high-accuracy weight measurement to S7-300 systems. Its flexible calibration, diagnostic features, and seamless PLC integration suit many industrial weighing applications.