Siemens 7MH4900-3AA01 Weighing Module for SIMATIC S7-1200 and ET200SP Systems

Product Overview

The Siemens 7MH4900-3AA01 is a weighing module designed for use with SIMATIC S7-1200 PLCs and ET200SP distributed I/O systems. This module, part of the Siwarex WP321 series, interfaces directly with strain gauge load cells to provide high-precision weight measurement for industrial process and batch control applications. The module supports up to four 6-wire or 4-wire load cells in parallel, with a 24-bit sigma-delta analog-to-digital converter delivering accurate weight readings at selectable update rates. The 7MH4900-3AA01 integrates seamlessly into the PLC's I/O bus, making weight data available as standard process values for use in control logic, data logging, and operator displays.

Key Features of Siemens 7MH4900-3AA01

The weighing module features a high-resolution measurement system with internal resolution of up to 16 million counts, providing outstanding accuracy for applications ranging from checkweighing to tank level measurement. The module supports multiple weighing modes including gross weight, net weight, and tare functions, with configurable digital filters to suppress vibration and noise. The 7MH4900-3AA01 includes two digital inputs and two digital outputs, which can be used for limit value monitoring, setpoint control, or as general purpose I/O. The module is configurable using the SIMATIC Step 7 software with the dedicated Siwarex library, which provides function blocks for easy integration into the PLC program.

The module includes automatic calibration via the Siwarex software, eliminating the need for manual potentiometer adjustments. It supports both theoretical calibration (using load cell data sheet values) and practical calibration (using known test weights). The device provides comprehensive diagnostic information including load cell signal quality, temperature compensation, and supply voltage monitoring. The 7MH4900-3AA01 is designed for DIN rail mounting and connects to load cells via a shielded cable terminated on a plug-in screw terminal block. The module is electrically isolated between the load cell input and the PLC backplane, preventing ground loops from affecting measurement accuracy. The product complies with legal-for-trade standards in many countries when used with approved load cells and software settings.

Technical Specifications

• Manufacturer: Siemens AG

• Part Number: 7MH4900-3AA01

• Series: Siwarex WP321

• Product Type: Weighing Module

• Compatible Systems: SIMATIC S7-1200, ET200SP

• Number of Load Cells: Up to 4 in parallel (6-wire or 4-wire)

• Measurement Resolution: 24-bit sigma-delta

• Internal Resolution: Up to 16,000,000 counts

• Weighing Modes: Gross, net, tare

• Digital Inputs: 2 (24 V DC)

• Digital Outputs: 2 (24 V DC, 0.5 A)

• Update Rate: Selectable (50, 100, 200, 400, 800 Hz)

• Configuration Software: Step 7 with Siwarex library

• Calibration: Theoretical or practical via software

• Isolation: Between load cell input and backplane

• Power Supply: 24 V DC (from backplane or external)

• Terminal Type: Plug-in screw terminal

• Mounting: DIN rail

• Operating Temperature: -20 to +60 °C

• Degree of Protection: IP20

Functional Description

The 7MH4900-3AA01 weighing module excites the load cells with a precision voltage supply and measures the resulting millivolt signal proportional to the applied weight. The sigma-delta ADC converts the analog signal to a digital value with programmable gain and filtering. The module applies internal linearization and temperature compensation algorithms to produce a stable weight reading. The digital filtering options allow the user to balance noise rejection against response time: higher update rates with minimal filtering are suitable for fast checkweighing applications, while lower rates with aggressive filtering are better for tank weighing subject to vibration. The tare function subtracts the weight of an empty container; the net weight is the gross weight minus the tare. The digital outputs can be configured to switch at user-defined setpoints (e.g., high weight alarm, filling cutoff), reducing PLC program load. The module continuously monitors the load cell signal integrity; if a load cell fails or the cable is disconnected, the module sets a diagnostic bit and the measured weight goes to a defined error value, preventing erroneous control actions.

Applications

The Siemens 7MH4900-3AA01 weighing module is used in industrial process and packaging applications where accurate weight measurement is required. Typical uses include batching and blending systems (cement, plastics, chemicals), checkweighers on packaging lines, tank and hopper level measurement, filling machines for bottles or bags, and dynamic weighing on conveyors. The module's integration with S7-1200 PLCs allows weight data to be used directly in PID loops, batch sequencing, and production reporting. For machine builders, the Siwarex module provides a certified weighing solution that meets legal metrology requirements for trade applications.