Name: Siemens 7MH4950-1AA01 | Universal Weighing Module | SIMATIC S7-300 & ET 200M System | runsheng
Brand: Other

Description

Key Technical Specifications

Product Model: 7MH4950-1AA01
Manufacturer: Siemens
System Platform: SIMATIC S7-300
Number of Channels: 1 Measuring Channel
Module Width: 40 mm
Load Cell Compatibility: Strain gauges (DMS) with full bridge (1 to 4 mV/V)
Internal Resolution: 65,535 parts (16-bit)
Update Rate: 50 Hz (Internal processing)
Communication Interface: RS-232 (TTY) for PC adjustment (SIWATOOL)
Power Supply: 24V DC (from external supply or backplane)
Current Consumption: Typ. 150 mA
Cable Length (Max): 500m (using verified 6-wire technique and junction box)
Measuring Accuracy: 0.05%
Ex Approval: ATEX Zone 2 (requires specific enclosure conditions)
Connection Method: 20-pin Front Connector (Screw or Spring-loaded)

Technical Architecture & Functional Positioning

Architecture Analysis

The SIWAREX U (Universal) module represents a direct integration strategy for industrial weighing. Unlike standalone scale terminals that communicate with a PLC via slow fieldbus gateways, the 7MH4950-1AA01 sits directly on the SIMATIC S7-300 backplane (or ET 200M remote I/O station).

Technically, the module functions as a high-precision analog-to-digital converter specialized for the millivolt signals generated by strain gauge load cells. It provides a highly stable 6V DC excitation voltage to the load cells and measures the return signal ratio-metrically. This architecture eliminates the need for external transmitters and analog input cards, reducing signal noise susceptibility and cabling complexity. The module processes the weight values locally (filtering and scaling) before passing the net/gross weight data to the CPU via the P-bus, ensuring that the main PLC processor is not burdened with high-speed A/D conversion tasks.

From an engineering perspective, this design solves the “black box” problem. Since the weighing data is mapped directly into the PLC’s process image, engineers have total access to raw counts and status bits, facilitating tighter integration between the weighing process and the control logic (e.g., closing a valve precisely when a target weight is reached).

Typical Application Scenarios

Silo and Bunker Level Monitoring: Used for gravimetric level measurement where non-contact sensors (ultrasonic/radar) fail due to dust or angle of repose issues.
Platform Scales: Integration of floor scales directly into the manufacturing automation system for inventory tracking.
Crane Load Monitoring: Safety monitoring of lifted loads to prevent overload conditions in overhead cranes.
Force Measurement: Monitoring tension in web processing (paper/steel) or press-fit force monitoring in assembly lines.
Hazardous Area Weighing: Through the use of Ex-interface barriers (IS), the module can measure load cells located in hazardous zones (Zone 1/21) while remaining in the safe zone.

Quality Standards & Testing Procedures

Quality Philosophy

In process weighing, drift or non-linearity translates directly to lost product or quality deviations. A weighing module that fluctuates by even 0.1% can cause significant inventory discrepancies over time. Therefore, our testing protocol focuses heavily on signal stability and linearity verification.

Standardized Testing Protocol (SOP)

Visual Inspection & Terminal Check

We inspect the 20-pin front connector interface for any spread pins or oxidation. The shielding contact element (critical for noise immunity) is checked for mechanical integrity. The PCB is cleaned to remove conductive dust that could affect the high-impedance amplifier circuits.

Precision Bridge Simulation

We do not rely solely on connecting a standard load cell. Instead, we use a Precision Strain Gauge Simulator (Calibrator) to inject exact millivolt signals corresponding to 0%, 50%, and 100% load.

Zero Point Stability: Verifying the reading remains stable at 0 mV input without drift.
Linearity Verification: We apply signals in 0.5 mV/V increments (e.g., 1mV/V, 2mV/V) to ensure the digital output scales perfectly linearly across the full range.
Excitation Voltage Check: We measure the excitation output at the terminals. It must remain stable at 6V DC even under maximum load (driving multiple load cells parallel, up to 87 Ω total resistance).

Communication & Parameter Test

We connect the module to a test S7-300 rack and establish communication using SIWATOOL software via the RS-232 port and the PLC backplane. We verify that parameter downloads (e.g., filter settings, characteristic values) are stored correctly in the non-volatile memory.

24-Hour Stability Run

The module is kept powered with a static input signal for 24 hours to ensure no thermal drift occurs in the A/D converter after warm-up.

Compatibility & Installation Maintenance Guide

Engineering Notes

Installing SIWAREX modules requires more attention to cabling detail than standard digital I/O. The signals involved are in the microvolt range, making them highly susceptible to electromagnetic interference (EMI).

Pre-Replacement Checklist

Cabling Configuration (4-wire vs. 6-wire): Check if your installation uses 6-wire load cells (with Sense lines). If using 4-wire load cells, you must bridge the Excitation (+) to Sense (+) and Excitation (-) to Sense (-) at the terminal block. Failing to do this will result in a “Wire Break” error or wildly incorrect readings.
Shielding is Non-Negotiable: The load cell cable shield must be grounded using the specific shield connection element on the front connector. Do not just use a pigtail wire to ground; the shield must have large-area contact.
Potential Equalization: Ensure that the load cells and the S7-300 rack are at the same ground potential. Potential differences can flow through the shield, causing measurement errors or module damage.

Electrostatic Discharge (ESD) Protection

The input channels connect directly to sensitive instrumentation amplifiers. Always use a grounded wrist strap when handling the module or wiring the front connector. Static discharge into the measurement terminals can permanently offset the calibration.

System Compatibility Verification

The 7MH4950-1AA01 is compatible with the vast majority of S7-300 CPUs. However, ensure your hardware configuration (HW Config in Step 7 or TIA Portal) matches the physical module exactly. If you replace a 2-channel module (2AA01) with this 1-channel module, the hardware configuration must be updated.

Our Service & Commitment

We provide specialized support for the Siemens weighing ecosystem. We understand that SIWAREX modules are often critical control points in batching and inventory systems.

Our team consists of automation engineers familiar with both the PLC logic and the physics of weighing. We supply genuine, tested Siemens modules backed by a 12-month functional warranty. Every unit we ship has passed the simulator tests described above, ensuring it is ready for immediate calibration upon installation.

We offer global shipping options and can assist with technical questions regarding migration from older S7-300 systems to newer platforms if necessary. Whether you need a single replacement module to get a silo back online or spare parts for stock, we ensure quality and speed.

Global Supply & Technical Support

Global Automation Distribution

RUNSHENG Automation connects industrial users worldwide with critical spare parts for the SIMATIC S7-300 series. We support customers in Europe, the Americas, Asia, and the Middle East, providing a reliable source for legacy and active Siemens components.

Quality Assurance & Logistics

All SIWAREX modules undergo our strict SOP before dispatch. We package items in anti-static shielding and reinforced boxes to withstand international transit. We provide tracking numbers immediately upon shipment and support various Incoterms to suit your procurement requirements.

Expert Support

Our technical team can guide you through the troubleshooting process. If you are unsure whether the fault lies with the module or the load cells, we can provide guidance on how to measure the load cell resistance and signal voltage to isolate the issue before you purchase a replacement.