ABB IEMMU21 Module Mounting Unit (MMU)

Overview

The ABB IEMMU21 is a Module Mounting Unit (MMU) used in ABB Bailey INFI 90 / Symphony / Harmony distributed control systems (DCS). It is not a controller or I/O module itself, but a rack and backplane infrastructure unit that provides mechanical mounting, power distribution, and internal communication paths for multiple system modules.

In simple terms, it serves as the core chassis platform that allows ABB control and I/O modules to operate together inside a standardized industrial cabinet.

Main Function

The IEMMU21 is responsible for:

• Physically housing ABB INFI 90 system modules
• Providing backplane communication (Controlway / module bus)
• Distributing regulated DC power to installed modules
• Supporting system expansion in modular DCS architecture
• Ensuring stable electrical and mechanical integration of all cards

It is essential for building a complete ABB Bailey control rack.

Technical Specifications

Based on typical ABB Bailey system documentation:

• Manufacturer: ABB (Bailey / INFI 90 systems)
• Model: IEMMU21
• Product Type: Module Mounting Unit (Rack / Chassis)
• Module Capacity: Up to 12 system modules
• Mounting Type: Standard 19-inch industrial cabinet installation
• Backplane Functions: Module bus, I/O expander bus, Controlway signals
• Power Distribution: +5 VDC, ±15 VDC, +24 VDC regulated outputs
• Operating Environment: Industrial control room / cabinet environment
• System Compatibility: INFI 90, Symphony, Harmony platforms

Typical environmental ranges (industry standard for this unit class):

• Operating temperature: ~0°C to 70°C
• Storage temperature: -40°C to 85°C
• Humidity: up to 95% non-condensing

Structure and Design

The IEMMU21 is built as a rack-mounted chassis system, typically including:

• Module slots for up to 12 plug-in cards
• Backplane circuit board for signal routing
• Power distribution section for regulated voltages
• Connector interfaces for system communication buses
• Mechanical guide rails for secure module insertion
• Cabinet mounting frame (front or rear mount options depending on configuration)

This modular structure allows fast replacement of individual cards without shutting down the entire system in some configurations.

Working Principle

The IEMMU21 itself does not perform control logic. Instead, it functions as the foundation layer of the control system.

Its operation can be described as:

1. System modules (I/O, control, communication cards) are inserted into slots
2. The backplane connects all modules electrically and logically
3. Power supply is distributed to each module through regulated rails
4. Communication signals (Controlway / module bus) are shared across modules
5. The controller modules coordinate process control through this shared infrastructure

This architecture enables a highly scalable distributed control system.

Applications

The ABB IEMMU21 is used in industrial plants requiring high-reliability DCS systems, such as:

• Power generation plants (steam, gas, hydro)
• Oil and gas facilities
• Petrochemical and refinery systems
• Water and wastewater treatment plants
• Heavy manufacturing industries
• Boiler and turbine control systems
• Process automation systems using ABB Bailey INFI 90

It is especially common in legacy ABB DCS installations still operating worldwide.

Installation Guidelines

To ensure stable operation:

• Install in a standard 19-inch industrial control cabinet
• Ensure proper grounding of the rack system
• Maintain adequate airflow for heat dissipation
• Insert modules only in powered-down conditions unless system supports hot-swap
• Verify correct backplane alignment before module insertion
• Follow ABB INFI 90 installation standards strictly

Maintenance Recommendations

Although the IEMMU21 is a passive hardware unit, maintenance is still important:

• Inspect backplane connectors for wear or corrosion
• Ensure modules are firmly seated in slots
• Keep cabinet free from dust and moisture
• Check power supply stability regularly
• Replace damaged chassis rails or connectors if mechanical wear appears
• Periodically verify system communication integrity

Advantages

• Provides standardized modular DCS architecture
• Supports up to 12 control and I/O modules
• Stable power and communication distribution
• High reliability for continuous industrial operation
• Easy expansion and system scalability
• Long lifecycle design suitable for legacy ABB systems
• Simplifies maintenance and module replacement

Conclusion

The ABB IEMMU21 Module Mounting Unit is a foundational rack component used in ABB Bailey INFI 90 distributed control systems. It provides the physical structure, power distribution, and communication backplane required for multiple industrial control modules to operate as a unified system.