The ABB RLM01 PROFIBUS DP/FMS Redundancy Link Module is a high-reliability communication redundancy device designed for industrial automation control systems. This module is primarily used to convert a simple, non-redundant PROFIBUS line into two mutually redundant lines (A and B), significantly enhancing the reliability and fault tolerance of fieldbus communication. The RLM01 is suitable for applications with stringent requirements for system continuity, such as power, chemical, marine, maritime systems, and various process automation control fields.
The RLM01 supports PROFIBUS DP and FMS protocols, with transmission rates ranging from 9.6 kBit/s to 12 MBit/s. It features automatic line selection, communication status monitoring, repeater functionality, redundant power supply support, and fault alarm capabilities. The compact module is designed for easy installation on standard DIN rails and is suitable for various industrial environments.
2. Key Features
| Feature Category | Specific Function Description |
| Redundancy Conversion | Converts one non-redundant PROFIBUS line (M) into two redundant lines (A/B), with bidirectional data transmission |
| Communication Protocol | Supports PROFIBUS DP/FMS, compliant with DIN 19245 standard |
| Transmission Rate | 9.6 kBit/s to 12 MBit/s, configurable via rotary switch |
| Line Selection | Automatically detects and selects available lines, supports failover |
| Monitoring Function | Real-time monitoring of communication status and errors on all three lines (A, B, M) |
| Repeater Function | Signal regeneration capability, supports long-distance communication extension |
| Power Supply Design | Supports single or redundant 24 VDC power supply, with power monitoring |
| Alarm Output | Provides potential-free alarm contact for remote fault diagnosis |
| Installation Method | Standard DIN rail mounting, suitable for industrial cabinets |
| Environmental Adaptability | Complies with industrial environment standards, supports marine and maritime applications (GL certified) |
3. Functional Description
The RLM01 module features three independent RS 485 interfaces (A, B, M), each supporting full-duplex communication. The module provides electrical isolation between the three interfaces and between the interfaces and the power supply, meeting the VDE 0110 functional isolation standard with a test voltage of 500 Veff.
The module incorporates intelligent line switching logic:
When data is received from line A or B with a correct telegram start, it is routed to line M;
If line A experiences a telegram start error, the control logic automatically switches to line B;
Data received from line M is simultaneously transmitted to both lines A and B, achieving broadcast-style redundant output.
The RLM01 also includes repeater functionality, which reshapes and amplifies received signals, supporting longer-distance PROFIBUS network extension. Each interface can connect up to 31 PROFIBUS stations. When used with repeaters and fiber optic converters, the network scale can be further expanded.
4. Installation and Configuration
Safety Notes
Must be powered by safety extra-low voltage (24 VDC); alarm contact power must also comply;
According to UL/CSA standards, external protective devices (e.g., fuses, rated 1.6A–10A) are required;
Installation location must ensure good heat dissipation, with >30 mm clearance around the module;
Do not connect bus lines that are partially or entirely installed outdoors directly to the module; use fiber optics and optical-electrical converters for electrical isolation.
Mechanical Installation
The module is designed for vertical mounting on a 35 mm DIN rail. To install, first place the rear clips on the top edge of the rail, then gently press the bottom of the module until the clips lock audibly. Maintain at least 30 mm clearance above and below the module for heat dissipation.
Electrical Connections
PROFIBUS Interfaces: A, B, M are all 9-pin Sub-D female connectors, with pin assignments compliant with PROFIBUS standard:
Pins 3, 8: Data positive/negative (RxD/TxD)
Pins 5, 6: External termination resistor power (DGND, VP)
Power and Alarm Terminals: Use an 8-pin pluggable terminal block for quick replacement:
| Terminal | Description | Function |
| 1 | E | Protective Earth (PE) |
| 2 | M | Power ground (single supply) |
| 3 | L1+ | +24 V power positive |
| 4 | L1+ | Jumper to L2+ when using single supply |
| 5 | M | Redundant power ground |
| 6 | L2+ | Redundant power positive |
| 7 | F2 | Potential-free alarm contact (normally closed) |
| 8 | F1 | Potential-free alarm contact (normally closed) |
Baud Rate Setting
Set the baud rate using the front panel rotary switch, with 10 positions (0–9) as follows:
| Switch Position | Baud Rate | Unit |
| 0 | 12 | MBit/s |
| 1 | 6 | MBit/s |
| 2 | 3 | MBit/s |
| 3 | 1.5 | MBit/s |
| 4 | 500 | kBit/s |
| 5 | 187.5 | kBit/s |
| 6 | 93.75 | kBit/s |
| 7 | 19.2 | kBit/s |
| 8 | 9.6 | kBit/s |
| 9 | No function | – |
After setting, briefly press the reset button to activate the configuration.
5. Monitoring and Diagnostics
The RLM01 front panel features multi-color LED indicators for communication and power status:
Communication Status LEDs (Act/Err)
Power LED (Pwr)
Green Pwr: Power normal
Off: Power abnormal
Alarm Contact
The module provides a set of potential-free normally closed contacts (F1–F2), which remain closed during normal operation and open in case of communication or power failure, allowing connection to PLC or supervisory systems for remote diagnostics.
Typical Status Indications
| Line A Act/Err | Line B Act/Err | Alarm Contact | Meaning |
| Yellow/Off | Yellow/Off | Closed | Normal communication on all three lines |
| Yellow/Off | Off/Red | Open | Normal communication on A & M only; B faulty |
| Off/Red | Yellow/Off | Open | Normal communication on B & M only; A faulty |
| Yellow/Yellow | Off/Off | Closed | Normal communication on A & B only; M faulty |
6. Application Scenarios
Industrial Process Control: Used in chemical, power, water treatment systems requiring high communication reliability;
Marine & Maritime Systems: GL certified, suitable for ship automation control systems;
Production Line Automation: Supports PROFIBUS DP network redundancy, improves equipment availability;
Energy Management: Suitable for substations, power plants, and other critical infrastructure;
Transportation Automation: Such as tunnel control, railway signaling systems.
