A-B 1756-RM2 Redundancy Module

Allen-Bradley 1756-RM2 ControlLogix Redundancy Module

 

 

The Allen-Bradley 1756-RM2 is a ControlLogix Redundancy Enhanced Module used to build high-availability (hot standby) PLC systems in Rockwell ControlLogix architectures.

It is NOT a CPU and NOT an I/O module—it is a system synchronization and switchover module that allows two PLC chassis to operate as a redundant pair.

Product Information

• Brand: Allen-Bradley (Rockwell Automation)
• Model: 1756-RM2
• Series: ControlLogix 5570 / redundancy system
• Type: Redundancy module (Enhanced)
• System use: Dual-chassis hot standby architecture
• Connection: Fiber-optic redundancy cable (1756-RMC1 / RMC3 / RMC10)

What it actually does

The RM2 module is responsible for:

• Synchronizing Primary PLC chassis ↔ Secondary PLC chassis
• Copying controller memory and system state in real time
• Monitoring system health of both racks
• Triggering automatic switchover if primary fails
• Managing redundancy communication via fiber link

System Role (Simple)

Primary chassis (PLC + I/O + network)
↕ (fiber sync via RM2)
Secondary chassis (standby copy)

If primary fails → secondary takes over in milliseconds

Key Features

• High-speed redundancy synchronization (up to ~1 Gbps class performance)
• Fast switchover (typically tens of milliseconds depending configuration)
• Fiber-optic link between chassis
• Supports ControlLogix 5570 redundancy architecture (L7x series commonly used)
• SIL2-capable redundancy architecture support (system dependent)
• Plug-in module inside 1756 chassis

Typical System Requirements

A full redundancy system usually needs:

• 2 × ControlLogix chassis (identical hardware)
• 2 × PLC CPUs (e.g., 1756-L71 / L73 / L7x series)
• 2 × 1756-RM2 modules (one per rack)
• Fiber cable: 1756-RMC1 / RMC3 / RMC10
• Matching communication modules (EtherNet/IP / ControlNet)

Typical Applications

• Oil & gas production systems
• Power generation control systems
• Chemical process plants
• Water treatment facilities
• Continuous manufacturing lines
• Any system requiring no downtime (high availability)

Advantages

• Eliminates single PLC failure risk
• Enables hot standby switchover
• Keeps process running during controller failure
• Industrial-grade reliability for 24/7 operations
• Integrated into Rockwell ControlLogix ecosystem

Important Notes

• Not PLC CPU ❌
• Not I/O module ❌
• Not power supply ❌
• Not communication module ❌
• It is a system redundancy synchronization module ✔

Precautions (very important for real systems)

• Both PLC chassis must be identical configuration
• Firmware versions must be compatible
• Fiber cable must be correctly paired (CH1–CH1 / CH2–CH2)
• Network topology mistakes can break sync (very common issue)
• GuardLogix safety systems are not compatible with redundancy mode
• Misconfiguration can cause secondary “disqualified” status