Allen-Bradley 1756-SRM ControlLogix System Redundancy Module

The Allen-Bradley 1756-SRM is a ControlLogix System Redundancy Module used to synchronize primary and secondary ControlLogix controllers in redundant automation architectures. Installed as a matched pair across redundant chassis, the module manages redundancy communications through dedicated fiber-optic links, enabling automatic switchover between controllers while maintaining identical hardware configurations.

Technical Specifications

Redundant Controller Architecture

The 1756-SRM is designed for ControlLogix redundant control systems that use a primary chassis and a synchronized secondary chassis. Each chassis requires an identical redundancy module installation and matching slot assignments to maintain synchronization.

The module continuously exchanges controller and system status information between the redundant chassis. If the primary controller becomes unavailable, the redundant system can transfer control to the secondary chassis according to the configured redundancy strategy.

Fiber-Optic Synchronization

Communication between redundancy modules is established through duplex fiber-optic connections. The module uses dedicated 1757-SRC series redundancy cables available in multiple lengths, allowing physical separation between primary and secondary systems while maintaining high-speed synchronization.

The integrated fiber-optic ports are dedicated to redundancy communication and are separate from standard ControlLogix network communications.

Supported ControlLogix Platforms

The 1756-SRM is intended for legacy ControlLogix redundancy systems using compatible controllers and communication modules.

Confirmed compatible controllers include:

• ControlLogix5555 (1756-L55 series)

• ControlLogix5560 (1756-L61, 1756-L62, 1756-L63)

Supported network modules include:

• 1756-ENBT EtherNet/IP bridge

• 1756-CNB ControlNet bridge

• 1756-CNBR Redundant ControlNet bridge

Compatibility should always be verified against the installed controller firmware, redundancy revision, and system architecture before replacement or expansion.

System Design Requirements

A key design requirement of the 1756-SRM redundancy architecture is that the primary and secondary chassis must use identical slot configurations. In addition, local I/O modules are not permitted within the synchronized redundant chassis structure.

Instead, field signals are transferred through remote I/O networks, allowing both redundant controllers to maintain a consistent view of system operation while supporting deterministic redundancy behavior.

Hardware Integration

The module draws operating power directly from the ControlLogix chassis backplane and does not require a separate external power source. A built-in primary status relay provides an additional method for external indication of redundancy state and controller ownership.

Because redundancy performance depends on both chassis remaining synchronized, hardware replacement should be performed using matching module revisions and approved redundancy components.

Frequently Asked Questions

Q: How many chassis slots does the 1756-SRM require?

A: The module occupies two chassis slots. The slot arrangement must be identical in both the primary and secondary redundant chassis.

Q: What communication method is used between redundant systems?

A: The module uses duplex fiber-optic communication through dedicated 1757-SRC redundancy cables available in lengths from 1 m to 100 m.

Q: Can local I/O modules be installed in a 1756-SRM redundant chassis?

A: No. The documented redundancy architecture requires system I/O to be implemented through remote I/O networks rather than local chassis I/O modules.

Q: Which ControlLogix controllers are supported?

A: Confirmed compatible processors include the ControlLogix5555 (1756-L55 series) and ControlLogix5560 controllers including 1756-L61, 1756-L62, and 1756-L63 models.