Product Overview

The Allen-Bradley 1785-BEM is a specialized Backup Communication Expansion Module designed for high-availability PLC-5® control systems. This module is engineered to provide a robust hardware link between primary and secondary processors, facilitating rapid data synchronization and seamless failover capabilities. By ensuring that the standby processor's data table is a mirror image of the active unit, the 1785-BEM eliminates process interruptions and prevents data loss during critical system transitions.

Occupying a single slot in the 1771 I/O chassis, the 1785-BEM operates as a high-speed bridge for processor-to-processor communication. It is a vital component for mission-critical industrial applications where system downtime can lead to significant production losses or safety hazards. This module allows the backup processor to monitor the health of the primary unit and take command of the I/O racks instantly should a fault occur.

Technical Performance and Connectivity

The 1785-BEM is optimized for high-speed data throughput, supporting communication rates up to 1.2 Mega Baud. It utilizes the 1771-CD cable connector to interface with Data Highway Plus (DH+) and Remote I/O (RIO) networks, ensuring that the redundancy link does not become a bottleneck in the control architecture.

• Processor Interface: Supports 32-bit input transfers to the processor image table for enhanced data handling.

• Configurability: Features a four-position DIP switch assembly, allowing technicians to easily define module behavior and network settings without complex software overhead.

• Field Integration: Equipped with a dedicated field wiring arm (supporting 26-14 AWG wire) for secure signal connections.

• Integrated Relay: Includes a factory-installed 0.25A relay at 24V DC to provide hardware-level status signaling or interlock functionality.

Technical Specifications

• Manufacturer: Allen-Bradley (Rockwell Automation)

• Model Number: 1785-BEM

• Module Type: Backup Communication Expansion

• Compatible System: PLC-5 (1771 Universal I/O)

• Backplane Current Draw: 0.6A @ 5V DC

• Communication Speed: Up to 1.2 Mbps

• Network Compatibility: DH+ (Data Highway Plus) and RIO (Remote I/O)

• Cable Interface: 1771-CD Connector

• Operating Temperature: 0°C to 60°C (32°F to 140°F)

• Storage Temperature: -40°C to 85°C (-40°F to 185°F)

• Certifications: CE, UL, CSA

• Weight: 0.52 kg

• Dimensions: 3 x 14.7 x 26 cm

Reliability and System Safety

• Hot-Backup Support: Facilitates real-time synchronization of the processor's memory, ensuring the standby CPU is always ready for immediate takeover.

• Diagnostic Indicators: The module works in tandem with the primary controller's status LEDs (PROC, FORCE, COMM) to provide a comprehensive overview of the redundancy state.

• Physical Protection: Designed with specific backplane keying to prevent accidental insertion into incorrect slots, protecting the delicate logic circuitry.

• Industrial Hardening: Built to withstand the rigorous temperatures and electrical noise common in heavy manufacturing environments.