Allen-Bradley 1785-BEM PLC-5 Backup Communication Module

Description

The Allen-Bradley 1785-BEM is a backup expansion module designed specifically for use with the 1785-BCM series B module in high-reliability industrial automation applications. It serves as an integral part of a redundant architecture, providing the capability to switch two additional configurable communication channels when deployed alongside a PLC-5/40, PLC-5/60, or PLC-5/80 processor.

Operating within a dual-processor system, the 1785-BEM acts as a hardware bridge that isolates the standby system while enabling the active system to govern remote operations. By maintaining separate primary and secondary control paths, the module facilitates high-speed link synchronization and fault isolation, supporting consistent system up-time and minimized process disturbance during control switchover.

Features

• Expands backup switching capabilities to accommodate up to two additional configurable communication channels.
• Provides dedicated physical ports to interface seamlessly with remote I/O (RIO) links and Data Highway Plus (DH+) networks.
• Includes an integrated customer relay rated at 0.25A at 24V DC resistive for driving external warning devices or status indicators.
• Utilizes a user-accessible 6-position switch assembly (SW2) located at the bottom of the module for field configuration of channel operating modes and baud rates.
• Integrates directly with the 1785-BCM module through the local I/O chassis backplane interface for reliable backplane communication.
• Features front-panel status indicators (RUN and FLT) for real-time visual hardware diagnostics and switch configuration error detection.
• Mounts using a standard 1771-WG wiring arm to permit module installation and replacement without disturbing field wiring.

Applications

• Redundant PLC control environments utilizing Allen-Bradley PLC-5/40, PLC-5/60, or PLC-5/80 processors.
• Process industries requiring high up-time and minimal disturbance switchovers (e.g., oil and gas, chemical processing, water treatment).
• Data Highway Plus network partitioning and automated link backup routing.
• Remote I/O link switching for critical distributed automation nodes.

Technical Specifications

Installation Guidelines

• Module Placement: Insert the 1785-BEM module into the right-most slot of the first I/O module group within the local I/O chassis. The module must reside in the exact same module group (as defined by 2-slot addressing) as its companion 1785-BCM module.
• Chassis Addressing Configuration: Ensure that the local I/O chassis backplane switch assembly is configured for 1/2-slot addressing. Failure to select 1/2-slot addressing when a 1785-BEM is installed will result in a processor fault.
• Switch Synchronization: The configuration switches on switch assembly SW2 must be set to the identical physical positions on both the primary and secondary 1785-BEM modules. Discrepancies between the two modules will cause the green RUN indicator to blink on the secondary unit and can prevent system switchover.
• Electrostatic Discharge (ESD) Protection: Wear an approved grounded wrist strap and avoid touching backplane connector pins or circuit components when setting switch parameters or handling the unit.
• Chassis Keying: Utilize plastic keying bands on the backplane connector to guard against incorrect module insertion. Place the keying bands between positions 8 and 10, and between positions 34 and 36.
• Inductive Load Suppression: The customer relay contacts are rated for resistive loads. If the relay is utilized to switch external inductive loads, additional external surge suppression devices must be installed to protect the internal contacts.