Allen-Bradley 1785-BCM | Backup Communications Module

Overview

The 1785-BCM from Allen-Bradley is a Backup Communications Module designed for PLC5 processor systems. It provides high-speed switchover of communication paths using Data Highway Plus (DH+) and Remote I/O (RIO) protocols. The module ensures redundancy and reliability in industrial control systems.

Technical Specifications

The 1785-BCM fits a single 1771 I/O chassis slot and weighs 0.3 lbs (0.1 kg). Its dimensions are 4.5 x 12.4 x 6.3 in (115 x 317 x 160 mm). It requires no external power and communicates via DH+/RIO and High-Speed Serial Link (HSSL) at up to 1.2 Megabaud. This module contains two DH+/RIO interfaces and one HSSL interface, supporting high-speed switchover.

Compatibility

The 1785-BCM is compatible with PLC-5/15 series B, -5/20, -5/25, -5/30, -5/40, and -5/60 processors. It uses a 1771-WG wiring arm and keying between 8–10 and 34–36 pins for proper installation. Each channel features diagnostic LEDs for status monitoring.

Performance and Reliability

This module provides communication redundancy via internal 4K word buffers. Switchover triggers include power failure, processor error, partner module fault, or manual RUN/PROGRAM transitions. Typical AC and DC signal delays are 29±8 ms and 30±7 ms, respectively. It supports up to 4 modules per system.

Environmental and Certification

The 1785-BCM operates between 0°C and 60°C (32°F to 140°F). It features conformal coating for environmental protection and meets UL Industrial Control, CSA Class 1 Division 2, and RoHS/WEEE compliance standards.

Key Benefits

The 1785-BCM ensures high-speed, reliable switchover in PLC5 redundancy applications. Its compact design, robust interfaces, and compliance with industrial certifications make it essential for critical control systems.