A-B Allen-Bradley

Model: 1785-BCM

Full Name: Backup Communication Module

Brand: Allen‑Bradley (Rockwell Automation)

Compatible Platform: PLC‑5 Series Modular PLC System

The 1785-BCM is a dedicated bridge communication module designed to build hot standby redundant systems for Allen‑Bradley PLC‑5 controllers. Its core functions are listed below:

1. Core Link Module for PLC‑5 Primary/Backup Redundancy System Two 1785-BCM modules are installed separately in two independent PLC‑5 racks to establish a dedicated high-speed redundant communication link between primary and backup CPUs. While the primary controller runs normally, the module continuously synchronizes all control programs and register status data to the standby CPU in real time.
2. Bumpless Switchover to Guarantee Uninterrupted Production In case of hardware failure, power abnormality or rack breakdown of the primary CPU, the 1785-BCM executes fast switchover logic. The standby controller instantly takes over all control tasks and maintains the original output logic, interlocks, timing sequences and analog regulation status without machine shutdown or misoperation. It is widely used in power plants, water treatment, chemical plants and other continuous process industries that cannot tolerate downtime.
3. Bidirectional Synchronization of Programs, Data and I/O Images Via its exclusive high-speed bus, the module synchronizes ladder logic programs, integer/float registers, timers, counters and remote I/O image zones between the two PLCs. Upon switchover, the standby CPU holds identical real-time field data as the primary unit to prevent loss of process parameters and equipment out-of-control risks.
4. Redundant Link Self-Diagnosis & Fault Alarm Output The module continuously monitors the communication channel between primary and backup racks. If the cable breaks, the module malfunctions or synchronization abnormality occurs, it sends fault alarm signals to local I/O or upper monitoring systems to prompt maintenance, avoiding single-point failure without backup protection.
5. Full Compatibility with All PLC‑5 CPUs & Expansion Racks It works with all PLC‑5 CPU models including 1785-L11, L20, L30, L40, L80, etc. It supports synchronization of local racks and remote expansion I/O racks, with no restriction on the quantity of digital I/O, analog modules and motion modules. It is compatible with legacy AB DH+ and Data Highway fieldbus networks.
6. Isolated Independent Communication Channel Against Industrial Interference The module adopts a dedicated separate redundant communication cable, physically isolated from plant DH+ and EtherNet/IP monitoring networks. Electromagnetic noise generated by inverters and motors will not corrupt synchronization data, ensuring stable and reliable redundant links.
7. OEM Replacement Spare Part for Legacy PLC Systems The 1785-BCM is an obsolete dedicated spare part for the vintage PLC‑5 platform. During maintenance and retrofitting of old chemical, metallurgical and energy equipment, replacing a faulty BCM module restores PLC hot standby redundancy without full PLC system replacement, greatly cutting renovation investment.

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