Allen‑Bradley 1785-BCM PLC‑5 Backup Communication Module
1785-BCM is a single-slot backup communication module exclusively built for Rockwell Allen‑Bradley PLC‑5 redundant control systems, installed inside standard 1771 I/O chassis. It establishes high-speed inter-processor links between primary and standby PLC‑5 controllers via HSSL (High-Speed Serial Link) at 1.2 Megabaud, synchronizing complete data table, remote I/O input and logic runtime data in real time. The module integrates dual DH+/RIO communication ports to maintain consistent field bus links during processor failover, eliminating production interruption risks for critical continuous-process plants.
This legacy communication card has been officially discontinued by Rockwell Automation, serving as a vital aftermarket spare for aging PLC‑5 redundant control racks deployed in power, petrochemical and pulp & paper facilities. Equipped with front-panel diagnostic LEDs and configurable DIP switches, it supports automatic bumpless switchover within 50ms once the primary processor fails. It works with 1771-W series wiring arms for convenient field communication cable termination and maintenance.
Main Specifications
- Brand: Allen‑Bradley (Rockwell Automation); Model: 1785-BCM; Type: PLC‑5 Redundant Backup Communication Module
- Chassis Requirement: Single standard slot for 1771 series I/O rack; Backplane Power: 1A @ 5VDC
- Communication Interfaces: 1×HSSL inter-processor link, 2×DH+/Remote I/O bus ports
- Transmission Rate: HSSL max 1.2 Megabaud; Built-in 4K-word data buffer for redundant data caching
- Compatible Processors: PLC‑5/15, /20, /25, /30, /40, /60 series controllers for redundant architecture
- Environmental Rating: Operating 0℃ ~ +60℃, 5%–95% RH non-condensing, UL & CE industrial certified
- Configuration & Diagnosis: 4-position DIP setting switches; Multi-color LED indicators for link, fault and run status
- Matching Accessories: 1771-W wiring arms, 1771-CD serial cables, 1785-BEM extended backup modules
Core Functions
- Achieve real-time high-speed data synchronization between primary and standby PLC‑5 processors, copying all input image tables, timer/counter values and process parameters to the backup unit continuously.
- Enable automatic bumpless redundant switchover within 50ms upon primary processor fault, power loss or manual maintenance, retaining all field DH+ and Remote I/O communication channels intact.
- Dual isolated DH+/RIO ports deliver stable bidirectional communication with field transmitters, valve actuators and remote auxiliary PLC stations without signal dropout during redundancy switching.
- Front-panel LED diagnostic lights display HSSL link status, bus communication activity and module hardware faults to accelerate on-site troubleshooting of redundant system failures.
- Configurable DIP switches adjust communication baud rates, redundant mode and fault alarm logic to match different PLC‑5 processor versions and process control requirements.
- Standard 1771 backplane electrical definition supports plug-and-play replacement; only simple redundant communication parameter setup is required after module swap.
Application Scenarios
- Redundant communication spare replacement for Allen‑Bradley PLC‑5 critical control racks in thermal power plants, combined cycle power stations and hydropower automation rooms.
- Fault-tolerant communication spare parts for petrochemical refineries, natural gas transmission pipelines and continuous chemical reaction production lines.
- Maintenance backup inventory for pulp & paper mills, metallurgical rolling lines and pharmaceutical batch manufacturing process control cabinets.
- Matching redundant accessory paired with dual PLC‑5 processors, 1771 analog/digital I/O modules and DH+ remote field stations.
- Retrofit replacement components for aging redundant PLC‑5 systems suffering inter-processor sync failure, DH+ communication dropouts and slow switchover faults.
Global aftermarket spare stock for process industry maintenance teams managing discontinued Rockwell PLC‑5 redundant automation hardware worldwide.
