PRODUCT INFORMATION
Product Overview
The Allen-Bradley 1785-BCM is a Backup Communications Module designed for the PLC-5 control system. This single-slot module delivers communication redundancy between a primary processor and a standby controller. It provides backup functionality for Data Highway Plus and Remote I/O networks. A dedicated high-speed serial link executes data transfer at speeds up to 1.2 Megabaud. The module incorporates an internal buffer capacity of up to 4K words.
The module initiates switchover during power interruptions, processor errors, or companion module faults. Switchover can also trigger through manual operating mode transitions between Run, Program, or Test. Standard 1771 I/O chassis backplanes host the module in local configurations. Input and output modules cannot reside in the same local chassis alongside this backup unit. Remote I/O stations must house all process input and output hardware instead.
TECHNICAL DATA
Technical Specifications
| Parameter | Specification |
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| Category | PLCs |
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| Weight | 2.38 lbs (1.08 kg) |
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| Wiring Arm | 1771-WG |
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| DH+/RIO Interface | 2 |
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| High Speed Serial Link Interface | 1 |
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| Module Type | Backup Communications Module |
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| Compatible processors | PLC-5/15 series B, -5/20, -5/25, -5/30, -5/40, and -5/60 |
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| Module Location | 1771 I/O Chassis (local) |
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| Keying Locations | between 8 and 10 between 34 and 36 |
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| Customer Relay | .25A @ 24V dc (resistive) |
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| HSSL | communicates with 1.2 Megabaud |
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| Operational Temperature | 0 Deg. Celsius to 60 Deg. Celsius (32 Deg. Fahrenheit to 140 Degree Fahrenheit) |
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| Certification | UL – Industrial Control Equipment, E65584 CSA – Class 1, Division 2, LR69960C |
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| Bus Communcations | Local PCI 2.3 Compliant |
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| LEDs | 1 Diagnosis/Status Per Channel |
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| Max Modules Per System | 4 |
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| Coating | Conformal Coat |
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| Required External Power | None |
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| Typical AC Signal Delay (Off) | 29 (+/- 8) ms |
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| Typical DC Signal Delay (Off) | 30 (+/- 7) ms |
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| UPC | 10612598117472 |
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TECHNICAL INFORMATION
Features
- Single-slot module enabling communication redundancy between primary and standby PLC-5 processors.
- High-speed serial link supporting data synchronization at up to 1.2 Megabaud.
- Dual Data Highway Plus (DH+) and Remote I/O (RIO) communication interfaces.
- Internal data buffer storing up to 4K words of critical operational data.
- Automated switchover triggered by processor errors, power losses, or partner module faults.
- Diagnostic and status indication through dedicated per-channel LED indicators.
- External field wiring supported via a 1771-WG wiring arm assembly.
- Conformal coating protection for industrial operating environments.
TECHNICAL INFORMATION
Applications
- Hot-standby redundant controller architectures using Allen-Bradley PLC-5 platforms.
- Continuous industrial processes requiring fault-tolerant Data Highway Plus communication links.
- Remote I/O supervisory systems demanding rapid switchover upon controller disruption.
TECHNICAL INFORMATION
Connections / Interfaces
- One high-speed serial link interface port.
- Two Data Highway Plus and Remote I/O interface channels.
- Connection to field communication lines via 1771-WG wiring arm.
TECHNICAL INFORMATION
Installation Guidelines
- Install the module into a local 1771 I/O chassis containing the local PLC-5 processor.
- Set backplane keying bands between pins 8 and 10 or pins 34 and 36.
- Maintain an empty local chassis without any standard input or output modules installed.
- Secure field connections using the designated 1771-WG wiring arm.
TECHNICAL INFORMATION
Configuration Guidelines
- Configure high-speed serial link parameters to support 1.2 Megabaud data synchronization.
- Establish primary and standby processor addresses across DH+ and RIO network channels.
- Map internal 4K word buffer registers for inter-processor status and data tracking.
TECHNICAL INFORMATION
Compatibility
- Compatible with Allen-Bradley PLC-5/15 series B processors.
- Operates with PLC-5/20 and PLC-5/25 controllers.
- Connects with PLC-5/30, PLC-5/40, and PLC-5/60 processors.
- Installs directly into 1771 I/O chassis slots alongside the main controller.
TECHNICAL INFORMATION
Compliance and Certifications
- UL Listed Industrial Control Equipment (E65584).
- CSA Certified Class 1, Division 2 (LR69960C).
TECHNICAL RESOURCES
Documentation
COMPATIBILITY
Compatibility & Replacement
COMPATIBLE MODELSCompatible with Allen-Bradley PLC-5/15 series B processors. Operates with PLC-5/20 and PLC-5/25 controllers. Connects with PLC-5/30, PLC-5/40, and PLC-5/60 processors. Installs directly into 1771 I/O chassis slots alongside the main controller.
Verify compatibility against the exact Part Number, revision and manufacturer documentation before ordering.
PART VERIFICATION
Before You Order
Industrial automation parts can share similar model names, suffixes and revisions. Confirm the complete manufacturer Part Number before purchase.
01Check Part NumberMatch the complete ordering code on the existing unit.
02Confirm RevisionCheck suffix, hardware or firmware when applicable.
03Send a Label PhotoWe can review a clear label photo when identification is uncertain.
ORDERING PROCESS
From RFQ to Dispatch
1Submit Part Number2Verify Product Identity3Confirm Stock & Condition4Issue Quotation5Dispatch & Tracking
PRODUCT FAQ
Frequently Asked Questions
What is the primary role of the Allen-Bradley 1785-BCM module?
The module provides communication redundancy and automated switchover capability between a primary and standby PLC-5 processor.
Which PLC-5 processors work with the 1785-BCM?
It is compatible with PLC-5/15 Series B, PLC-5/20, PLC-5/25, PLC-5/30, PLC-5/40, and PLC-5/60 processors.
Can standard I/O modules share the chassis with the 1785-BCM?
No. Standard input and output modules cannot reside in the chassis where the processor and 1785-BCM operate.
What wiring arm is required for the 1785-BCM module?
The module utilizes the 1771-WG wiring arm for communication cable termination.
