Allen-Bradley PLC-5 1785 Series Modules | 1785-L30B Processor

Technical Dossier

Product Details And Specifications

Allen-Bradley PLC-5 1785 Series Technical Notes

The Allen-Bradley PLC-5 platform, catalogued under the 1785 series, is one of the most widely deployed programmable logic controller families in global heavy industry. Installed across petrochemical complexes, nuclear power generation facilities, offshore oil platforms, and continuous-process refineries, the PLC-5 architecture established the benchmark for mid-to-large scale discrete and process control from the mid-1980s through the 2000s. Its modular backplane design, deterministic scan-cycle execution, and broad I/O compatibility made it the default selection for engineers specifying control systems in environments where uptime is measured in decades, not years. Installed base estimates place active PLC-5 deployments in the hundreds of thousands of nodes globally, with a significant proportion operating in facilities where a full DCS migration is not economically or operationally viable within the current planning horizon.

The Evolution of PLC-5 / 1785 Architecture

The PLC-5 family was introduced by Allen-Bradley (now Rockwell Automation) in 1983 as a successor to the PLC-2 and PLC-3 platforms. The original 1785-L series processors used a proprietary Data Highway (DH) communication bus and a fixed-rack backplane architecture. By the late 1980s, the platform expanded to support Data Highway Plus (DH+), enabling peer-to-peer messaging across multi-processor networks — a capability that became foundational for distributed manufacturing architectures of that era.

The 1990s introduced the Enhanced PLC-5 (E-series) processors, which added ControlNet and Ethernet/IP communication options, extended memory addressing, and improved instruction set coverage including PID, SFC, and structured text elements. The 1785-L series processors are segmented by memory capacity: L11 through L80 designations correspond to increasing user program and data table memory allocations. The 1785-L30B, for example, provides 32K words of user memory with a standard 1771 I/O chassis interface.

Rockwell Automation announced end-of-life for the PLC-5 platform in 2022, with last-time-buy windows closing for most catalog numbers. The recommended migration path is to the ControlLogix 1756 platform. However, given the scale of installed base and the capital cost of migration in regulated industries, demand for 1785-series spare parts and replacement processors remains structurally elevated. DriveKNMS maintains dedicated inventory and sourcing channels for this lifecycle phase.

PLC-5 1785 Series Full Catalog & Functionalities (SKU List)

The following catalog numbers represent verified, commonly stocked SKUs within the Allen-Bradley 1785 / PLC-5 family. Classifications follow Rockwell Automation's functional taxonomy.

Processors — Standard & Enhanced

• 1785-L11B: PLC-5/11 processor, 8K word user memory, DH+ port, 1771 I/O chassis.
• 1785-L20B: PLC-5/20 processor, 16K word user memory, dual DH+ ports.
• 1785-L30B: PLC-5/30 processor, 32K word user memory, DH+ and RS-232 ports, 1771 backplane.
• 1785-L40B: PLC-5/40 processor, 48K word user memory, extended I/O addressing.
• 1785-L40L: PLC-5/40L processor, 48K word user memory, local chassis only configuration.
• 1785-L60B: PLC-5/60 processor, 64K word user memory, high-density I/O support.
• 1785-L80B: PLC-5/80 processor, 100K word user memory, maximum memory configuration in standard series.
• 1785-L20E: PLC-5/20E Enhanced processor, 16K word, Ethernet/IP port integrated.
• 1785-L40E: PLC-5/40E Enhanced processor, 48K word, Ethernet/IP and DH+ dual-port.
• 1785-L60E: PLC-5/60E Enhanced processor, 64K word, Ethernet/IP, ControlNet-ready.
• 1785-L80E: PLC-5/80E Enhanced processor, 100K word, full network option set.

Communication & Network Adapters

• 1785-CHBM: Channel B communication module, DH+ network bridge interface for multi-rack systems.
• 1785-KA: DH+ to RS-232 communication adapter, serial gateway for legacy HMI integration.
• 1785-KA5: Enhanced DH+ communication adapter, supports higher baud rates and extended node counts.

Power Supply & Chassis Support

• 1771-P4S1: 1771 I/O chassis power supply, 5VDC/8A, standard rack power for PLC-5 I/O modules.
• 1771-A1B: 4-slot 1771 I/O chassis, base expansion rack for PLC-5 remote I/O configurations.

Quality Control for the 1785 PLC-5 Range

The 1785-series processor and communication modules present specific technical challenges in quality verification due to their backplane-dependent architecture and proprietary communication protocols. Standard bench power-on tests are insufficient to validate functional integrity of PLC-5 processors; full validation requires protocol-level communication testing on a live 1771 backplane.