The Allen-Bradley 1785-L20C, also cataloged as the 1785-L20C PLC-5/20 ControlNet Processor Module, operates as a dedicated hardware component for controller execution and ControlNet communication within PLC-5 automation systems.

Model Identification and Configuration

The 1785-L20C is a PLC-5 processor module with integrated ControlNet communication capability. It provides controller processing functions, local I/O management, and network communication through the PLC-5 architecture.

The module includes one serial communication port, one coprocessor port, and 16 K memory capacity. It supports PLC-5 chassis-based configurations using 1771 I/O modules with selectable digital and analog I/O addressing structures.

Hardware Specifications

PLC-5 ControlNet Communication Architecture

The 1785-L20C integrates ControlNet communication functions directly into the PLC-5 processor platform. The processor manages application execution, I/O scanning, and network data exchange through the PLC-5 backplane and ControlNet interface.

For deterministic industrial networks, proper ControlNet scheduling, node configuration, and firmware compatibility are required. The processor maintains communication timing through the ControlNet network structure while executing ladder logic and controller tasks.

The PLC-5 architecture supports I/O density scaling through 1771 module selection. Digital and analog I/O capacity depends on chassis configuration, addressing mode, and installed modules.

Processor and Backplane Integration

The 1785-L20C receives operating power through the PLC-5 chassis system and communicates with installed I/O modules through the backplane bus. The processor supports firmware-controlled execution and module configuration stored within the PLC-5 controller environment.

Backplane bus communication velocity and controller scan performance depend on processor loading, installed I/O modules, and configured communication tasks.

The module provides compatibility with PLC-5 system components while maintaining the original controller hardware structure.

Ordering Information

• 1 x Allen-Bradley 1785-L20C PLC-5/20 ControlNet Processor Module
• 1 x Processor module hardware assembly
• 1 x Installed communication interface assembly
• User manuals, programming software, and programming cables are not included unless specifically listed

Field Installation Guidelines

Controller Preparation

• Remove chassis power before installing or removing the processor module.
• Confirm PLC-5 chassis type and ControlNet network configuration before replacement.
• Verify firmware compatibility with the existing controller application.

Module Installation

• Align the processor module with the PLC-5 chassis slot connector.
• Secure the module according to the chassis mounting requirements.
• Confirm backplane connection integrity before restoring power.

Network Connection

• Check ControlNet node addressing before network startup.
• Use appropriate shielded communication cabling for ControlNet connections.
• Verify communication status through controller diagnostics after power restoration.

Frequently Asked Questions

• What function does the 1785-L20C provide in a PLC-5 system?

The module performs PLC-5 controller processing, I/O management, and ControlNet communication functions within compatible PLC-5 chassis systems.

• How much memory is available in the 1785-L20C?

The processor module provides 16 K memory capacity for PLC-5 application execution and controller data storage.

• Does the 1785-L20C support ControlNet communication?

Yes. The module includes integrated ControlNet communication capability for PLC-5 network integration.

• What I/O addressing method does the module support?

The processor supports 2-slot addressing for compatible 1771 I/O module configurations.

• What power source does the 1785-L20C use?

The processor operates through the PLC-5 chassis power system. System power configuration depends on the installed chassis supply.

• Can digital and analog I/O modules be used with this processor?

Yes. The processor supports digital I/O modules with 16/32-point configurations and analog modules with 4/8-channel configurations.

• Does the processor support hot swapping?

Hot swapping is not specified for this processor module. Power removal is recommended before hardware installation changes.

• What factors affect PLC-5 processor communication performance?

Network configuration, controller scan load, installed I/O modules, and ControlNet scheduling parameters affect communication performance.