A-B 1785-L40B PLC Controller

Allen-Bradley 1785-L40B PLC-5/40 Enhanced Processor

 

 

The Allen-Bradley 1785-L40B is a PLC-5/40 Enhanced processor module, part of the classic PLC-5 control system (1771 platform). It is a central PLC CPU (controller) used in large industrial automation systems before ControlLogix became the standard.

It manages remote I/O (RIO), DH+ networks, and full plant-wide control logic execution.

Key Specifications

• Model: 1785-L40B
• Brand: Allen-Bradley (Rockwell Automation)
• Series: PLC-5/40 Enhanced Processor
• Type: PLC CPU (Central Processing Unit)
• User Memory: 48K words SRAM
• I/O Capacity: up to 2048 total I/O (any mix)
• Max Analog I/O: up to 2048
• Communication Ports:
  • 4 × DH+ / Remote I/O channels
  • 1 × configurable serial port (RS-232/RS-422/RS-423)
• Backplane Load: ~3.3A @ 5V
• Mounting: 1771 chassis slot (CPU slot)
• Programming Software: RSLogix 5
• Architecture: PLC-5 / 1771 I/O system

What it does (simple explanation)

Field I/O Modules (1771 rack)
↓
1785-L40B CPU (brain)
↓
DH+ / RIO network
↓
Drives / HMIs / remote racks
↓
Factory equipment control

It is the main controller of the PLC-5 system, responsible for executing all logic and coordinating I/O.

Main Functions

• Executes ladder logic programs
• Controls large-scale machines and process systems
• Manages remote I/O scanning (RIO)
• Communicates via DH+ industrial network
• Coordinates distributed plant automation
• Handles inter-PLC communication

Key Features

• High reliability for heavy industrial environments
• Supports multiple communication channels (DH+ + RIO + serial)
• Large memory capacity for complex programs (48K words)
• Designed for plant-wide automation systems
• Compatible with extensive 1771 I/O modules
• Real-time deterministic control system

Applications

• Oil & gas facilities
• Power plants
• Steel and metallurgy plants
• Water treatment systems
• Large conveyor systems
• Process automation lines
• Heavy manufacturing equipment