Allen-Bradley 96844671 1785-LT3 PLC-5/12 Processor Module

The Allen-Bradley 96844671 (catalog number 1785-LT3) is a PLC-5/12 processor module for the PLC-5 programmable controller family. This compact processor provides 24K of user memory and includes built-in Data Highway Plus (DH+) and Remote I/O (RIO) communication ports. It is designed for small to medium control applications requiring reliable performance and connectivity in a legacy PLC-5 system. The module is powered by 24 V DC and mounts in a PLC-5 chassis. It supports all PLC-5 I/O modules and communication options.

Technical Specifications

Processor type: PLC-5/12. User memory: 24K words (RAM, battery-backed). I/O capacity: up to 1024 discrete I/O points (with expansion). Communication ports: DH+ (Data Highway Plus) and RIO (Remote I/O). Power supply: 24 V DC (20.4-28.8 V), consumption 0.5 A. Dimensions: 220 mm × 100 mm × 40 mm. Weight: 0.8 kg. Operating temperature: 0°C to 60°C. Storage: -40°C to 85°C. Humidity: 5-95% non-condensing. The module includes a real-time clock (battery-backed) and a key switch for RUN/PROG/REM. Programming: via RSLogix 5 or AI-5 software using DH+ or serial port (with adapter).

Key Features

• PLC-5/12 processor with 24K user memory

• Built-in DH+ and RIO communication ports

• Supports up to 1024 discrete I/O points

• 24 V DC power supply

• Battery-backed RAM and real-time clock

• Key switch for RUN/PROG/REM modes

• Compatible with all PLC-5 I/O modules

• Programming with RSLogix 5 software

DH+ and RIO Communication

The built-in Data Highway Plus (DH+) port supports peer-to-peer communication with other PLC-5s, HMIs, and programming terminals at speeds up to 57.6 kbps. The Remote I/O (RIO) port allows connection to remote I/O racks (1771) and other RIO devices, reducing field wiring. Both ports are optically isolated for noise immunity. The processor can act as a master on the RIO network and as a node on the DH+ network.

Programming and Software

The 1785-LT3 is programmed using Rockwell Automation RSLogix 5 (or older AI-5 software). The programming cable connects to the DH+ port (via a 1784-PCMK card) or to the serial port using a 1784-CP10 cable. The ladder logic language supports all standard PLC-5 instructions, including PID, high-speed counters, and advanced math functions. The processor supports file-based addressing and data handling.

Applications

The 1785-LT3 is used in small to medium industrial control applications such as packaging machinery, conveyor systems, material handling, and process control. It is often found in legacy PLC-5 systems where a compact processor is required. Typical industries include automotive, food and beverage, and general manufacturing.

Installation and Wiring

Mount the processor in a PLC-5 chassis (e.g., 1771-A1B, -A2B, -A4B). Connect 24V DC power to the L+ and L- terminals. Connect the DH+ network to the 9-pin D-sub connector (blue hose). Connect the RIO network to the 9-pin D-sub connector (yellow hose). The processor automatically detects the chassis size and I/O modules. Use the key switch to select RUN (executes program), PROG (program mode), or REM (remote control).

Memory and Battery

The user program and data are stored in battery-backed RAM. The battery (1770-XY) should be replaced every 2-3 years to prevent memory loss. An optional EEPROM module can be installed for permanent program storage. The processor supports up to 1024 discrete I/O points and 256 analog I/O points (with additional modules).

Maintenance and Diagnostics

LED indicators: POWER (green), RUN (green), CPU FAULT (red), COMM (yellow). Use RSLogix 5 to read the diagnostic buffer. The processor includes a watchdog timer to detect program faults. If the CPU FAULT LED is on, check the program for errors or memory corruption.

Conclusion

The Allen-Bradley 96844671 1785-LT3 PLC-5/12 processor module provides a reliable, compact solution for legacy PLC-5 control systems. Its 24K memory, built-in DH+/RIO communication, and 24V DC power make it suitable for small to medium industrial automation applications.