Allen-Bradley 1785-LT4 PLC-5/10 Processor Module

The 1785-LT4 is a PLC-5/10 family processor, offering 6K words of user program memory and 2K words of data table memory. It is designed for small to medium discrete and process control applications. The module features a built-in DH+ (Data Highway Plus) port for peer-to-peer communication with other PLC-5s, SLC 5/04, and HMI terminals. It also includes a separate RS-232 port for programming via a terminal or modem.

Memory and Processing

• User Program Memory: 6K words (12K bytes) of CMOS RAM with lithium battery backup

• Data Table Memory: 2K words

• Program Scan Time: 1 ms/K word typical for bit logic

• I/O Capacity: 512 discrete I/O points (maximum), 128 analog channels

• Remote I/O Support: Yes, up to 3 channels via 1771-ASB adapters

Communication Ports

The DH+ port runs at 57.6 kbps or 115.2 kbps, supporting up to 64 nodes over 10,000 feet (using Belden 9463 cable). The RS-232 port (DF1 protocol) can be used for point-to-point communication, ASCII printouts, or as a bridge to other networks. A third port (remote I/O scanner) is integrated into the backplane interface and supports up to 32 racks of 1771 I/O.

Physical and Environmental

Dimensions: 245 mm × 80 mm × 135 mm (Height × Width × Depth). Weight: 1.1 kg. Power requirements: 1.5 A at 5 VDC from backplane. Operating temperature: 0 to 60°C; storage: -40 to 85°C. The module includes a key switch (RUN/PROG/REM) and a battery access door. The lithium battery (1770-XY) typically lasts 5 years.

Programming and Software

The 1785-LT4 is programmed using RSLogix 5 (version 6.0 or higher) or DOS-based AI series software. It supports relay ladder logic with advanced instructions such as PID, shift registers, and sequencers. The processor can store a program comment database in its memory if enabled (reduces available program space by 20%).

Typical Applications

Commonly found in legacy machine control, material handling, and packaging lines. The 1785-LT4 is best suited for systems where I/O is distributed via remote racks, and where DH+ networking is already established. For replacement, the 1785-LT4 can be substituted by 1785-L30B or newer ControlLogix processors with appropriate adapters.