Allen-Bradley 1785-L20B PLC-5/20 Processor Module

The 1785-L20B is a PLC-5/20 family processor offering 20K words of user program memory and 10K words of data table memory. It is designed for medium to large discrete and process control applications. The module features a built-in DH+ (Data Highway Plus) port for peer-to-peer communication at 57.6/115.2 kbps, a dedicated remote I/O scanner port for 1771 I/O racks, and an RS-232 port for programming or ASCII communication. The processor supports up to 4096 discrete I/O and 1024 analog I/O points.

Memory and Processing

• User Program Memory: 20K words (40K bytes) of CMOS RAM with lithium battery backup (1770-XY)

• Data Table Memory: 10K words

• Program Scan Time: 1 ms/K word typical (0.5 ms with fast I/O scan enabled)

• Maximum I/O: 4096 discrete, 1024 analog via remote I/O network

• Built-in Diagnostics: Watchdog timer, power-fail detection, memory checksum

Communication Ports

The DH+ port supports up to 64 nodes over 10,000 feet (Belden 9463 cable). The remote I/O port (blue hose) can drive up to 32 racks of 1771 I/O with baud rates selectable from 57.6 to 230.4 kbps. The RS-232 port (DF1 protocol) can connect to a modem, printer, or operator interface. An optional DH-485 network module can be added via the backplane.

Physical and Environmental

Dimensions: 245 mm × 80 mm × 135 mm (H × W × D). Weight: 1.2 kg. Power requirements: 1.8 A at 5 VDC from backplane. Operating temperature: 0 to 60°C; storage: -40 to 85°C; humidity: 5–95% non-condensing. The processor includes a key switch (RUN/PROG/REM) and a battery access door. The lithium battery typically lasts 5 years.

Programming and Compatibility

Programmed using RSLogix 5 (version 4.0 or higher) or DOS-based AI. Supports relay ladder logic with advanced instructions such as PID (up to 16 loops), block transfers, and subroutine calls. The 1785-L20B is compatible with all 1771 I/O modules and 1771-ASB remote adapters. For expansions, memory can be upgraded to 32K using an optional memory cartridge (1785-ME32).

Applications

Commonly found in water/wastewater treatment, mining conveyors, and packaging lines. The 1785-L20B can be used as a main controller or as a remote I/O processor. For replacement in new projects, consider ControlLogix 5570 with 1756-DHRIO module. The module is UL 508 and CSA certified.