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

The Allen-Bradley 1785-L20B, also cataloged as the 1785-L20B PLC-5/20 Processor Module, operates as a dedicated hardware component for central logic processing and I/O execution within Allen-Bradley 1771 Universal I/O chassis platforms.

Hardware Specifications

Deterministic Backplane Bus & Network Architecture

The 1785-L20B processor seats directly into Slot 0 of a 1771 Universal I/O chassis, interfacing across the backplane bus to execute sequential and regulatory control tasks across 512 total input and output points. Featuring dedicated Data Highway Plus (DH+) and configurable Remote I/O (RIO) communication channels, the module maintains deterministic network update velocity across distributed chassis links and peer processor networks. Application code and data registers are stored in 16k words of onboard SRAM, with memory retention sustained during main power loss via a battery backup system.

Frequently Asked Questions

Q: What is the backplane current draw for the 1785-L20B processor module?

A: The 1785-L20B draws 2.3 A at 5 VDC from the 1771 backplane power supply, resulting in a maximum power dissipation of 12 W.

Q: How are communication channels configured on the 1785-L20B processor?

A: The processor provides one dedicated Data Highway Plus (DH+) port, one configurable DH+/Remote I/O port (set as either Scanner or Adapter mode), and one multi-standard serial port supporting RS-232, RS-422A, and RS-423A interfaces.

Field Installation Guidelines

1. Power Isolation: Disconnect primary power to the 1771 chassis power supply before installing or removing the 1785-L20B module to prevent backplane pin damage and electrical transients.
2. Chassis Alignment: Slide the module into the primary slot (Slot 0) of the 1771 chassis along the top and bottom card guides until the backplane connectors fully engage, then secure the chassis retaining latch.
3. Communication Cabling: Route DH+ and Remote I/O twinaxial cables in grounded metallic conduits isolated from high-voltage AC lines to prevent EMI noise pickup.
4. Shield Grounding: Terminate cable shield conductors strictly at a single chassis earth-ground point to avoid high-frequency ground loops.
5. Battery Installation: Verify that the onboard lithium battery harness is properly connected prior to initial system power-up to ensure program persistence.