Allen-Bradley 1761-L20BWB-5A MicroLogix 1000 Programmable Controller

The Allen-Bradley 1761-L20BWB-5A is a compact fixed-configuration Programmable Logic Controller (PLC) from the MicroLogix 1000 product line. Powered directly by a 24 VDC main power supply, it integrates twenty I/O points—comprising twelve 24 VDC discrete inputs, eight relay contact outputs, four analog inputs, and one analog output—into a single base chassis for small-scale industrial automation applications.

DC Power Supply and Integrated I/O Architecture

Designed for 24 VDC control systems, the controller consumes approximately 7 Watts of real power at nominal line voltage. The discrete section features twelve 24 VDC sinking/sourcing inputs and eight electromechanical relay outputs capable of switching 24 to 125 VDC or 0 to 240 VAC load circuits. Built-in analog channels support differential voltage (-10.5 to +10.5 VDC) and current (-21 to +21 mA) signals, alongside a single analog output delivering 0 to 10 VDC or 4 to 20 mA for process variable regulation.

Program Processing and Communication Interfaces

The unit incorporates an internal 1K EEPROM memory supporting up to 737 instruction words and 437 data table words for executive control programs, counters, and timers. Communication is established through an integrated RS-232 port supporting DF1 point-to-point and network protocols (DH-485, DeviceNet, or EtherNet/IP via external converters). An integrated Master Control Relay (MCR) circuit provides hardwired safety shutdown capability across output groups.

Technical Specifications

Grounding and Wiring Conditions

When installing the 1761-L20BWB-5A, note that the 24 VDC user supply ground terminal and the module chassis ground screw are connected internally. Installers must connect the chassis ground screw directly to a low-impedance industrial earth ground prior to energizing the controller. Terminal block connections accommodate #14 to #22 AWG solid wires or #16 to #22 AWG stranded wires.

Frequently Asked Questions

Q: What is the main operational difference between the 1761-L20BWB-5A and the 1761-L20BWA-5A? A: The 1761-L20BWB-5A is powered by a 24 VDC main power supply, whereas the 1761-L20BWA-5A requires a 120/240 VAC main power source. Both units utilize 24 VDC digital inputs and share identical analog channel layouts.

Q: Are the 24 VDC user supply ground and chassis ground isolated on this controller? A: No, the 24 VDC user power input ground and the chassis ground terminal are connected internally on the 1761-L20BWB-5A.