Allen-Bradley 1771-A4B Module Rack

Allen-Bradley 1771-A4B Module Rack is a 16-slot I/O chassis designed for 1771-series modules used in PLC-5 and SLC 500 control systems. The A4B designation identifies a 16-slot backplane that supports up to sixteen modules in a single chassis, integrating with the 1771 backplane and providing the electrical interconnections that let I/O modules communicate with the controller. This module rack serves as the physical backbone for modular I/O in legacy automation installations, enabling centralized mounting of digital and analog I/O modules in a compact enclosure. It is listed in product catalogs and literature as the 1771-A4B I/O chassis and is described as a 16-slot rack suitable for use with 1771-series components, making it a common choice for expanding PLC-5/SLC 500 I/O capacity. Typical applications include machine control, process automation, and general factory automation where modular I/O expansion is needed in a single chassis. The 16-slot format supports scalable I/O configurations in one enclosure, contributing to organized panel layouts and straightforward wiring in industrial environments.