Qbus

Qbus, or Q-Bus, was a computer bus architecture developed by Digital Equipment Corporation (DEC) in the late 1970s for use with PDP-11 and early VAX systems. It served as an enhanced successor to the Unibus, offering higher performance and expanded addressing while maintaining a modular backplane concept that allowed multiple plug-in cards to share a single bus.

Technically, the Q-bus used parallel signaling with dedicated address, data, and control pathways, enabling devices such as disk controllers, magnetic tape controllers, network interfaces, terminal adapters, and memory expansion cards to be attached to the same backplane. It supported a range of backplane sizes and slots and was designed to accommodate DEC and third-party peripherals, providing a scalable I/O subsystem for midrange machines.

Q-bus was widely utilized on several PDP-11 configurations, including models used in business, engineering, and research environments, and was also employed in some early VAX-based systems. In many installations it coexisted with Unibus devices, and DEC offered documentation, adapters, and firmware support to facilitate integration of diverse peripherals on the same system.

Decline and legacy: As DEC introduced newer system architectures and buses in the late 1980s, Q-bus gradually fell out of favor in favor of other interfaces better aligned with evolving hardware and software needs. Today, Q-bus is primarily of historical interest, preserved in museums and by computer collectors. Some hobbyists emulate or recreate Q-bus environments for educational or archival purposes, illustrating an important phase in the evolution of minicomputer I/O architectures.