Compatibility
The design of the Q-bus was very closely related to the design of the Unibus both in spirit and in detailed implementation. Adapters were available from Digital and from third parties that allowed Q-bus devices to be connected to Unibus-based computers and vice-versa. A number of I/O devices were available in either Unibus or Q-bus flavors; some of these devices had minor differences while many others were essentially identical.
Read more about this topic: Q-Bus