RS-232 is a control protocol used by AV gear, primarily in the home automation market. RS-232 control is a wired protocol that is loved by custom integrators because the locking connectors rarely fall out and the connections are fast and robust.
Systems ranging from home theater electronics to home automation control systems like Crestron, AMX and Control4 use RS-232 as their preferred way of component to component communication.
Many new AV Receivers and AV Preamps come with RS-232.
Another option found on newer equipment is control over Ethernet.
See a preview of Crestron’s MC3 3-Series Control System.
Read a review of the Denon AVR-5308 HDCI AV Receiver with RS-232.