Sony introduced this protocol to control reel-to-reel type C video tape recorders (VTR) as well as videocassette recorders (VCR).
It uses an DE-9 D-Sub connector with 9 pins (hence the name), where bi-directional communication takes place over a four wire cable according to the RS-422 standard.
Editing machines relied heavily on the 9-Pin Protocol to remotely control automatic players and recorders.
In the new millennium, RS-422 is slowly phased out in favor of Ethernet for control functions.
However its simple way to perform troubleshooting means it will stay around for a long time.