The Sony 9-Pin Protocol or P1 protocol is a two-way communications protocol to control advanced video recorders. 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.
While nowadays all post-production editing is done with a non-linear editing system, in those days editing was done linearly, using online editing. Editing machines relied heavily on the 9-Pin Protocol to remotely control automatic players and recorders.
Sony's standard also specifies a pinout:
|Pin||on master||on slave|
|1||Frame ground||Frame ground|
|2||Receive A||Transmit A|
|3||Transmit B||Receive B|
|4||Transmit common||Receive common|
|5||Spare or ground||Spare or ground|
|6||Receive common||Transmit common|
|7||Receive B||Transmit B|
|8||Transmit A||Receive A|
|9||Frame ground||Frame ground|
This 9-pin RS-422 pinout has become a de facto standard, used by most brands in the broadcast industry. 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.
- Sony 9-Pin Remote Protocol (Archived) 
- Protocol of Remote-1 (9-pin) Connector, 2nd Edition, Sony, document number 9-977-544-13