RJ9 is a four-contact 4P4C plug typically used for telephone handsets or for serial RS232 connections. (RJ stands for Registered Jack, BUT this RJ9 4P4C connector is not a registered jack).

In astronomy equipment this plug is often used as the serial RS232 port for hand controllers, to connect the hand controller with the mount (RJ9 ports at both the hand controller and the mount) or for firmware upgrades of the mount and the hand controller.

Other denotations for this plug are: RJ10, RJ22 

See RS232 cable and RS232 Serial Port chapters. 

See also: Modular connector - Wikipedia

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