The 8 position 8 contact (8P8C),  commonly referred to as RJ45 in the context of Ethernet and category 5 cables connector, is a modular connector commonly used to terminate twisted pair and multi-conductor flat cable. These connectors are commonly used for 

• Ethernet over twisted pair, 
• registered jacks and other telephone applications, 
• RS-232 serial communication using the ANSI/TIA-568 (formerly TIA/EIA-568) and Yost standards, and other applications 
• involving unshielded twisted pair, shielded twisted pair, and multi-conductor flat cable.

At astronomy equipment it is also used for the EQDirect connection between mounts and computers.

See Wikipedia for more information

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