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Michael A. Terrell

wrote:

Maybe I'm an old fuddy duddy. I see an RJ45 on an amplifier and I say to myself "Why ?".

You've never seen a 'RJ45' on any amplifier, since that is a
telephone company standard for an application for the 8P8C connector.

"RJ45 is the common name for an 8P8C modular connector using 8
conductors that was also used for both RJ48 and RJ61 registered jacks.
The "RJ45" physical connector is standardized as the IEC 60603-7 8P8C
modular connector with different "categories" of performance, with all
eight conductors present. A similar standard jack once used for
modem/data connections, the RJ45S, used a "keyed" variety of the 8P8C
body with an extra tab that prevents it mating with other connectors;
the visual difference compared to the more common 8P8C is subtle, but it
is a different connector. The original RJ45S keyed 8P2C modular
connector had pins 5 and 4 wired for tip and ring of a single telephone
line and pins 7 and 8 shorting a programming resistor, but is obsolete
today.

Electronics catalogs commonly advertise 8P8C modular connectors as
"RJ45". An installer can wire the jack to any pin-out or use it as part
of a generic structured cabling system such as ISO/IEC 15018 or ISO/IEC
11801 using 8P8C patch panels for both phone and data. Virtually all
electronic equipment which uses an 8P8C connector (or possibly any 8P
connector at all) will document it as an "RJ45" connector."

The same plug is used for the RJ31X for alarm systems, but uses a
special 'shorting' 8P8C connector to allow a dial-up 'communicator' to
seize the phone line and call the central station of the monitoring
service.