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While searching for a cable to connect a router to my pc via serial I realized, that most cables available on Amazon are male USB to male. But when I look at most routers and switches they also have male connectors, meaning one needs an extra adapter to connect such cables to their serial ports. Am I missing a use case for serial ports? Is there any reason why there are few cables with a female serial connection?

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The RS-232 serial specification was designed as a way to connect dumb computer terminals (a.k.a. "Data Terminal Equipment" or DTEs) to modems (a.k.a. "Data Communication Equipment" or DCEs). Later, PCs running terminal emulator software became the usual DTEs. A DTE's DB-25 or DE-9 connector was always male, and a DCE's connector was always female. You can connect two DTE's back-to-back via serial, but you need what's called a "null modem" cable or adapter (female-to-female with crossed-over wiring). Routers and switches are DTEs too, so they have male DE-9 serial ports.

The reason you don't see USB Type-A male to DE-9 female serial adapters is because that would only be the correct cable if your PC was going to act as a DCE (modem).

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