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  • What's the purpose of this (apart from testing hardware or drivers for expected behavior)? I mean, why someone would develop a useless thing? Commented Sep 8, 2016 at 20:57
  • Serial is not a packet lobbing network, and must connect to something. You could do this with a USB serial cable and wiring the appropriate RX/TX/etc. lines to the appropriate TX/RX/etc. lines on the rpi. Commented Sep 8, 2016 at 20:57
  • Also! Beware that the (usually) red wire on USB serial thingies is 5V, which can be bad news for 3.3V things it might accidentally be plugged into. Also, old serial ports on desktops may run at 12V, which can be bad news for 5V or 3.3V things that might be connected to that. Level shifters may help, along with appropriate caution and reading tech specs. Commented Sep 9, 2016 at 15:18
  • @Serge Why should computers have such a useless thing as local loopback? You can easily do that with two network ports and a short piece of network cable, right? Localhost has its uses and a local serial "localhost" loopback would be useful to those working with hardware and software development that interfaces to serial devices. Especially if the hardware is rare, expensive, or still under development. Commented Mar 17 at 16:53
  • @JimJRHarris don't confuse network loopback interface and serial port with a loopback plug or a serial in loopback mode. The OP tries to do something like ssh-ing, which requires two-way communications, over a one-way communication channel. Commented Apr 10 at 22:20