Editing Loopback (section)

==Serial interfaces==
A serial communications [[transceiver]] can use loopback for testing its functionality. For example, a device's ''transmit'' pin connected to its ''receive'' pin will result in the device receiving exactly what it transmits. Moving this looping connection to the remote end of a cable adds the cable to this test. Moving it to the far end of a modem link extends the test further. This is a common troubleshooting technique and is often combined with a specialized test device that sends specific patterns and counts any errors that come back (see ''[[Bit Error Rate Test]]''). Some devices include built-in loopback capability.

A simple serial interface loopback test, called ''paperclip test'', is sometimes used to identify serial ports of a computer and verify operation. It utilizes a terminal emulator application to send characters, with flow control set to off, to the serial port and receive the same back. For this purpose, a paperclip is used to short pin 2 to pin 3 (the receive and transmit pins) on a standard [[RS-232]] interface using [[D-subminiature]] DE-9 or DB-25 connectors.