Types

• One type of loopback test involves using a jumper or a special plug known as the "wrap plug," which can be inserted in the port of a computer or other communications item.
  
  

Function

• The function of a jumper cable or "wrap plug," in conjunction with a "terminal" program, is to transmit data from the computer's (serial) port, through the plug, then back to the port.
  
  

Features

• These tests can be performed by using "terminal" programs such as the Hyperterminal. If your device is working properly, you should be able to type something (data) on your keyboard, then see your words appear on the terminal window/screen, according to hosenose.com. If everything is connected properly and you don't see anything on the screen, your device probably has malfunctioned.