A loopback connector is just a piece of hardware that you plug into the serial or parallel ports in the back of your computer. The wiring inside the loopback connector allows the diagnostic program to send and receive signals, verifying that the port is working. You can buy loopback connectors at many electronic parts stores. In PCDiagnostics there are several tests that require "loopback connectors." What are these and where do I get them? (KB002077)Modified on: Wed, Jun 29, 2016 at 5:20 PMDid you find it helpful? Can you please tell us how we can improve this article?