Loopback Test. A loopback test can be used to troubleshoot serial communications. It can show problems in the serial port, the cable, or the software generating the messages without having to connect to third party hardware. By connecting the proper wires, a loopback test can verify the operation of serial communication. How do you perform a loopback test with Tera Term to test a serial communication system? Putty or similar terminal programs can be. Type in the TCP port #. Putty can be used for testing any USB to TTL adapter easily through the loopback test. Cdfs File Converter here. This is a great way to check whether your USB to TTL adapter is faulty or not.