You can test a serial port (also known as a COM port) through a PuTTY loopback. You will need a loopback connector to do this. If you haven’t got one of those, you can improvise with a paperclip. To do this your port needs to be a female coupler. If it isn’t, get a male-to-female adapter and plug it in. Unwind a paper clip and stick one end in pin 2 and the other in pin 3. Then do the following:
Open PuTTY and go to the Configuration screen and click on Session in the left-hand tree-structure menu.
In the Serial Line field, enter the label of the port that you are testing (eg COM3). Enter 9600 for Speed and click on the Serial radio button for Connection Type.
Click on the Open button. This will start the session.
Type anything in at the prompt. If those characters appear, your serial port is working.