Solution:

Here are the COM port test program for DOS. It's easy for you to clarify the problem. 1) Select the card. 2) Setting the 'Active Port', 'Address', 'INT Level', ...... 3) Diagnose all the item (also include the function 'Terminal Emulation'). You can get the 'loopback' wiring message on the screen, just make the 'loopback' plug and 'port-to-port' cable for test. Please notice that RS-485 can't use (don't have) the 'loopback' plug for test, because it's a half duplex interface. All the test items must be pass if the port is a good one. (It's recommended to use address 300H, 308H, 310H, 318H, ... , etc.)