Why did I write it? I needed to "Reverse Engineer" (hack) a command & communications protocol. I was able to use CommLink to put my computer between the computer I was hacking and it's modem. I captured every byte of data and eventually was able to write a replacement program. Yes, I actually had rights to the protocols, but nobody could find the protocol document (until AFTER I did it the hard way...). 

COMMLINK can display all data on-screen as it happens, can log all captured data to a file, and can change both the logging and displaying on-the-fly. COMMLINK allows you to "mark" the log file with comments, and allows you to identify the ports you are using with descriptive names. The serial ports used by COMMLINK can be configured to the full capabilities of Windows 3.1. Speeds up to 115200, any combination of hardware or software flow control, and selectable buffer sizes. Comes with "bonus" Excel file with macro to help you display and analyze your captured data. Windows 3.1 and Windows 95. Needs VBRUN300.DLL.