Free software to monitor serial port communication [closed]
I have used the Free Serial Port Monitor, by HHD Software, to monitor communications with an old magnetic card reader.
They also have a paid version if you prefer.
You could check if Portmon still works. I'm legacy-free here, so I can't test it myself ;).
It appears that the everpresent Mark Russinovich has crafted such a tool, called Portmon.