This is something I did maybe more than 20 years ago, and it is still around, it was even used recently to troubleshoot UART Cable on LPKF PCB mill.

Two DB9 connectors, 2 resistors and 4 LED's. All LEDs go to pin 5 (GND) resistors go to pins 2 and 3. Easy uh?

If you ever have the need to troubleshoot a RS232C connection.. and you have no tools, and have NOT made this one to help you, then I can not help either.

For me this simple tool has been valuable last xx years.