Explain what those "many" circuits are, because I don't think
they use the same circuit configuration for the serial port.


Now it is time to really check them out.
Get yourself an old file transfer program that works with serial ports, and test each and every single one. If at least one serial port is functional between the computers, then ignore using any other serial port, and test your circuit again.


How do you know for sure it damaged it?
You should check Pins 2, 3, and 5 of the port. Pin 5 is ground. Pin 2 or Pin 3 is the transmit pin, and the other pin is the receive pin. All you need to do is connect the receive pin to an LED through a 1K resistor, and connect that to ground. The LED and 1K are connected in series.

in Windows, add a modem in control panel and make it uses the same port. When the LED is hooked up and the modem test begins (you can activate this under Port Diagnostics I think), It will try to commmunicate with the modem. Expect an error message 2 minutes later. During the test, watch the connected LED. If you saw it blinking a number of times, at different speeds, then the port is probably working.

Another thing you could do is a loopback test. Connect the receive pin to the transmit pin through a 1K resistor. I use resistors to prevent massive current.

Now run a terminal program allowing the use of com ports. Recent versions of putty work well for this. When the terminal window opens, type in some random characters. (you won't see a prompt).
Whatever you type in should appear on the screen, even with the echo setting in the terminal software set to off.

So tell me, are your ports REALLY screwed up?