Please explain with more detail what you are attempting to do.
The standard RS-232 serial port on a PC computer cannot sample the receive-data input at a steady rate. Remember that the UART waits for a start bit and then samples several data bits, then it waits for another start bit, ...
You will get data, but it won't be very useful. The UART stops sampling after each received data byte, and then waits for the next start bit. The UART doesn't tell you how long it waited. You will get 8 uniform samples, unknown delay, 8 uniform samples, unknown delay, etc. Those unknown delays will cause trouble when you try to estimate the signal frequency.