As shown, the PC will put negative voltage on the data pin, which is
probably causing problems.
I don't see any level conversion happening.

I have seen some "hack" jobs which connect the "ground" of the uC to
the data pin and connect Ground of the PC to the uC input, but I don't
recommend it.

Although it's not routinely done anymore, the RS-232 spec allows some
fairly high voltages (+/- 24V IIRC) on those lines.  There are several
reasons that you shouldn't directly connect a uC to an RS-232 port.
--=20
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