Ok, I've said it before, but repeating it one more time won't hurt: when it comes to electronics or hardware, I am completely ignorant. So, yesterday morning, I spent the best part of 3 hours trying to plug the STM32's UART ports on a PC DE9 interface, trying to get the code to work and fiddling with Minicom's settings.
Then I looked it up. As it turns out, signals from the microcontroller are encoded using TTL (where a voltage close to 0V indicates a zero, while a voltage close to the chip's power supply voltage indicates a one), while RS232 relies on a negative voltage to indicate a zero.
*cough* I felt really silly.
So I looked up how to interface both. There are many schematics and explanation on the web; this explanation is quite good, and I ended up using their schematics and some parts I already had to build one (well, I didn't add the LEDs because I didn't see much point in these).
Here's the result:
And, if that can help, here are the schematics for the board itself (not exactly the one shown above, as I'd made a bit of a mistake):
And the list of parts:
|R1 R2 R3 R4 R5||10kΩ|