It's really in how the DB-9 or DB-25 connector is wired. What is an input on a DCE is an output on a DTE, and vice-versa. This allows a DTE to "talk" to a DCE via a straight-through cable. You can have DTE communicate with DTE or DCE to DCE, but you then need to use a "null modem" cable, which has connections crossed (various configurations are used for such "null modem" cables.)
There's more to it than that, but I don't feel like typing that much
urm.. i pretty much understand what's DCE and what's DTE.
but... my project, the MCU actually sends data to PC via MAX232 driver IC.
so, is MCU the DCE device or MAX232 the DCE device (because it is the one that actually act as an input to the PC).
No is correct if you communicate DCE to DCE or DTE to DTE you will need a tail-circuit buffer placed between the two. And is better if you chose asyncronous mode to communicate.
Transmit and receive leads (2 or 3) can be reversed depending on the use of the equipment DCE or a DTE. Look rs232 data is bipolar 3 to +12 volts indicates an ON or 0-state condition while -3 to -12 volts indicates an OFF 1-state condition. Computers ignores the negative level and accepts a zero voltage level as the OFF state. This means the ON state can be achieved with a less positive voltage.So circuits powered by 5 vdc are capable of driving rs232 circuits directly. However the rage will be reduce because of the less potencial but it will work with your device.