I answered my own question about the 0v/5v thing (example here: https://www.sparkfun.com/tutorials/215 - the whole reason we need a Max232 is because the signaling is 0v/5v for the line-level and +13/-13 for the RS-232 level).

But does that mean the chip connected to the TX is always the one supplying the voltage? So for example if I accidentally connect a TX to a TX then it's two voltage suppliers competing?