RS-232 signal level inversion
The normal configuration for an RS-232 link between two pieces of equipment is this:







simulate this circuit &ndash; Schematic created using CircuitLab

As shown above as well as changing the voltage levels the line drivers (typically a MAX232 or some similar part) also invert the signals. As long as you use line drivers at both ends of the link (or skip them completely and run the link at logic levels) then you don't need to worry about the signal inversion.

There are also some cheap hacks of RS-232 drivers used for hobbyist systems which invert the signal but don't give the voltage levels required by the RS-232 specification and so you may sometimes see signal levels of 0 to 5 V rather than the -3V to +3V minimum range required by the RS-232 specification. While technically a violation of the specification these will normally work fine at lower speeds.

Hopefully this also explains why you need to be careful not to connect the TTL level serial interface you get out of most processors directly to an RS-232 port. Not only will the signal be inverted and so not work as a data link, few processors can take -15V on their IO pins without going pop.