If you like low-level stuff, this is for you. A micro-controller with a built-in voltage comparator can be used for a cool little hack: use the comparator as a RS-232 serial receiver without the need for a RS-232 to TTL adaptor.

For those of you unfamiliar with what I just said, here’s a little primer. Most microprocessors with I/O pins support TTL: 0V is negative, 5V is positive. RS-232 (aka your PC’s serial port) specified that -15V to -3V is positive (a mark) and +3V to +15V is negative (a space). Incidentaly, most PC implementations will actually not meet that spec as a mark is only about -1V.

How does the little hack work?  Connect the RS-232 RX pin to the Voltage Comparator Pin on your microprocessor.  Set the reference voltage to be about 3V.  Depending on your particular, anything above 3V will be considered a 1 and bellow 3V will be a 0.  Hook-Up an Interrupt Service Routine to read the comparator output and bit-bang read the bytes.  Note that only fairly slow baud rates is supported with bit banging; 19,200 bauds is the upper limit with this technique.

Why would you ever want to do this?  Small physical package, with no spare circuit board space is one possible reason…