incested sisters

To keep things simple, the three outputs are latched when they arrive and no attempt at synchronising them has been made (it's a moot point whether this would be a good idea or counterproductive); also no input latches are provided (again this may or may not be a good thing). For ease of interfacing to external circuitry a +5V power supply is used and the RS232 signals (up to +/-15V) must be converted to compatible levels. The most platform independent way to do this is to use something like a MAX232 chip to guarantee RS232 conformant signals from a single +5V supply. For the more adventurous the whole circuit can be powered from the RS232 signals themselves. This should work on most machines that provide DTR and/or RTS; note that it's not necessary to program these lines as they normally idle at between +8V to +12V and should be able to supply the small current required for the circuit. My prototype uses this approach: it has a two transistor voltage regulator (used because it has a lower dropout voltage than a 78L05 for example); a single transistor RS232 receiver and uses an op-amp as an RS232 transmitter. The TXD line is rectified and smoothed to provide the negative supply for the op-amp; the positive supply is taken from DTR/RTS.