RS-232 to TTL Translator
A simple terminal program on a PC can have a lot of uses. The hang-up is that most of those uses involve communicating with a device running on 5 volts. I've done several schemes over the years but this one is my favorites.
The switch is used to join the TTL side send and receive circuits for 'network' style communication. In the network position, it also engages the 'anti-crosstalk' system so that data sent out from the computer will not come back in. This eliminates the software overhead of discarding the reflected data.
The complete schematic is available
for download. Since it is in a GIF file, hold down the shift key
when you click the link to force the browser to download instead of displaying
the file (at least that's what Netscape does).