http://pclt.cis.yale.edu/pclt/COMISDN/STRTSTOP.HTM
...from the good olde Win31, "This is the communications configuration panel from Windows Terminal. It raises a number of interesting questions. Why would someone use 5 bits per character? Because a 5-bit code was used by very early Teletype equipment that was already obsolete in the 1950's. What is the right number of Stop Bits? Well, if you have a Teletype Model 33, the right answer is 2. If you have a Teletype Model 35, the right answer is 1.5. However, no device built in the last 20 years has needed more than 1 stop bit. What is Xon/Xoff Flow Control? XON and XOFF are byte values. The Teletype had a device to read punched paper tape. The XON character turned the tape reader on, and the XOFF character turned it off. Long after the last paper tape was burned, computers have maintained the tradition that XOFF can optionally mean "stop sending data," in which case XON means "begin sending again." What is parity? Before modems did error correction, parity provided a simple mechanism to detect characters corrupted by phone line noise. Today it is unnecessary and is typically disabled.
So in current use, the correct setting for the COM port is always 8-bit characters, no parity, 1 stop bit, hardware pacing (more about that later) and some speed faster that the native transmission speed of the modem. The panel to configure the COM port is left around because everyone is scared to get rid of it. The user should not be scared when presented by a large package of useless options. "...
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