> Most (if not all) equipment operating at 300 bps or faster allow only require
> the use of a single stop bit, so they only need 10 bits per character.  Thus
> at 300 bps they can transmit 30 characters per second.

Actually, I've seen a variety of items which require two--I've even coded
some.  The most common instance where this is the case is with software-only
polled receive routines.  Typically, when I write such a routine it would
offer the user a choice of either 9600-n-8-1 or 19200-n-8-2; the stop bit time
is when i actually get to process the data, so if I need more than 50us to
stash a byte the extra stop bit allows the link rate to run at 19200 rather
than having to slow all the bits down to 9600.