Worked on serial I/O over the weekend, verified LDP-1450 command

I improved the serial I/O code over the weekend for Dexter and optimized it a bit more.  I also plugged in my real LDP-1450 to my PC and wrote a small "game" driver to talk to it.  My main purpose was to test the "repeat" command on the 1450 to see if it behaved similarly as it does on the LDP-1000A (which I do not have).  I did indeed verify that the "repeat" command seems to be identical on the 1450 as it is on the 1000A.  I also discovered that the 1450 won't work correctly if DTR is disabled.  I am trying to move in the direction of adding support to Dexter for the 1000A/1450 players.  But "real life" stuff keeps getting in the way.  Maybe I'll have some more time over the holiday vacation coming up.