TH> I want to control the DSR and/or the CTS line of a serial port
  > that a serial mouse is connected to. When I probed these pins, I
  > found some unexpected things.
 
TH> First, when the computer is first turned on, all of the output
  > lines (TD, DSR, and CTS) light both the 'hi' and 'lo' indicaters
  > on my logic probe. The odd thing is, the legend does not list
  > this condition, so I have to assume that it is a square wave with
  > a duty cycle that appears to be about 50-percent.
ES> Well, that may be so, or the lines could be in a floating
  > condition.
RS-232 ports do not float. The control lines are either asserted
(high) or negated (low).
ES> Without connecting a scope hard to say exactly what they are
  > doing. The DSR and CTS are inputs to the serial port so they
  > could be in any state really...
 
Oops. I ment to say RTS, not CTS; and DTR not DSR. Sorry. Just the
same I am refering to output lines of the port.
TH> PS - how does a serial mouse get its power. My guess is the
  > toggling state on the TD line. But that's just a *guess*.  I've seen circuits that use the RTS/CTS lines to power themselves
  > off the serial port, like to run a MAX232 chip. I really dunno
  > how the mouse powers itself though.
I've figured this out. They are powered from either the RTS or DTR
lines. Which one depends on the maker.
 
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