Hi;
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.
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.
Now, when I run Windows, or if I simple run something like "copy
 COMx:" I can see the data on TD, and CTS and DSR assume a
steady high state. They remain high after exiting Windows, or when the
copy command completed execution.
*My* challange is that I want to force either of the control lines
'lo.' But, I don't know how to do that. I tried using the INT 014
service, but that simply returns the control lines to this alternating
state. I just want to negate the control lines. Using service 5
(extended control word) doesn't work either.
I want to avoid direct register manipulation, but if someone knows the
programming model of the 8250, I'll consider it. My concern is
compatibility. It *must* work with all PCs.
Anyone messed with manipulating control lines before?
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*. 
 
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