Subj : Re: DTR Output on COM port
To   : borland.public.cpp.borlandcpp
From : Bill
Date : Sat Jul 30 2005 03:00 pm

"Jack Sawatzky" <j-s@telusplanet.net> wrote in message
news:42e99481@newsgroups.borland.com...
>
> >One thing you're doing wrong is looking at DSR, which is an input :-)
You
> >might have better luck with pin 4 on a 9-pin connector, or pin 20 on a
> >25-pin connector.
> >
> >HTH,
> >Bill
> >
> >
> >
> >
> Hi Bill,
> Actually I made a mistake in my query.  It IS pin number 4 on the DB9
> that I was
> monitoring with my meter, and getting nothing but  0 volts.  So the
> basic question
> remains.
> Jack

Ok then - moving on to the code...

Your code looks fine to me, very similar to code I use to explicitly control
the DTR line.  For what it's worth, here is where my code differs from the
code you show:

1) I use FILE_ATTRIBUTE_NORMAL rather than 0 for the dwFlagsAndAttributes
parameter in the CreateFile call.
2) I check the return from CreateFile against INVALID_FILE_HANDLE (though I
assume you do too).
3) I call CloseHandle when I'm done (I assume you do too).
4) I don't bother with GetCommState or SetCommState if I'm only interested
in controlling the DTR line.
5) If I do use the DCB (because I want to use the port for comms at the same
time) then I set the fDtrControl flag to DTR_CONTROL_DISABLE.

I don't know which, if any, of these differences might affect the
effectiveness of your code.

Finally, I'm _still_ suspicious of your voltage measurements.  Every PC
comms port I have looked at swings positive and negative at the output pins
(typically +/-12V) rather than zero (required by RS-232?).  Zero suggests
either you are not making a connection, or you are not looking at an output.

HTH,
Bill

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