user6841: I was told, that when computer opens port, it sends DTR (Data Terminal Ready) signal to Arduino, which initiates Arduino reboot.
This looks like part of DTR / DSR flow control. According to this, in response to DTR, Arduino should no only reboot, but also raise DSR (Data Set Ready) signal when it is ready to receive data
And computer should wait for it.
Does it happen so?

user4143: No, this doesn't happen.
DTR is used purely for resetting the board.  Personally I would have chosen the RTS signal instead, but the wise guys at Arduino decided on DTR.  
The Arduino has no knowledge of the DTR signal, it only knows that it has been rebooted.  Whether that reboot came from DTR, or from the reset button, or wherever, it has no knowledge.
So the DTR is being (in my opinion ab-) used to perform the resetting in order to make uploading a sketch easier and that is all.

user612: No, it does not, and can not for the simple reason that there is no DSR line connected to the processor.
Generally host software / serial driver configuration ignores DSR.
On the boards with an 8u2/16u2 the custom firmware therein can report whatever virtual state of the signal it is programmed to. On those with hardware USB-serial they will report however the pin is hard wired, or whatever value it is pulled to or floating at.  
None of these (possibly excepting a Leonardo type board or an ARM one with native USB) reflect anything about the state of the target processor, so software hoping to talk to an Arduino should ignore the DSR signal.

6841: <p>I was told, that when computer opens port, it sends DTR (Data Terminal Ready) signal to Arduino, which initiates Arduino reboot.</p>

<p>This looks like part of DTR / DSR flow control. According to this, in response to DTR, Arduino should no only reboot, but also raise DSR (Data Set Ready) signal when it is ready to receive data</p>

<p>And computer should wait for it.</p>

<p>Does it happen so?</p>
 4143: <p>No, this doesn't happen.</p>

<p>DTR is used purely for resetting the board.  Personally I would have chosen the RTS signal instead, but the wise guys at Arduino decided on DTR.  </p>

<p>The Arduino has no knowledge of the DTR signal, it only knows that it has been rebooted.  Whether that reboot came from DTR, or from the reset button, or wherever, it has no knowledge.</p>

<p>So the DTR is being (in my opinion ab-) used to perform the resetting in order to make uploading a sketch easier and that is all.</p>
 612: <p>No, it does not, and can not for the simple reason that there is no DSR line connected to the processor.</p>

<p>Generally host software / serial driver configuration ignores DSR.</p>

<p>On the boards with an 8u2/16u2 the custom firmware therein can report whatever virtual state of the signal it is programmed to. On those with hardware USB-serial they will report however the pin is hard wired, or whatever value it is pulled to or floating at.  </p>

<p>None of these (possibly excepting a Leonardo type board or an ARM one with native USB) reflect anything about the state of the target processor, so software hoping to talk to an Arduino should ignore the DSR signal.</p>
