under Linux

[jlp]

My Windows computer died few days ago. I'm under Linux now and I have
noticed a difference.
I have to press the reset button in order to receive the START
message. It wasn't necessary with the Windows USB/Serial driver.

[Reza Naima]

Sorry for the delay in getting back to you.  I think it might have to do with how the serial driver opens the connection.   It needs to toggle the !DTR line on the serial interface to cause a reset.   The DTR is the data terminal ready line, and according to http://tldp.org/HOWTO/Serial-HOWTO-20.html

Although Linux doesn't support DTR/DSR flow control, it can be obtained by connecting the RTS/CTS pins at the PC to the DSR/DTR pins at the device that uses DTR/DSR flow control. DTR flow control is the same as DTR/DSR flow control but it's only one-way and only uses the DTR pin at the device. Many text terminals and some printers use DTR/DSR (or just DTR) flow control. In the future, Linux may support DTR/DSR flow control. The software has already been written but it's not clear when (or if) it will incorporated into the serial driver.

The normal use of DTR and DSR (not for flow control) is as follows: A device asserting DTR says that its powered on and ready to operate. For a modem, the meaning of a DTR signal from the PC depends on how the modem is configured. Negating DTR is sometimes called "hanging up" but it doesn't always do this. One way to "hang up" (negate DTR) is to set the baud rate to 0 using the command "stty 0". Trying to do this from a "foreign" terminal may not work due to the two-interface problem. See Two interfaces at a terminal. For internal modem-serial_ports it worked OK with a port using minicom but didn't work if the port was using wvdial. Why?

So i'm thinking it has to do with the linux implementation of RXTX, or that it needs to be set explicitly on the linux platform.  I can't find anything explicit in the documentation, but looking through the source I found these methods:

final public class RXTXPort extends SerialPort {
......

        /**
        *  Line status methods
        */
        /**
        *  @returns true if DTR is set
        */
        public native boolean isDTR();
        /**
        *  @param state
        */
        public native void setDTR( boolean state );
      


You might want to use this as a method to reset the device on startup, as well as to restart the device if it ever jams or needs restarting.  Let me know how it goes.  The connection is capacitively coupled so it doesn't matter what the state end up as - it just needs to be toggled from high to low (though the pin is listed as an inverted pin so you might have to set the state as HIGH).  

Reza