title: Sending serial data out Arduino's USB port
body: <p>I am using an Arduino-compatible board with USB host shield to send and receive serial data throught its USB port. </p>
  
  <p>Note for any beginners out there, this nice video explains how to do Arduino serial transmission over the USB: <a href="http://www.youtube.com/watch?v=KYWCkdrCUKg" rel="noreferrer">http://www.youtube.com/watch?v=KYWCkdrCUKg</a></p>
  
  <p>The Software I am using is Arduino 1.0 on my PC to write, compile and upload the firmware to the Arduino board.  I can successfully send and receive serial data between the Arduino and the PC using the Arduino software's "Serial Monitor." The Arduino board's TX light flashes for each byte sent up to the PC.  So far so good.</p>
  
  <p>My problem occurs when I disconnect my Arduino's USB cable from the PC and connect to my own USB device -- a FTDI compatible USB slave.  </p>
  
  <p>(My Arduino board now has an external power supply to provide USB power.  The firmware flashes the Digital Pin 13 LED, so I know that the firmware is running.  I checked USB Pin 1 up on the slave so I know it's getting the 5V.)</p>
  
  <p>But now, when the same firmware attempts to send some bytes over to my USB device, the TX light will not flash and nothing gets transmitted.  I am thinking that maybe the USB data+ and data- lines might need to be swapped, but I don't know.  Can anyone offer some advice?  </p>