On the Arduino Duemilanove, the serial lines are tied to the USB chip. Can one still use the serial transmit line to send data to another serial device? That is, if one were to ignore the USB chip and connector, and just look at the TX output line, could one use it to control another device?
yes
Yes indeed, Arduino serial output pin can be used safely from a hardware point of view. Not sure how the PC's USB driver responds to a bunch of unserviced data being sent to it, buffer full at least? Maybe if no program has the USB serial port opened then no problems?
However trying to use the Arduino input pin might likely be a problem in that it is being driven by the USB chip and I'm not sure that if an external device tried to drive this line that it might damage the USB output pin? Normally you can't wire two outputs together. Anyone know for sure if the USB chip's serial output pin is safe to override?
Lefty
The transmit line from the FTDI USB chip is connected via a series resistor so you can override it fairly easily.
Note that the FTDI chip does not tristate the output so when serial traffic is received it will try to toggle the line but again this is via a fairly large value resistor.
If the line is loaded by another output you will not be able to load new firmware. This may not be a problem if you can disconnect the other output during the programming cycle.