Using the TXD and RXD pins on the Arduino uno

If you want to use the rxd and txd pins on the uno, doesn't txd from the usb to serial converter that is onboard the Uno conflict with a txd that you supply?

Is the txd on the usb-> serial open collector or something like that?

It would seem that if you tie 2 outputs (TXDs) together there would be a problem.

Thanks

There are 1k resistors between the USB/Serial converter and the AVR RxD/TxD pins. This should mean that any other serial device you connect should be able to "easily" overpower the existing signal, with no damage to the USB controller. The USB output essentially ends up acting as a pull-up resistor. (OTOH, 1k@5V is 5mA, which is more than some modern chips "like" to provide, so some care is required.)

For some shields it happens that it is unable to upload the sketch with connected shield to the Arduino. After disconnecting the shield, everything works again.

unable to upload the sketch

Ah; yes, I should have mentioned that. The effect of the 1k resistors is that is that it is very easy for an external device that uses pin 0/1 to end up interfering with the "upload" function of the the arduino IDE.