Can not upload, start program or reset Arduino while TX, RX pin are connected

Hello everybody.
When I use UART communication between Arduino and other UART devices, I get these problems:

  1. I can not upload program into Arduino when its UART pins are connected with any other devices. I can only upload the program after I plugged out those UART pins. (Some body tell me it's common and isn't a error)

  2. After I upload the program into Arduino, I reconnet, turn it on but it doesn't work. (I can also make my Arduino work by turn it on when the UART pins are plugged out, then I plug them in after Arduino is turned on)

  3. It doesn't work, so I reset it, but it doesn't either. And like the first problem, I have to plug out all UART pins in order to reset it, it is reset, but then I have to plug in those pins again to make it work.
    Can anybody tell me if I did wrong? My hardware isn't good or somethings? And how to fix them. Thank you.
    p/s: Please also spot any my English misstake.

What are the other UART devices? Perhaps you can use one of the Software Serial libraries for them.
Or change to a Mega which has 3 more hardware serial ports.

CrossRoads:
What are the other UART devices? Perhaps you can use one of the Software Serial libraries for them.
Or change to a Mega which has 3 more hardware serial ports.

I use GPS Ublox Neo 6M, HC 11 and other Arduino sir. And I tried with 2 UNOs, Mega ADK and 2 DUEs but it still happended all the times.

You really need to provide a diagram showing how you have everything connected. Without that information it is impossible to help.

If you were using the Mega properly I can't see how you could have had a problem.

...R