Any data you send over the serial connection (e.g. using Serial.print()) will go out both over the TX pin (pin 1) and over the USB connection to the computer. If you don't need to talk to the other device, you can simply not connect the TX pin to anything and just send data to the computer. Any data that comes in from the USB connection or the RX pin (pin 0) will be available with Serial.read(). So you could just connect the RX pin to the device and then read data from it with Serial.read() and send it to the computer with Serial.print(). If you need to both write to and read from the device, you might need to use a software serial connection; see the SoftwareSerial library: http://www.arduino.cc/en/Reference/SoftwareSerial