Serial question : Going from Mega to Uno
Arduino UNO only has one hardware UART, hence only one Serial available, which is linked to USB, but you could still use it for something else by connecting directly to D0 &amp; D1. 

However, if you need 2 separate, independent, UART then go for SoftwareSerial for the second one.

Now, if you need to connect 2 devices, one to exclusively get data from, the other exclusively to send data to, then you can use just one UART, i.e. Serial for both devices, as long as both links work at the same speed and settings (parity...) 

Also you must ensure that USB won't receive anything otherwise you would get it and confuse it with input from the first device.

The non USB device providing input to the Uno should be connected to D0 (RX).