Hi @marcardui
The serial input doesn't require a pull-up resistor. Furthermore, should also be high impedance, so shouldn't load the TTL-234X-3V3 in any way.
If you've got a multi-meter you could test the Arduino D7 input (without the TTL-234X-3V3 connected) by using a 10k resistor. In pull-up configuration it should pull the input to 3.3V and in pull-down to 0V.
Hello all and MartinL,
Yesterday I did not verify correctly if I had a line that sets D7 back to GPIO, or repurpose it. I found it today and corrected this error...and it works!!! 
What a relief! I would like to thanks everyone who helped me on this and especially MartinL. Without your help I am not sure I could have done it. The sercom and pads story is a very complicated one. Thanks to all!