There is a pin labeled as “D1” on the serial communication header (rst,tx,rx,vdd,D1,gnd). Why is this pin for?
Because the other D1 at the busa and busb are digital 2 and digital 7, and for Arduino Tx is Digital1. Or is not connected?
Okay, first of all the FTDI does not use the pin in that location. Normally that pin is left empty (say on the Arduino USB Serial Light). However on the Versalino, and the Versalino FTDI the interrupt pin is brought out to this pin to help with certain control applications.
On the Versalino the PGM port has BUSA.D1 pulled out to it for interrupt access, that is pin 2 in straight Arduino code.