For my new PCB, I want to use a 7-series XILINX FPGA. I was a little bit unsure of the JTAG interface so I looked up some schematics of XILINX evaluation boards.
I found the Arty S7 50 schematic.
The JTAG interface of the Arty S7 50 is depicted below (can be found on page 6 in the schematic).
I think the Zener diodes have the purpose to protect the I/Os of a voltage that would be too big. Zener diodes have different breakdown voltages. I guess the depicted Zener diodes have a breakdown voltage that is bigger than 3.3V. Is this correct? So when I use Zener diodes with a breakdown voltage of around 5V I should be good.