Before insert VEX series cards into your computer, you should do below things.
1.Power on your computer(without VEX series cards)

2.Check your own ttyS device number.
  (1)#dmesg | grep ttyS 

     You will get information like below, in this case, my ttyS device number is 2(ttyS0 & ttyS1).

     serial8250: ttyS0 at I/O 0x3f8 (irq = 4) is a 16550A
     serial8250: ttyS1 at I/O 0x2f8 (irq = 3) is a 16550A
     00:05: ttyS0 at I/O 0x3f8 (irq = 4) is a 16550A
     00:06: ttyS1 at I/O 0x2f8 (irq = 3) is a 16550A
  
3.Modify grub file
  (1)Find line which you load your bzImage(vmlinuz)
  (2)Add "8250.nr_uarts=2" in end of line  (2 means your ttyS device number)

  For example in Fedora 21

  (part of content in /boot/grub2/grub.cfg)
  linux16 /boot/vmlinuz-3.17.4-301.fc21.i686 root=/dev/sda6 ro rhgb quiet LANG=en_US.UTF-8 8250.nr_uarts=2

  Add "8250.nr_uarts=2" end of the line.

4.Update grub
  Use command "update-grub" to refresh grub file.
  If you don't have command "update-grub" like Fedora 31, you can use below command to refresh grub file.
  grub2-mkconfig -o /boot/grub2/grub.cfg	   (BIOS system)
  grub2-mkconfig -o /boot/efi/EFI/fedora/grub.cfg  (UEFI system)

5.Power off your computer and insert your VEX series cards.

6.Build & insmod driver.

