Solution
This limit is coming from the implementation of the NI-Serial driver being on top of the native Linux driver.
NI-Serial driver until version 21.3 leverages the 8250 driver for serial ports in Linux System. The CONFIG_SERIAL_8250_NR_UARTS parameter in the configuration kernel file defines the maximum number of serial ports handled by the kernel.
To detect more than four serial ports, do the following:
- Edit /etc/default/grub and add 8250.nr_uarts=<number of ports> to the line starting GRUB_CMDLINE_LINUX=... You can use VI Text Editor if you are not familiar with text editing in Linux.
- Rebuild the grub.cfg file by running the grub2-mkconfig -o command as root for different machine:
| Machines | Commands |
|---|
| BIOS-based | grub2-mkconfig -o /boot/grub2/grub.cfg |
| UEFI-based | grub2-mkconfig -o /boot/efi/EFI/redhat/grub.cfg |
| CentOS | grub2-mkconfig -o /boot/grub2/grub.cfg |
| RHEL | grub2-mkconfig -o /boot/efi/EFI/redhat/grub.cfg |
3. Reboot
NI-Serial driver version 21.5 and newer do not rely on the 8250 driver for 843x-series hardware, and therefore can address more ports than configured in the kernel parameters.