Hello:

On 6 May 2026 at 11:08, terryc wrote:

> My 2c guess is does /dev/ttyS4 exist?
Just a guess and it may *not* apply.
But it could point in the right direction:

[quote]
Detect More Than Four Serial Ports in Linux Desktop
Updated May 1, 2025
[/quote]

[url]https://knowledge.ni.com/KnowledgeArticleDetails?id=kA03q000001Dn
ZLCA0&l=ko-KR[/url]

TL;DR

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= ...

Best,

A.