Ubuntu server 18.04 uart 4 not working

We bought two new rock pi 4b board.
Whenever we trying to access uart 4 some time working some time not working if we reboot also same issue.
sudo cat /dev/ttyS4
cat: /dev/ttyS4: Input/output error

Note - old borad working fine
Why we getting error ?
How to resolve ?


Because uart (except 0) is disabled by default

Manually enabled - /boot/hw_intfc.conf
intfc:uart4= on


But still same issue.
Other boards also we did same thing working fine.
New boards only we facing this issue.
how to solve ?


Hi Jolla,
Thank you for your reply,
Now it’s working.
I having few doubts if you know please clarify.

How /boot/hw_intfc.conf file is working ?
When uart driver check this file ?

Thanks & Regards

hw_intf.conf controls what device tree is loaded at boot time. device tree specify the hardware resource kernel can use. you can check below for more info.


Hi jack,
How can we use uart2(debug) for our application ?
Our application contains two uart.
If possible to use uart2 for our data sending purpose.