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Nanopi M4 UART4 TX not working

sbela
in Rockchip

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I have a nanopi m4 and I want to use the /dev/ttyS4 (UART4) for communication, but only the RX is working, and the TX does not,

but when I set the GPIO nr. 40 (the TX pin) to output through sysfs I can change the voltage level from high to low and back...

Please help, what am I missing here, that I can not transmit anything ?

I am using the latest friendlycore.

7 hours ago, sbela said:

I am using the latest friendlycore.

Armbian has nothing to do with FriendlyCore. That image is made by FriendlyElec and not by Armbian.
For FriendlyCore information go to the wiki for it. http://wiki.friendlyarm.com/wiki/index.php/FriendlyCore_(based_on_ubuntu-core_with_Qt)
For NanoPi M4 info (with the serial interface info) http://wiki.friendlyarm.com/wiki/index.php/NanoPi_M4
Otherwise their forum, but I know there is low chance of getting an answer. But all info you need should be in the wiki. 

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