Fortsetzung der Diskussion von How can I activate the UART Ports on CON3:
I did not receive any comments on my question up to now. May be, I did not state clearly what the Problem is.
As stated before, I'm running a BananaPi Pro A20 Dual Core Board. Due to the fact, that I had a lot of trouble with the SD-Card in the past, I changed the Boot-Info of the System. The System now boots from a SSD-Drive. Therefore I had to manipulate the boot instructions such that the SD-Card is not the Boot Device anymore. It is just there, to tell the machine that the Boot Device is the SSD-Drive from now on. The effect is, that the SD-Card is not accessible anymore.
If I want to configure the UART-Interface on the GPIO-Port, I have to make changes on the "config.txt", which is located on the SD-Card only.
Is there anybody to advice me how to get the UART on CON3 (GPIO) activated?