This post covers configure the Raspberry Pi to interface with the TNC-Pi. Configuration of AX25 and other ham radio protocols will be covered elsewhere…
Source for this info is the TNC-Pi User Manual:
Make sure Pi OS is up to date:
sudo apt-get update
sudo apt-get upgrade
To use the serial port to connect the TNC-Pi to the Raspberry Pi, make the following configuration changes:
1. Edit the cmdline.txt file in the boot directory, make the following changes:
a. Remove the following: console=ttyAMA0, 115200
b. Remove the following: kgdboc=ttyAMA0, 115200
Note: cmdline.txt is a single line of text.
2. Edit the inittab file in the etc directory, remove the line that says:
T0:23:respawn:/sbin/getty –L ttyAMA0 115200 vt100
Reboot the Pi so the changes take effect.