The PI GPIO pins operate at 3.3 volts, and the PI is likely to be damaged by applying 5 volts from a normal TTL port. You will need a level converter on the TNC to PI lead. I used a simple resistor divider. Most TTL inputs are happy with the lower voltage from the PI.
Normally the PI linux kernel allocates the serial port as a console. You need to disable this to use it for packet.
I've used the linux ax.25 and netrom code, and the linux node. I haven't tried jnos.
From: Raspberry_Pi_4-Ham_RADIO@... [mailto:Raspberry_Pi_4-Ham_RADIO@...] On Behalf Of ve2hom
Sent: 08 September 2012 18:30
Subject: [Raspberry_Pi_4-Ham_RADIO] raspberry pi for packet radio