When communicating over UART, use a terminal emulator like tio to verify command/response behavior. To debug commands used in a chat script, open two terminals: one running cat /dev/ttyS0 to listen, and the other sending commands with echo -e "AT\r" > /dev/ttyS0. This setup allows for validation of the command sequence and modem responses before integrating with pppd.

If issues persist, double-check serial port configuration. A known working configuration is:

sudo stty -F /dev/ttyS0 115200 cs8 -cstopb -parenb -crtscts

The configuration can differ depending on the modem or processor. To learn more about serial port configuration, see the Linux manual page. To confirm the configuration: reboot, reconfigure, and retest with cat and echo before retrying PPP. More details on serial configuration are available in the stty man page.

Note that only one program can access a serial port at a time. Do not run cat /dev/ttyS0 or tio concurrently with pppd or chat.