Terminal TTY – Connecting Modern Linux Distributions to Physical Terminals

The system is allowing you to log in over serial because you have a line like this in /etc/inittab:

T0:23:respawn:/sbin/getty -L ttyS0 9600 vt100

That says to run a a getty on ttyS0 in runlevels 2 and 3, and to respawn it when it exists. 9600 is of course the bits per second, and vt100 is the terminal type. You'll also notice gettys on tty1–6 as well, those are the kernel VGA virtual consoles (which is probably connected to the small screen).

You can write to the VGA console running your program on it, or alternatively opening it (generally after commenting out the inittab lines running getty there). You could also use the frame buffer libraries (or the kernel framebuffer interface directly). Could even start X on it, I suppose.

Not sure what the easiest way to mirror the output on both ttys is.

The modem rules should match ACTION=="add", SUBSYSTEM=="tty" combined with idVendor, idProduct and port_number attributes:

ACTION=="add", SUBSYSTEM=="tty", ATTRS{idVendor}=="1199", ATTRS{idProduct}=="0028", ATTRS{port_number}=="0", SYMLINK+="ttyPort0"
ACTION=="add", SUBSYSTEM=="tty", ATTRS{idVendor}=="1199", ATTRS{idProduct}=="0028", ATTRS{port_number}=="1", SYMLINK+="ttyPort1"
ACTION=="add", SUBSYSTEM=="tty", ATTRS{idVendor}=="1199", ATTRS{idProduct}=="0028", ATTRS{port_number}=="2", SYMLINK+="ttyPort2"
ACTION=="add", SUBSYSTEM=="tty", ATTRS{idVendor}=="1199", ATTRS{idProduct}=="0028", ATTRS{port_number}=="3", SYMLINK+="ttyPort3"

Also check if any of the existing symlinks in /dev/serial/by-id (helps if your modem has a unique serial number) or /dev/serial/by-path (works if you always plug in your modem in to the same USB port) would work for your purpose.