How do I host a shell for a serial terminal?

I am building a dumb terminal with an arduino and I can now see the output of serial connections. How do I get a login/shell? I have tried screen /dev/ttyACM0 and that lets me type on the host computer and see the results on the terminal screen, but no shell. Using screen's :exec /bin/sh command starts a shell on that controlling xterm window but no longer outputs to the serial line. various attempts with agetty -L /dev/ttyACM0 9600 just sit and hang for a minute then exits without ever having put anything on the screen. I have tried cu -l /dev/ttyACM0 -s 9600 and again it hangs with no screen interaction. I have connected minicom to echo keys onto the arduino similar to screen but could not figure out how to get a shell. To be clear I am trying to access my main computer via the arduino based terminal, and I can not find a way to make my main computer offer a shell session to the arduino over a USB serial connection.