It's a "dummy centronics" driver that opens a linux named pipe and sends it directly to the ap2000 driver. In the code it's "/dev/lp0" but you could change that to any filename and just mkfifo. When I was fiddling with it, I changed it to a "lp0" filename in my directory.
It's really slick in that you can send it data from any linux application, like doing stuff from the command line: echo "hello" > /dev/lp0
ASCII animations (Twilight Zone, Outer Limits) playing in VT240 emulation. I had to add XON/XOFF flow control support to null_modem -- with hardware control, firmware misbehaves and sets baud rate 1200 for some reason -- offending code is the MOV: