[dcb]

actually reading looks pretty straightforward. If you use the external interrupts, one on the rising edge and one on the falling edge and keep track of the time between them, the 1's will be short blips and the zeros will be longer, just need to figure out a threshold in between them.

then stream the bits received into some thing to keep track of the messages. It can do things like "oh, I got 9 ones in a row, this must be a new message block"

you can set both pins to input and gang them together and protect with a resister and 5v zener, like the guino.

The output pin basically needs to drive a transistor to toggle 12v to the aldl port.

A timer interrupt to stream the output bits to aldl might simplify timing, or complicate it.