Check out the ISL83071E - It's an RS422/RS485 transceiver from Intersil. They're $1.68 each and there are samples available. There are also several variants with different speeds, duplex modes, etc. available. Check out the data sheet for more info:
http://www.intersil.com/data/fn/fn6115.pdf
Just get a handful of these, wire them up to the TTL interface and you're all set (from a signal prospective that is).
You'll still have to implement some sort of media access/collision control/avoidance in software.