RS 485

RS-485 does require a control signal:

"In contrast to EIA-422, which has a single driver circuit which cannot be switched off, EIA-485 drivers need to be put in transmit mode explicitly by asserting a signal to the driver. This allows EIA-485 to implement linear topologies using only two wires. The equipment located along a set of EIA-485 wires are interchangeably called nodes, stations and devices. [1]"

Here is post on the subject: http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1257538661

Lefty