RS485 uses twisted pair (A+, B-) signaling, just like RS422 (but, unlike RS422, it supports multiple 'masters' on the same bus), often with a ground (shield) wire (C). Full duplex uses 2 pairs, one for Tx the other for Rx, however bi-directional 'half-duplex' is supported over a single pair (so is more common, especially for long cable runs, which can be up to 1km !). Driver voltages are typically +/-2.5v to +/-5v (max +/-6v) = the minimium 'specified' reciever difference is +/- 1.5v however most work down to +/-200mV (the max. voltage, refernced to chip ground, on any pin is -7 to +12). 'Idle' state is usually A more positive than B (which is taken as a '1') and (since the drivers are 'tri-state', idle is 'guaratneed' with the use of 'bias' resistotrs). You can, in fact, 'get away with' using TTL 0,+5 and +5,0 as the two states

Termination

Ideally an RS485 system consists of a single linear cable (no branches) with 120 ohm resistors connected across the 2 wires at each end of the cable. RS485 can handle speeds of over 10 Mbits per second and line lengths of over 1 km. If you are operating anywhere near these values you must arrange your wiring close to the ideal. For many applications where baud rates are slow (say 9600 baud), and lines are only tens of meters long, this is not essential. The wiring requirements can then be relaxed to allow cables to run in star arrangement from the computer and the terminating resistors become optional.

Usage

RS485 is used for the 'Profibus' and 'Modbus' interface standards, and extensivly used for CCTV camera control (for example,