This breakout board is the interface between a serial TTL and a RS485
bus, a serial interface too but in balanced configuration consisting of
two wires referred to ground, rather than unbalanced (only one conductor
connected to ground) as in the classic TTL.
The RS485 is a simplex data communication line specifically designed to
operate in electrical noisy environments and on long distances, up to
1.2 km.
Its performances about electrical noise immunity and long operating
distances are due to the current loop and to the high signaling voltage
range implemented by the protocol. RS485 drivers (transmitting stages)
send to the two wires a differential voltage to ground , having so a
mutual wire-to-wire signal voltage that is double the level obtainable
on the unbalanced line.
RS485 is a bus, so it is
possible to connect in parallel (to the two line wires) many devices,
the farthest of those must be terminated with a 120 Ohm stub resistor
between A and B wire. The maximum data rate is 35Mbps up to 10m distance
or 100kbps at 1200m. |