The Electronics Industry Association (EIA) has developed standards
for data communication. EIA standards where originally marked with the
"RS" means that it is a recommended standard, but the standards are now
generally indicated as "EIA" standards.
See also the TIA, standards
RS485 is a truly multi-point network with up to 32 drivers and 32
receivers on a 2-wire or 4-wire bus.
It is a "party-line" communication system there the addressing is handled
by the Remote Computer unit.
RS485 is a differential voltage loop interface (balanced differential signal);
differential data transmission reduces the effects of ground shifts and induced noise signals.
logical "1" (Mark) is represented by a negative voltage of -1.5V to -6V,
logical "0" (Space) is represented by a positive voltage of +1.5V to +6V,
receiver input resistance is more than 12k Ohm,
true multi drop standard, up to 32 transmitters can be connected to up to 32 receivers,
the data rate is up to 10 Mbps,
cable length could be up to 1.3 km (4000 feet),
the connector is a 37 pin D-Sub connector,
Twisted pair wire with a characteristic impedance of 120 ohms is recommended with a
120 ohm termination at each end of the communications line.
The common-mode voltage range is -7V to +12V.
Cat5 cable (shielded twisted pair (STP) or unshielded twisted pair (UTP)) generally
exceeds the recommendations for RS-422 and RS-485 systems.
Most RS 485 systems use Master/Slave architecture, where each slave unit has
its unique address and responds only to packets addressed to this unit. Each node can communicate independently.
A 485 network can be configured as a two-wire or a four-wire system. In the two-wire
network the transmitter and receiver of each device are connected to one single twisted pair.
All devices are connected to a single twisted pair. All connected ports (including the Master) must
have drivers with a tri-state output. Communication goes over the single line in both directions.
Data transmission from more than one device must be prevented by software.
Four-wire networks have one master port, there the transmitter is connected to each of
the slave receivers on one twisted pair. The slave transmitters are all connected
to the master receiver on the second twisted pair. The master does not need to have a tri-state output,
since the slave devices are all transmitting over the second twisted pair. These second pair is
intended only to send data from slaves to the master. Multipoint-communication is possible with these systems.
The two-wire systems are limited to half-duplex communication and require attention to a
turn-around delay. Four-wire systems allow full-duplex operation, but are limited to the master-slave configuration.
The differential signal does not require a signal ground to communicate, but the ground
connection serves aa other important purpose. With distances of up to thousands of feet
significant differences in the ground voltage level are possible. The RS-485 networks can
mostly supply a correct data transmission with a voltage difference of -7 to +12 Volts.
If the ground voltages differ more than that, data could get lost and the ports could get damaged.
The signal ground connection in the cable ties the signal grounds of each of the nodes together to one
single common ground. If the differences in signal grounds exceeding, optical isolation is the best choice.