Advantech ADAM-5511 Manual page 316

EIA RS-485 is the industry's most widely used bidirectional, balanced
transmission line standard. It is specifically developed for industrial
multi-drop systems that should be able to transmit and receive data at
high rates or over long distances.
The specifications of the EIA RS-485 protocol are as follows:
• Maximum line length per segment: 1200 meters (4000 feet)
• Throughput of 10 Mbaud and beyond -Differential transmission
(balanced lines) with high resistance against noise
• Maximum 32 nodes per segment
• Bi-directional master-slave communication over a single set of
twisted-pair cables
• Parallel connected nodes, true multi-drop
ADAM-5510/P31 systems are fully isolated and use just a single set of
twisted pair wires to send and receive! Since the nodes are connected
in parallel they can be freely disconnected from the host without
affecting the functioning of the remaining nodes. An industry stan-
dard, shielded twisted pair is preferable due to the high noise ratio of
the environment.
When nodes communicate through the network, no sending conflicts
can occur since a simple command/response sequence is used. There
is always one initiator (with no address) and many slaves (with
addresses). In this case, the master is a personal computer that is
connected with its serial, RS-232, port to an ADAM RS-232/RS-485
converter. The slaves are the ADAM-5510/P31 systems. When
systems are not transmitting data, they are in listen mode. The host
computer initiates a command/response sequence with one of the
systems. Commands normally contain the address of the module the
host wants to communicate with. The system with the matching
address carries out the command and sends its response to the host.