RS-485 Network
E-2
MIR
6
000 Series User’s Manual
EIA RS-485 is 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:
-Max 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
M
IR
modules 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. In industry 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 address).
In this case the master is a personal computer that is connected with its
serial, RS-232, port to an M
IR
RS-232/RS-485 converter. The slaves
are the M
IR
I/O modules. When modules are not transmitting data,
they are in listen mode. The host computer initiates a
command/response sequence with one of the modules. Commands
normally contain the address of the module the host wants to
communicate with. The module with the matching address carries out
the command and sends its response to the host.
