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Hardware Platform 10 Installation Guide
24
RS-485 (option only)
The RS-485 standard
The RS-485 communication protocol is the industry's most widely used bidirectional,
balanced transmission line standard. It is specifically designed for industrial multi-drop
systems that should be able to transmit and receive data at high rates or over long
distances.
RS-485 (RS stands for recommended standards) is accepted as a standard in 1983 by the
EIA (Electronic Industries Association). The full name of the latest revision is TIA/EIA RS-
485-A, published March 3, 1998 which is equivalent to the ISO 8482 standard.
The characteristics of the RS-485 protocol are as follows:
S bi-directional master-slave communication over a single twisted-pair (half-duplex)
S up to 32 nodes per segment
S maximum line length per segment of 1200 meters (4000 feet).
By using repeaters, multiple networks (segments) can be chained together to
accommodate virtually an unlimited number of nodes.
As with the implementation of many standards, there is a difference between theory and
practice. Though the RS-485 standard mentions a maximum of 32 nodes and a line
lenght of 1200 meters, in practice it depends on the used transceivers, cable quality and
network topology. For a deeper understanding of RS-485 in real-world applications, check
the list of recommended literature at the end of this manual.
The connections
RS-485 is not difficult, though some care has to be taken when adding a controller as a
new device on the bus. This chapter explains some of the basic terms and wiring strategy
though it does not intend to go into to much detail.
To learn more about the practical use of RS-485 please refer to many excellent resources
on this subject. Find a list of recommended literature at the end of this installation
manual.
Masters and Slaves
RS-485 is used for multi-point communications: more devices may be connected to a
single signal cable. Most RS-485 systems use the Master/Slave architecture, where each
slave unit has its unique address and responds only to packets addressed to this unit.
These packets are generated by the master (e.g. PC), which periodically polls all
connected slave units. In Master/Slave systems, slaves never start the communications.
In Multi-Master systems, each node can initiate its own transmission creating the
potential for data collisions. This type of system requires a more sophisticated method of
error detection, including methods such as line contention detection, acknowledgment of
transmission and a system for resending corrupted data. This is not specified in the
RS-485 standard.