|
 by
Bob Perrin
Start
• Arm Yourself •
RS-485 101 • Getting
Grounded • Shielding
• Topology •
Termination • Idle-state
Biasing • Transients
• Review Time •
Sources
ARM YOURSELF
Before jumping headlong
into any endeavor, it’s a good idea to research
your topic, and RS-485 is no different. Before sinking
thousands of dollars into a network, get a hold
of the documents listed in the references
section and
study them well.
Two documents that
aren’t free are the standard and the application
guidelines for the standard. The full name of the
RS-485 standard currently is TIA/EIA-485-A. The
last revision was March 3, 1998. The Telecommunications
Industry Association (TIA), in association with
the Electronic Industries Alliance (EIA), also publishes
a telecommunications systems bulletin (TSB89) entitled
Application Guidelines for TIA/EIA-485-A.
The standard is 17
pages long and only defines the characteristics
of the line drivers and receivers. Nothing is said
about transmission lines and network topology. Three
of the 17 pages comprise Annex A, which is an informative
addendum to the standard but is not considered by
the TIA/EIA to be part of the standard. Annex A
offers only the briefest of guidelines regarding
application of RS-485 devices.
TSB89 is 23 pages long
and is dedicated to explaining how to apply the
devices defined in TIA/EIA-485-A to a physical network.
Reading these two documents
will rapidly cure anyone of blind faith in the RS-485
standard. Having the documents available for reference
is handy when evaluating physical parts and performance
tradeoffs in real applications.
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THE
ART AND SCIENCE OF RS-485