Rs485Post104

TableOfContents
Category:Sasecurity
http://groups.google.com/group/comp.arch.embedded/browse_frm/thread/22511591b16678dd/4184893c4c3f9888?lnk=st&q=RS485&rnum=1#4184893c4c3f9888
Paul Keinanen says the same about signal grounds than Computer Journal article on RS485:
In this configuration only two wires are required. No signal grounds
wires nor any connection to local frame ground are required, but the
transceiver supply must be floating. The problem with this system
especially in multidrop systems is that the terminating resistors are
at the end of the bus, but the bias resistors must be used at each
station and are effectively in parallel, loading the bus. This may
limit the number of stations connected to the bus.
=== Post 116 ===
Steve E. Bennett
<http://www.svconline.com/news/avinstall_implementing_electronicsystem...>
There is currently some discussion on AES48.
<http://www.aes.org/standards/b_comments/comments-draft-aes48-xxxx.cfm>
This document is on Industrial Ethernet.
http://www.graybar.com/Industrial/Industrial_Networking/whitepapers/B...
This one is a facilities departments standard.
<http://www.utsystem.edu/fpc/docs/electrical_mechanical/electrical/16120
Cable Wire and Connectors 600V.doc>
So, there is plenty of material there that states that the signal cable
shields shall only be connected at one end. Like I have stated before, each
situation should be carefully considered, calculations performed, safety
assessments performed (yes, even for the simple audio lead between intercom
units) and the complet circumstances fuly understood before you go in with
a half baked notion of what is right. Circumstances can change things quite
drastically. You just need to understand the underlying physics of the
situation a bit.
http://www.electric-boat-association.org.uk/
=== Post 117 ===
Here's a blurb on equipment to test the ground and shield continuity at comm cable splices (where you are saying grounds
don't exist).
 * http://www.tequipment.net/WilcomTeleTransT124.asp
Here are a couple of specifications for terminating cables
to building ground systems:
 * http://www.nrao.edu/evla/geninfo/memoseries/evlamemo41.pdf
 * http://www.fac.ilstu.edu/Facilities_Services/Fac_Plan_Home/DesignGuid...
Here's short URL for that last one,
 * http://tinyurl.com/b9gu2
Here's an even more authoritative treatment of grounding,
 * http://www.usda.gov/rus/telecom/publications/word_files/1751f810.doc
All of these describe the grounding at *both* ends of a cable.
-- 
=== Notes post 116 and 117 ===
Both post 116 and 117 Floyd L. Davidson and Bennett would agree that grounding inside a premise at only one end
is a simply fast convenient way of avoiding ground loops and other noise. But if CPE are seperated across the road Davidson
says that both sides must be grounded. Bennett and Steve at fivetrees http://www.fivetrees.com disagree, they say it must always be grounded at only one end.
=== Links ===
 * http://www.svconline.com/news/avinstall_implementing_electronicsystem...>
 * http://www.aes.org/standards/b_comments/comments-draft-aes48-xxxx.cfm>
 * http://www.graybar.com/Industrial/Industrial_Networking/whitepapers/B...
 * http://www.utsystem.edu/fpc/docs/electrical_mechanical/electrical/16120
 * http://www.tequipment.net/WilcomTeleTransT124.asp
 * http://www.nrao.edu/evla/geninfo/memoseries/evlamemo41.pdf
 * http://www.fac.ilstu.edu/Facilities_Services/Fac_Plan_Home/DesignGuid...
 * http://tinyurl.com/b9gu2
 * http://www.usda.gov/rus/telecom/publications/word_files/1751f810.doc