"Dennis Clark" <dlc@io.frii.com> schreef in bericht news:406df1b1$0$199$75868355@news.frii.net...> Paul Keinanen <keinanen@sci.fi> wrote: > : On 02 Apr 2004 05:03:14 GMT, Dennis Clark <dlc@io.frii.com> wrote: > > Paul, > > I've read the app notes on total galvanic isolation and have a question > for those who may have more experience than I in that area. My network > will be quite short (less than three meters total) and all of the nodes > as well as the cabling will be on the same supply, in the same box and > near the same noise. In those circumstances it seems to me that total > galvanic isolation is a pretty elaborate design, and not really useful > since the board is as likely to be "hit" as the cable by any static > blast or motor CMI. Am I off base with that thinking or with total > isolation actually be of value in this environment?If everything is inside the same 'machine' you don't need heavy protection at all. 1K2 - 120 - 1K2 between +5, datalines A and B and GND. You can use the unprotected LTC485. If you feel better with more protection, there is the LTC1485 and a bunch of others. -- Thanks, Frank. (remove 'x' and 'invalid' when replying by email)
bombproof RS485 chips?
Started by ●April 2, 2004
Reply by ●April 3, 20042004-04-03
Reply by ●April 3, 20042004-04-03
In article <406df1b1$0$199$75868355@news.frii.net>
dlc@io.frii.com "Dennis Clark" writes:
> Paul Keinanen <keinanen@sci.fi> wrote:
> : On 02 Apr 2004 05:03:14 GMT, Dennis Clark <dlc@io.frii.com> wrote:
>
> Paul,
>
> I've read the app notes on total galvanic isolation and have a question
> for those who may have more experience than I in that area. My network
> will be quite short (less than three meters total) and all of the nodes
> as well as the cabling will be on the same supply, in the same box and
> near the same noise. In those circumstances it seems to me that total
> galvanic isolation is a pretty elaborate design, and not really useful
> since the board is as likely to be "hit" as the cable by any static
> blast or motor CMI. Am I off base with that thinking or with total
> isolation actually be of value in this environment?
The environment sounds like it won't be as hostile as the ones I am used
to dealing with (even the biggest vacuum pump and stepper motors, being
controlled down the same cable bundle, where I work causes me no problems
on over 180m of network run).
Having the shorter cable run and the fact that all nodes are on the same
power supply will help you a lot. You can protect the node electronics
by enclosing them in their own metal shroud and ensuring that that shroud
is bonded to the outer case (it's boxes within boxes type protection).
You will need to ensure that each node stub-length is as short as
possible to the interface drivers (to minimise impedance mis-matching
errors) so no long T-off's.
You may well get away without galvanic isolation (but I would put that
in place anyway - especially to protect the rest of the electronics).
It may seem like a lot of extra effort but is well worth doing as it
will save money down the line if the worst happens and you lose node.
> DLC
>
> :> I'm working a project where I've got several RS485 nodes cabled together
> :>in about as hostile environment as I can imagine. High powered vacuum
> :>pumps, high current stepper motors, high static discharge possibilities,
> :>etc.
>
> : Galvanic isolation takes care of most of the common mode problems and
> : the twisted pair cables attenuate quite a lot of the differential
> : voltages. The way cable shields are grounded can also be critical in
> : some situations.
>
> : Large distances between the PCB tracks are of course needed as well as
> : suitable optoisolators and power supplies to maintain the isolation
> : even at discharge voltages of several kilovolts.
>
> : Paul
> :
>
--
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Paul E. Bennett ....................<email://peb@amleth.demon.co.uk>
Forth based HIDECS Consultancy .....<http://www.amleth.demon.co.uk/>
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Reply by ●April 3, 20042004-04-03
"Dennis Clark" <dlc@io.frii.com> wrote in message news:406df0e7$0$199$75868355@news.frii.net...> I didn't make myself clear enough. I know that other features will > be needed to make a good system, not just a "magic" driver. But, a > chain is only as strong as its weakest link. I'm looking for a driver > chip that won't be that weakest link. I'm already planning on shielded > cabling, twisted pairs, cabled common line, transorbs, etc. No stone is > being left unturned.To echo what's already been said, so long as you ensure that the transient energy is dumped BEFORE it hits the transceiver, it should be fine. Steve http://www.sfdesign.co.uk http://www.fivetrees.com
