Reply by Steve at fivetrees●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.ukhttp://www.fivetrees.com
Reply by Paul E. Bennett●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
> :
>
--
********************************************************************
Paul E. Bennett ....................<email://peb@amleth.demon.co.uk>
Forth based HIDECS Consultancy .....<http://www.amleth.demon.co.uk/>
Mob: +44 (0)7811-639972 .........NOW AVAILABLE:- HIDECS COURSE......
Tel: +44 (0)1235-811095 .... see http://www.feabhas.com for details.
Going Forth Safely ..... EBA. www.electric-boat-association.org.uk..
********************************************************************
Reply by Frank Bemelman●April 3, 20042004-04-03
"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)
Reply by Dennis Clark●April 2, 20042004-04-02
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?
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
:
--
============================================================================
* Dennis Clark dlc@frii.com www.techtoystoday.com *
* "Programming and Customizing the OOPic Microcontroller" Mcgraw-Hill 2003 *
============================================================================
Reply by Dennis Clark●April 2, 20042004-04-02
CWatters <colin.watters@pandorabox.be> wrote:
: Perhaps you need to add some extra static protection measures of your own?
OK,
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.
With that all in mind...
DLC
: "Dennis Clark" <dlc@io.frii.com> wrote in message
: news:406cf412$0$197$75868355@news.frii.net...
:> Hi all,
:>
:> 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. Can anyone suggest an 8 pin part that is REALLY bombproof to static
:> and CMI? I know that the TI 75176B is right out, but who is in?
:>
:> thanks,
:> DLC
:> --
:>
: ============================================================================
:> * Dennis Clark dlc@frii.com www.techtoystoday.com
: *
:> * "Programming and Customizing the OOPic Microcontroller" Mcgraw-Hill 2003
: *
:>
: ============================================================================
--
============================================================================
* Dennis Clark dlc@frii.com www.techtoystoday.com *
* "Programming and Customizing the OOPic Microcontroller" Mcgraw-Hill 2003 *
============================================================================
Reply by Paul E. Bennett●April 2, 20042004-04-02
In article <406cf412$0$197$75868355@news.frii.net>
dlc@io.frii.com "Dennis Clark" writes:
> Hi all,
>
> 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. Can anyone suggest an 8 pin part that is REALLY bombproof to static
> and CMI? I know that the TI 75176B is right out, but who is in?
There are two articles on my website that will give you the merest
clue about what you are dealing with. The transient figure and the
table are from Railway Industry standards. Equipment I have done for
London Underground deals with that extremely hostile environment very
well. The articles are called "Reading the World" and "Writing the
World" and are linked from "articles" in my Forth pages (but don't
let that put you off - they are only there as I wrote the articles
for Forthwrite).
The RS485 chips out there in the commercial domain may even be
unsuitable when you need to add protection components that will stand
up to the environment you are in. This was the case for the LU
applications I was dealing with due to the constraints imposed by the
re-use of old auto-couplers. I ended up designing a higher voltage
version of the RS485 interface in discrete components for the driver
side and a well guarded op-amp for the receiver. That interface,
during some severe testing, withstood an 18kV directly applied
transient for 0.1us without even flinching. This was well beyond
the specification it was designed to and was actually an accident
of test set-up. I ride the line where the systems are installed on
occassions and will be doing so this Sunday (feeling quite safe).
If you must have it as small as possible then look at the App-Notes
from Linear Technology, which were a good read on the subject. I
think you may have to look at proper galvanic isolation to ensure you
protect the microprocessor/controller and may even have to consider
whether the interface chips become sacrificial in exceptional
circumstances.
As a quicky clue (I suggest you still read up on transient energy
managemnent though) you will end up clamping and sinking transient
energy spikes that would otherwise destroy your circuits. The energy
has to go somewhere and either a Transorb Diode or big resistor is
going to have to take the brunt of this energy surge. Capacitance in
parallel with the Transorb but in series with some resistance will
help to slow down the transient energy groth in your circuits. However,
you may also need to PSPICE* your interfaces to understand what this
may do to the signal you wish to get through.
Naturally, you will have to decide where your trade-offs are within
your system. Only you can do that though. PSPICE can become a very
good friend in such circumstances (but do not think you can hurry it).
In doing the interfaces for the LU job I ran hundreds of PSPICE
simulations for each and every interface type. I usually ended up
building and testing two or three versions of each to confirm what
I saw in PSPICE.
I suspect that when you have gone through this quite arduous mill
once you should be better set to tackle even greater challenges like
when you get asked to design and build systems that are guaranteed
to run for 30 years with very little or no maintenance and will only
require rapid fault-finding to an LRU* replacement if they do fail.
*PSPICE is a circuit analysis tool that will run all the normal set
of calculations for your circuit. This is often attached to packages
like OrCAD, UltiCAP and Proteus schematic capture tools. This can be
an enourous amount of work if you have to enter models of the components
you are using (although many manufacturers have these models available
ready to use.
*LRU Line Replaceable Unit. Essentially a plug in module of the
system.
--
********************************************************************
Paul E. Bennett ....................<email://peb@amleth.demon.co.uk>
Forth based HIDECS Consultancy .....<http://www.amleth.demon.co.uk/>
Mob: +44 (0)7811-639972 .........NOW AVAILABLE:- HIDECS COURSE......
Tel: +44 (0)1235-811095 .... see http://www.feabhas.com for details.
Going Forth Safely ..... EBA. www.electric-boat-association.org.uk..
********************************************************************
Reply by sap●April 2, 20042004-04-02
think about using shielded / twisted high quality cables/EMI
filters/connectors/housing/PSU
then if you still need bombproof, forget 8 pin parts and design your own
using discrete components.
regards
"Dennis Clark" <dlc@io.frii.com> a �crit dans le message de
news:406cf412$0$197$75868355@news.frii.net...
> Hi all,
>
> 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. Can anyone suggest an 8 pin part that is REALLY bombproof to static
> and CMI? I know that the TI 75176B is right out, but who is in?
>
> thanks,
> DLC
> --
>
Reply by Watson A.Name "Watt Sun - the Dark Remover"●April 2, 20042004-04-02
Dennis Clark wrote:
> Hi all,
> 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. Can anyone suggest an 8 pin part that is REALLY bombproof to static
> and CMI? I know that the TI 75176B is right out, but who is in?
I would use an optoisolator on the receive end, to make it isolated from
the receive ground.
I don't know what the commercial controllers we use have, but I have
wires running all over the campus, in hostile envrionments, and they
never seem to have a problem. One cable runs into a bldg next to the
swimming pool, and the juice from the underground conduit wicked up thru
the wires inside the jacket, and then came out and dried onto the 66
blocks (for the telephone system). The juice dried and left a salt
encrustation over half of the block! Crazy, but it was still working,
but not as good as it was supposed to be..
> thanks,
> DLC
Reply by Leon Heller●April 2, 20042004-04-02
"Dennis Clark" <dlc@io.frii.com> wrote in message
news:406cf412$0$197$75868355@news.frii.net...
> Hi all,
>
> 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. Can anyone suggest an 8 pin part that is REALLY bombproof to static
> and CMI? I know that the TI 75176B is right out, but who is in?
Devices like Transguards are very useful for this.
Leon
Reply by Paul Keinanen●April 2, 20042004-04-02
On 02 Apr 2004 05:03:14 GMT, Dennis Clark <dlc@io.frii.com> wrote:
> 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