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'[EE]: RS485 -vs- Static'
2000\08\10@151333 by Shawn Yates

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Hello all,

       I see some discussions about RS485 batting around here every now and
then, so  I will pose a question and see what you all have to say.  We use
an SN75LBC185, which is a pretty simple littls RS485 driver/receiver with
some built in static protection.  The, we put a TVS (1.5KE6.8CA) from each
data port (A and B) to ground for added protection.  Yet we are losing the
drivers fairly regularly.  I have seen other devices with similar 485 chips,
but they are soldered to the board which tells me they dont have to replace
them very often.  Our are in sockets for ease of replacement.  I have read
Nationals AN-1057 Ten Ways to Bulletproff RS-485 Interfaces and TI's SLLA036
EIA-485 design notes.  I think I have complied with all their
recomendations.  Though it should not matter for static I will tell you our
data rate is about 5K.  The wire lengths get sort of long, but never over
the 500 Meter limit.

       I was hoping some of you could tell me how you protect your 485
network or what you have seen done.

Thanks

Shawn

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2000\08\10@152345 by M. Adam Davis

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I remember reading a story of someone who had those same exact issues.  They
started using a specific maxim driver/receiver which had quite a bit more
protection than 'normal' and their problems went away.

You might try other drivers, they really aren't all the same.  And some
semiconductors are going to die no matter how much you protect them...

I would start replacing failing chips with other brands or more robust ones and
see which chips you end up not replacing any longer.

-Adam

Shawn Yates wrote:
{Quote hidden}

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2000\08\10@153720 by Shawn Yates

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I will look into that.  The part I use now is from TI and is supposed to be
rated at:

+- 15kV Human Body
+- 8kV IEC1000-4-2 contact
+- 15kV IEC100-4-2 Air Gap

They are also 1/2 LU devices so I can use 60 on a group.

Shawn

{Original Message removed}

2000\08\10@184244 by David Huisman

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Shawn,

Can you describe more fully the output configuration.
Do you use the bias resistors at one end and terminations at the other with
nodes between not terminated.

Also, do you use a 100R in series with each ground ?

Regards

David Huisman
http://www.orbitcoms.com

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2000\08\10@191934 by Alan B. Pearce

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>I was hoping some of you could tell me how you protect your 485
>network or what you have seen done.

what sort of environment is the cabling run through? One experience a work
colleague had with a 20ma loop which kept blowing its driver comes to mind. The
wire was shielded twisted pair, but the shield was not connected. When connected
to ground at the driver end, the problems went away. The environment was one
that should not have been terribly noisy either.

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'[EE]: RE: RS485 -vs- Static'
2000\08\11@085403 by Shawn Yates

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David,

       The aires are run in a daisy chain (ie not at star config).
       The two far ends have a 100ohm resistor across the lines

       100R in series with what?  the TVS? No, the TVS's are connected
directly from the data line to ground.  Would a resistor in series with the
TVS act as a current sink when the voltage goes over the TVS rating?  the
TVS is rated at about 200 amps or 1500 watts so I would need one heck of a
resistor to be able to send that throught.

       Please let me know what you mean and why you asked.  I am open to
trying new things.

Shawn

{Original Message removed}

'[EE]: RS485 -vs- Static'
2000\08\11@090719 by Shawn Yates

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Alan,

       The enbironment is a typical office building.  The data pair is part
of a cable which has 2 22guage conductors used for data and 2 18 guage
conductors used for 12Vdc power.  All the conductors are stranded and the
whole cable is not sheilded.  I though sheilds would reduce your maximum
distance and maximum distance right?  Though if that would make it more
lightining resistant (never say immune!!) then we would have to make the
trade off.

       You said they connected the ground at the driver end.  I have a
multi point network and from what I know of sheilds you only supposed to
connect one end of the sheild.  Which end are you calling the 'driver' end?
In a multi point network would you suggest connecting all the shields to
each other and grounding it at the center most node to provide two short
drain paths?

       Again, any input is appreciated.

Shawn

{Original Message removed}

2000\08\11@093927 by Alan B. Pearce

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In a typical office building I think you will find that shielded wire is a
"must". Just think of all those fluorescent lights creating mains borne noise,
lift motors with chunky relays turning currents off and on through cables which
are probably never far from yours, especially if going between floors. These can
all induce potentially fatal spikes into devices which are directly connected to
the data lines. Do remember that a spike may not kill a device immediately, it
may die some time later, or it may be the cumulative effect of several spikes
which cause it to die.

On one occasion I had a number of VT100 terminals spread around two floors of an
office building, and I was using 19200 baud. The wires all had the screens
connected at the modem/multiplexor end, and left open at the terminal end. these
ran for several years without any communication errors ever being reported.

For your situation, as you have the power coming down the same line, I would
suggest connecting the screen to the box through a low value resistor, say 100
ohms, to stop the screen getting too great a potential away from the internal
wiring, but the resistor is a large enough value to be a significant impedance
to any earth loop currents so they dont interfere.

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2000\08\11@100405 by Shawn Yates

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Alan

       Thank you very much for the input.  I will try some of that and see
what happens.  Though I will say that the environment noise does not seem to
cause a problem, but you can not tell if it is weakening the drivers like
you say.  But failure is has been absolutely linked with lightning activity
in the vicinity of the building.

Shawn

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2000\08\11@100610 by W. K. Brown

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I agree that 4 wires and a shield are best.
Some decisions though :
1) Where to connect the 485 common (shield or the DC common).
2) Should the shield be over the whole cable or just the 485 pair?
Any Help here? (Let's cut to the chase!)
Regards,
Keith

       {Original Message removed}

2000\08\11@101416 by Shawn Yates

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Keith,

       What do you mean by the 485 common?

       I have a DC common which supplies power to the board and thus is the
common for the 485 chip istelf.  Than I have the proposed shield that, to
the best of my knowledge, should only be grounded at one single point.

Shawn

{Original Message removed}

2000\08\11@111535 by W. K. Brown

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part 1 1831 bytes content-type:text/plain;Let's say there are three wires for 485 to work (let's call them A B and
Common).
(This means the 485 is not AC coupled.)

The DC common is a logical place to connect Common at the host end.
Where do you connect the common to the cable shield (if you have one)?
To repeat in other terms, what should be shielded?

<<485-Com.gif>>
I have included the 485-Com.gif. to show some options.
I am not sure of the rules on attachments. I hope this is OK for those
interested.
The diagram does not show termination of transient protection.
Regards,
Keith
       {Original Message removed}
part 2 6436 bytes content-type:image/gif; (decode)


part 3 142 bytes
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2000\08\11@113626 by Shawn Yates

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hhmmm

       We happen to use the same power supply for all of our nodes, so they
all have the same DC ground.  But as I understood the 485 interface, its a 2
wire interface (A and B).  The fact the we have a common power supply is
almost concidental.



{Original Message removed}

2000\08\11@145735 by Shawn Yates

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hhmmm

       We happen to use the same power supply for all of our nodes, so they
all have the same DC ground.  But as I understood the 485 interface, its a 2
wire interface (A and B).  The fact the we have a common power supply is
almost concidental.



{Original Message removed}

2000\08\13@103404 by Nikolai Golovchenko

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I think this story was 'The Art and Science of RS-485':

http://www.chipcenter.com/circuitcellar/july99/c79bppdf.pdf

What cured their problems with static was MAX3095.

Anyway, it's an interesting article.

Nikolai

---- Original Message ----
From: M. Adam Davis
Sent: Thursday, August 10, 2000 22:22:31
 To: spam_OUTPICLISTTakeThisOuTspamMITVMA.MIT.EDU
Subj: [EE]: RS485 -vs- Static

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