1-wire network cabling question
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I'm currently working on a design to measure temperature in a forest from
the ground to the top of the tree canopy at 10 to 15 ft. intervals.
The design will use Peter Anderson's (and his students) Serial 512
Temperature Measurement System (http://www.phanderson.com/t512.html).
This uses DS1820's along a 1-wire network that feeds into a custom
programmed PIC that outputs the measuremensts through a serial port.
This subsystem will be connected to a stamp that acts as a datalogger.
My two questions regard running a twisted pair up into the air 100' or
150'. I'm concerned about static electricity and lightning.
1) Will I need a shielded twisted pair cable? If yes, then should the
shield be wired to earth gnd. or supply gnd? Or both and coupled in some
way to supply ground.
2) Seems I should connect the Stamp and Pic serial connections (9600
baud) via an optocoupler to isloate the Stamp. Anybody have any
suggestions for choice of optocoupler and schematic to do so.
Any other considerations I didn't think of - please give me a heads-up.
Thanks, Pete.
________________________________________________________
Peter W. Houlihan, (413) 538-3091, phouliha@m...
Mellon Postdoctoral Fellow, Center for Environmental Literacy
Mount Holyoke College, South Hadley, MA 01075
Visit our website at:
http://www.mtholyoke.edu/proj/cel/index.html
________________________________________________________
the ground to the top of the tree canopy at 10 to 15 ft. intervals.
The design will use Peter Anderson's (and his students) Serial 512
Temperature Measurement System (http://www.phanderson.com/t512.html).
This uses DS1820's along a 1-wire network that feeds into a custom
programmed PIC that outputs the measuremensts through a serial port.
This subsystem will be connected to a stamp that acts as a datalogger.
My two questions regard running a twisted pair up into the air 100' or
150'. I'm concerned about static electricity and lightning.
1) Will I need a shielded twisted pair cable? If yes, then should the
shield be wired to earth gnd. or supply gnd? Or both and coupled in some
way to supply ground.
2) Seems I should connect the Stamp and Pic serial connections (9600
baud) via an optocoupler to isloate the Stamp. Anybody have any
suggestions for choice of optocoupler and schematic to do so.
Any other considerations I didn't think of - please give me a heads-up.
Thanks, Pete.
________________________________________________________
Peter W. Houlihan, (413) 538-3091, phouliha@m...
Mellon Postdoctoral Fellow, Center for Environmental Literacy
Mount Holyoke College, South Hadley, MA 01075
Visit our website at:
http://www.mtholyoke.edu/proj/cel/index.html
________________________________________________________
Comments
anything from a lightning STRIKE, but it will bleed off the
everpresent static charges that build up on tall structures.
NOTHING will protect your system from a direct hit.
Having the powersupply (batteries?) tied to earth ground probably
isn't necessary or even desired.
At 11:21 AM -0400 on 5/16/02, Peter W Houlihan wrote
>I'm currently working on a design to measure temperature in a forest from
>the ground to the top of the tree canopy at 10 to 15 ft. intervals.
>
>The design will use Peter Anderson's (and his students) Serial 512
>Temperature Measurement System (http://www.phanderson.com/t512.html).
>This uses DS1820's along a 1-wire network that feeds into a custom
>programmed PIC that outputs the measuremensts through a serial port.
>This subsystem will be connected to a stamp that acts as a datalogger.
>
>My two questions regard running a twisted pair up into the air 100' or
>150'. I'm concerned about static electricity and lightning.
>
>1) Will I need a shielded twisted pair cable? If yes, then should the
>shield be wired to earth gnd. or supply gnd? Or both and coupled in some
>way to supply ground.
>
>2) Seems I should connect the Stamp and Pic serial connections (9600
>baud) via an optocoupler to isloate the Stamp. Anybody have any
>suggestions for choice of optocoupler and schematic to do so.
>
>Any other considerations I didn't think of - please give me a heads-up.
>
>Thanks, Pete.
--
.-. .-. .-. .-. .-. .-. .-. .-. .-. .-
\ / \ / \ N / \ C / \ S / \ S / \ M / \ / \ /
`-' `-' `-' `-' `-' `-' `-' `-' `-'
Chuck Britton Education is what is left when
britton@n... you have forgotten everything
North Carolina School of Science & Math you learned in school.
(919) 286-3366 x224 Albert Einstein, 1936
>the ground to the top of the tree canopy at 10 to 15 ft. intervals.
>
>The design will use Peter Anderson's (and his students) Serial 512
>Temperature Measurement System (http://www.phanderson.com/t512.html).
>This uses DS1820's along a 1-wire network that feeds into a custom
>programmed PIC that outputs the measuremensts through a serial port.
>This subsystem will be connected to a stamp that acts as a datalogger.
>
>
>My two questions regard running a twisted pair up into the air 100' or
>150'. I'm concerned about static electricity and lightning.
>
>1) Will I need a shielded twisted pair cable? If yes, then should the
>shield be wired to earth gnd. or supply gnd? Or both and coupled in some
>way to supply ground.
I agree with Chuck that the shield should be connected to a good
earth ground. Not much can be done about direct hits, but even
distant lightning strikes create huge electromagnetic events. If
wires go off in different directions into the trees, you will have a
huge antenna! The shield bleeds the induced currents safely (we hope)
off to ground. If you can, put small resistors and ferrite beads in
series with the signal and signal return lines (but not too much, see
below!). They will limit the magnitude of surges that spill over to
the inner conductors and thence into the chips. If you do connect
the system ground to the shield, do so at only one point at the
center of the star. I'm not sure you need to connect the earth and
supply grounds, but it might help if there turns out to be a high
error rate due to noise pickup. The shield acts like a big capacitor
plate in relation to the signal wires inside, and if there are
voltages on the shield, they are coupled as a common mode signal into
the signal wires. That could spell trouble if the noise level is
volts. Connecting the grounds (at one point near the PIC) could
clamp the common mode signal at that point. Magic. Sometimes it
works, sometimes not.
>2) Seems I should connect the Stamp and Pic serial connections (9600
>baud) via an optocoupler to isloate the Stamp. Anybody have any
>suggestions for choice of optocoupler and schematic to do so.
That would mean separate power supplies for the two systems (or an
inefficient power isolator). If you do have separate power supplies,
it is a simple matter to optoisolate. The transmit side drives the
led in an optoisolator (with a ~500 ohm series resistor), while the
other side of the optoisolator is a transistor or logic circuit
(perhaps requiring a pullup resistor) that provides a voltage to the
other side. Same thing for the other direction. The Stamp and PIC
are quite happy with standard logic levels. The optical isolation
would be essential if your Stamp is connected to the phone lines or
to the power mains, because those services are like the other part of
an even huger antenna system, and surges can blast right through on
the easiest path. It is less of a problem if your stamp is standing
by itself.
>
>Any other considerations I didn't think of - please give me a heads-up.
Remember that the one-wire protocol runs at a speed equivalent to
approximately 10k baud, with acknowledge pulses of ~60 microseconds,
and it is an open drain protocol. All that makes it very subject to
capacitance of cables, which smooths down the signal edges. Any
protection resistors you might install in series with the lines
degrade the response ever further. There are some clever "repeater"
circuits that you can put along the line to regenerate the signal.
Maybe some of the people more familiar with the one-wire stuff can
chime in with more detail about the current state of the art.
>
>Thanks, Pete.
>________________________________________________________
>Peter W. Houlihan, (413) 538-3091, phouliha@m...
>Mellon Postdoctoral Fellow, Center for Environmental Literacy
>Mount Holyoke College, South Hadley, MA 01075
>
>Visit our website at:
>http://www.mtholyoke.edu/proj/cel/index.html
>________________________________________________________
Your concerns about static (lightning) discharges are well taken.
Lightning, as you know, can travel in both directions: ground to cloud and
cloud to ground. I wonder if it might be better to
try and divert any static charges away from your twisted pair sensor line.
This
could perhaps be done by grounding a small cable and running it up a
an adjacent tree, positioning its upper end (terminated with a sharp point),
as high as possible above the tree canopy. This
would provide a low resistance pathway for any static electricity build up:
in effect,
it will serve as a lighting rod and hopefully bleed off or ground down, any
rising
electrical charges
before they become large enough to ionize air. Grounding your twisted pair
line - even if it
is shielded - should be avoided otherwise it will simply serve as another
lightning rod. Breaking up
the 150 foot line with optoisolators would possibly provide some additional
protection but would complicate the design. There are devices to "sniff out"
building
electric charges and these could be used to warn of impending lightning
strikes.
A less elegant but probably much safer option, would be to simply monitor
the weather closely
and take down your sensor array whenever storms are forecast.
Just some thoughts,
Sean
Original Message
From: "Peter W Houlihan" <phouliha@m...>
To: <basicstamps@yahoogroups.com>
Sent: Thursday, May 16, 2002 11:21 AM
Subject: [noparse][[/noparse]basicstamps] 1-WIRE NETWORK CABLING QUESTION
>
> I'm currently working on a design to measure temperature in a forest from
> the ground to the top of the tree canopy at 10 to 15 ft. intervals.
>
>
> My two questions regard running a twisted pair up into the air 100' or
> 150'. I'm concerned about static electricity and lightning.
>
> 1) Will I need a shielded twisted pair cable? If yes, then should the
> shield be wired to earth gnd. or supply gnd? Or both and coupled in some
> way to supply ground.
>
> 2) Seems I should connect the Stamp and Pic serial connections (9600
> baud) via an optocoupler to isloate the Stamp. Anybody have any
> suggestions for choice of optocoupler and schematic to do so.
>
> Any other considerations I didn't think of - please give me a heads-up.
>
> Thanks, Pete.
> ________________________________________________________
> Peter W. Houlihan, (413) 538-3091, phouliha@m...
> Mellon Postdoctoral Fellow, Center for Environmental Literacy
> Mount Holyoke College, South Hadley, MA 01075
>
> Visit our website at:
> http://www.mtholyoke.edu/proj/cel/index.html
> ________________________________________________________
>
>
>
>
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