Re: Lightning immunity for a computer connected to a weather station?
From: w_tom (w_tom1_at_hotmail.com)
Date: 12/13/04
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Date: Mon, 13 Dec 2004 00:35:12 -0500
If distance to rooftop remains short, then RS-232 is
sufficient assuming other external noise sources are not
strong. Even with shielded and twisted pair wires only 200
feet and using a baud rate of 9600 baud, we suffered
unacceptable noise problems.
Second, rooftop weather station and computer must be powered
from a power source sharing a common safety ground. This also
made necessary because RS-232 is common mode signalling - to
avoid ground loops. Voltage differences between computer
power source and weather station power source can create noise
in RS-232 signalling. Again, this caution only so you avoid
surprise problems. Simple ICs can interface computer serial
port to RS-485 cable meaning your choice was not 'fixed' by
the embedded computer.
Original problem is about protecting a computer from
lightning. That means RS-232 cable cannot connect direct from
computer to weather station. Cable from weather station to
computer must first connect down to earth ground. Notice how
CATV service or telephone connects to building. They do same
so that lightning strikes are made irrelevant.
If the path down to single point ground increases that
distance beyond the 100 feet, then consider RS-485. RS-485
only mentioned because that RS-232 cable may need be longer
than you originally thought. Make plans so that RS-485 can be
easily implemented later as you learn from experience (IOW run
a 4 wire cable - not a 3 wire).
Important facts were provided by but a few others. For
example, opto-isolation may only provide 2000 or 4000 volts
isolation. Lightning will overwhelm that protection - again -
if you don't provide lightning a better path to earth
ground. Ben Franklin's 1752 demonstration is essential for
understanding lightning protection. Many even hype surge
protector and opto-isolators without first learning how damage
happens.
John Popelish provided good solutions. No matter what you
do, without that single point earth ground, then no protection
exists. RS-232 cable first connects to single point earth
ground before rising back up to connect to embedded computer.
Overhead protection such as a lightning rod or catenary wire
can also connect that direct strike to earth ground. Why? So
that the direct lightning strike does not find earth ground
destructively via the computer.
This is what you want to avoid. As Ben Franklin
demonstrated in 1752, if lightning is provided a shorter, more
conductive path to earth, then lightning will not harm church
steeples - or your embedded computer. Every solution requires
that earth ground.
mark thomas wrote:
> I'm using an embedded serial driver, so I am limited by 50-100feet,
> which is more than enough to reach my roof from my computer...
>
> Anyways I don't have a choice, I purchased a microcontroller board
> that uses RS232, not RS485. And, this isn't really what I was
> asking.
- Next message: John Popelish: "Re: Lightning immunity for a computer connected to a weather station?"
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