From: Mike Fahrion
Date: December 6, 2004
Three Heinekens into my Red-Eye back to Chicago, after my
expensive emergency trouble-shooting gig
Dear Friend &
Subscriber:
Dang, I wish everyone
who ever touched a serial system subscribed to my newsletter.
Yeah, I know, I
go off on the occasional rant or distraction, but there’s
good information in these emails.
What I’m
about to tell you has been beaten into me over the years
through dozens of disasters and plenty o’ field applications.
And plenty of desperate support calls from engineers in
some God-forsaken place who really needed to get their system
up and running.
Some of those engineers
even dig up my home number and catch me nights and weekends.
(Well hey, if getting the bits and bytes flowing means that
they can catch that Saturday flight home to see the family
for the weekend, I don’t blame ‘em one bit.
Besides, I’m a sucker for troubleshooting.)
My panache for
troubleshooting data communications systems means I sometimes
get sucked into serious field work. I’m writing you
this note from the airplane on a red-eye return from one
of those rescue-work field visits at this very moment.
If only they had
listened…
Over the past decade
or so, I’ve written dozens of articles, app-notes
and tech guides on how to build a robust RS-485 communications
system. As I suspected, not all of you were paying attention.
If this particular customer had been listening they could
have saved hundreds of thousands of dollars in downtime
costs and I wouldn’t be writing this from 33,000 feet
up.
No, I’d be
home sleeping right now, warm, toasty and oblivious to the
world of ground loops, data collisions and improperly wired
RS-485 systems.
Hey, I enjoy panic
buttons and troubleshooting expeditions just as much as
the next guy. Making systems work does get my blood pumping,
and I get to escape from my Dilbert Cube. It’s fun
to parachute in as the “expert swat team” to
help save the day. Knowing they’re paying last-minute
air fare makes me feel important. (It smoothes over the
insecurities I’ve got left over from high school.)
And, the fact that
I have an entire exit row to myself and there appears to
be no danger of depleting the plane’s supply of Heineken
has me feeling pretty good about this particular visit right
now.
But that doesn’t
change the fact that if this customer had paid attention
to three basic principles of RS-485, none of us would have
been in this situation.
I don’t want
to spotlight this particular customer in a negative way
(ain’t gonna bite the hand that feeds me). But you
still deserve to know what they did wrong.
So here’s
the scoop.
This multi-node
RS-485 system covers a distance of a couple hundred meters,
indoors and outdoors, linking equipment from multiple vendors.
To add a bit of complication, this system is replicated
several hundred times across Asia. Lots of high tech, high
dollar equipment used in this automated service facility
came to a screeching halt when the RS-485 system failed.
Why did it fail?
3 simple reasons:
- There’s
no such thing as “2-wire RS-485.” The next
time you hear someone describe their RS-485 system as
“2-wire”, please do me a favor and give them
a poke in the ribs. RS-485 requires a differential pair
and a signal return line. Sure, it seems like it works
without the ground, but I can assure you that your system
is in peril until you connect that dedicated signal ground
connection. With this customer, I was easily able to demonstrate
that the noise level dropped dramatically when we added
a signal ground connection. Want
more info?
- Unless your
system happens to occupy some sort of Utopian world of
data communications, you need to isolate each RS-485 node.
Sure it costs a few bucks more for an isolated serial
converter such as 485LDRC9,
or an external RS-485 isolator like 485OP.
This particular customer reduced the cost of each $2,000,000
installation by almost $50 by choosing a non-isolated
model. An educated guess is that this decision cost them
over $250,000 in downtime. Non-isolated converters are
typically just fine for simple field service and benchtop
applications. But a real-world installation like this
one? No sirreee. I assure you that the money you spend
on our best-in-class isolated RS-485 converter is cheaper
than bringing yours-truly in on an all expenses paid trip
to save your bacon.
- Termination
and biasing – I can’t even guess how much
I’ve written and evangelized on this particular
topic. Every once in a while I read a competitor’s
well intentioned but misinformed newsletter telling me
how every RS-485 system should include termination resistors.
Bullhockey! This isn’t a matter of opinion, folks,
it’s simple laws of physics. 9 out of 10 RS-485
systems have no need for termination resistors to be installed.
IF you happen to be running a particularly high speed
system for very long distances, you may be one of the
rare few that qualifies. You can read tons of info on
this on our web
site. But, bottom line is don’t ever, ever,
under any circumstances, even consider, in your wildest
dreams, installing a termination resistor without changing
the bias resistors. So, remember this one lesson. If you
terminate, you need a soldering iron to modify the equipment.
If you don’t have a soldering iron handy, don’t
terminate. Unfortunately there are many makers of equipment
that happen to have an RS-485 port, and they install the
termination for you. As much as I’ve evangelized
on this topic I haven’t been successful in stopping
this reckless practice. So that means that you’re
gonna need a soldering iron to fix the biasing. Our support
staff will be happy to help you figure this out, and if
it happens to be our equipment, we’ll be happy to
make this change for you right at the factory.
There you have
it. Three quick lessons. Follow these three pieces of advice
and you’ll never be in a situation where your project
is bleeding tens of thousands of dollars a day, and I won’t
be back on this airplane in the wee hours depleting the
worlds supply of Heineken.
Happy Connections.
Mike Fahrion
