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'[PICLIST] [EE][PIC] Long distance I2C'
2000\10\03@115227 by Jason Wolfson

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I'm trying to run a remote I2C temp sensor over phone
cable 50m away into a PIC. It needs to be very reliable and cheap.

My first assumptions are: Run slow out of the PIC since I can
control the bit rate and therefore reduce effect of slew due to cable
capacitance.

Second, use a driver of some sort out at the I2C sensor to isolate
the I2C sensor chip from cable capacitance/impedance and protect from
induced transients.

Has anyone done this and know all the "gotcha's"?
Easiest way of implementing a driver/receiver circuit?
Transient protection? etc....


thanks,

Jason Wolfson

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2000\10\03@121755 by Don Hyde

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I'd be inclined to use an 8-pin PIC as the buffer, and let it to convert to
a different protocol (async serial as in RS-232) that's better suited to the
distance.  At a modest baud rate, you can probably drive and receive the
data with bare PIC I/O's with a little R and C on the driving end to keep
the spurious emissions down.

> {Original Message removed}

2000\10\03@125558 by Andy Howard

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----- Original Message -----
From: "Jason Wolfson" <jasonspamKILLspamLIPIDEX.COM>
To: <.....PICLISTKILLspamspam.....MITVMA.MIT.EDU>
Sent: Tuesday, October 03, 2000 4:51 PM
Subject: [EE][PIC] Long distance I2C


{Quote hidden}

You might like to look at the Philips I2C extender chip P82B715 and
http://www.semiconductors.com/acrobat/applicationnotes/AN444.pdf "Using the
P82B715 I 2 C extender on long cables"

From the summary:

The P82B715 I 2 C Buffer was designed to
extend the range of the local I 2 C bus out to
50 Meters. This application note describes
the results of testing the buffer on several
different types of cables to determine the
maximum operating distances possible. The
results are summarized in a table for easy
reference.

and www.semiconductors.com/acrobat/applicationnotes/AN452.pdf
Application Note AN452 "One Mile Long I2C Communication using the P82B715"

LONG DISTANCE I C DESIGN CONSIDERATIONS
Summary
I 2 C communication is possible over very long distances, but at
greatly reduced data rates. Tests were made and results shown for
cable lengths exceeding one mile. This report explores the design
considerations and trade-off needed to implement a successful long
distance I 2 C communication system.


Even if you don't use their chip there's quite a bit of useful information
there.

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2000\10\07@122634 by hgraf

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> I'm trying to run a remote I2C temp sensor over phone
> cable 50m away into a PIC. It needs to be very reliable and cheap.
>
> My first assumptions are: Run slow out of the PIC since I can
> control the bit rate and therefore reduce effect of slew due to cable
> capacitance.

       That's a good start, but remember you are dealing with a pretty nasty
transmission line, you'll have alot of reflection going on. I run about a
20m length using standard telephone cable. It works OK and is quite
reliable. What I have done is I read each device 10 times, drop the lowest
and highest result, and average the rest (it is also a temp sensor, the NS
LM75 in my case). Before I allow the result I check the range, if it is more
than a few degrees I try again. It works and seems very stable although I
don't know how many times it has had to retry. Good luck and let us know how
it turns out! :) TTYL

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