AVR-Chat

You are correct that twisted pair will only help if you're running a differential signal.    In fact, running SDA and SCL on the same pair could, in theory, make the signal worse.

Why bother with Cat5?  I'd probably use a ribbon cable.  You can tie every other conductor to ground if it makes you feel better (the electrons won't know either way).  I'd just hook it up and try it. 

Regards,



Tim Gilbert
JEM Innovation Inc.
303-926-9053 (office)
303-437-4342 (cell)
720-890-8582 (fax)
www.jeminnovation.com
www.pdksolutions.com

  ----- Original Message ----- 
  From: Steve Hodge 
  To: AVR-Chat@yahoogroups.com 
  Sent: Monday, March 30, 2009 7:04 PM
  Subject: [AVR-Chat] i2c and twisted pair


  I would like to use standard shielded Cat 5/6 cable to carry I2C signals
  between boards, i.e., between microcontrollers on separate boards.
  Distances are not the question here, because if they end up a factor I'll
  just use a PCA9517 buffer or something similar, but should someone ask they
  will be of the order of a few to several feet.

  The question is rather what conductors of the four twisted pairs should I
  use? At first, without thinking about it much I thought just use a single
  twisted pair for both SCL & SDA, but after thinking about it further for a
  few minutes, I now think that might be a bad idea, because SCL & SDA do not
  comprise a single differential signal.

  So now I'm thinking they should be on separate twisted pairs. Perhaps with
  GND as the other half of each pair.

  I'd appreciate any help on this. 

  Thanks, Steve


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