I2C over 30.5 meter wire

256

01:53:20

30.5 meters (100ft)  of a CAT5 twisted pair (which I'm familiar with) has about 50pF per meter which adds up to 1525pF. So how fast can the I2C/TWI clock run it and what are the risk? 

Note: This was a question on the Arduino forum.

The first thing I notice while playing with the simulation is a current spike when the open drain pulls down (I used an NPN BJT to keep the simulation simple, but the MCU has an N-CH MOSFET that pulls down). The current spike can be limited to something bellow the absolute max  value (e.g. <40mA on 328 AVR) if >125 Ohm is between the MCU and I2C bus. I would try 180 Ohm to limit current and a 3.3k bus pull-up which gives a detectable 312mV low level and looks like the clock could go a little faster than 10kHz.  

published 8 years ago

flowDAQ

8 years ago

50 pF per meter, I had written per foot.

zorgrian

8 years ago

I will investigate this but at first sight I feel that twisted pair is best used in a balanced mode. This would imply the best way to get serial information to transfer over twisted pair might be to use diferencial op amps. Possibly in both directions?

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