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Author Topic: I2C over RS422 using MAX485, will this circuit work?  (Read 385 times)
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Los Angeles
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I would like to use I2C to communicate between Arduino controllers using the MAX485 to convert the signal to RS422.

I drew a circuit of how I think it would work.   Does this look correct?

https://www.dropbox.com/s/31ir8kst2kcalkx/i2c_over_rs422.JPG

IC1 is a Texas Instruments SN74AHC125N, Buffers & Line Drivers Tri-State Quad Bus.
HiZ = High Impedance.

The same circuit would be used for the SDA and SCL.

The cable will have 2 twisted pairs.  This should extend the I2C range from 10 feet to 4900 feet.

Eric
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As the receiver is always enabled I would say that this circuit drives itself as soon as SDA went low once.

Do you know that I2C extenders exist that allows I2C bus lengths of over 50m? (http://www.nxp.com/documents/data_sheet/P82B715.pdf)
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nr Bundaberg, Australia
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As the receiver is always enabled I would say that this circuit drives itself as soon as SDA went low once.
Yes I think that's right, looks like a feedback loop to me.

Any reason not to use the I2C buffer chips? If you need > 50m there are versions that split the SDA and SCL signals into separate lines so you can use RS-485 or other drivers, the PCA9600 for example.

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Rob
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Rob Gray aka the GRAYnomad www.robgray.com

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Thanks.   The I2C extender looks like it should work.
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