How to use a LAN cable for I2C and SPI?

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How to use a LAN cable for I2C and SPI?

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mark5
Contributor I

I have a situation where I need to have an I2C master drive multiple I2C slaves over distances of 1 foot to 50 feet. Specifically I want to use CAT-6 shielded cable to run the following:

  • Power: 12v, GND
  • I2C: SDA, SCL
  • SPI: MOSI, MISO, CLK, CS

I'm currently looking at the P82B715 for the I2C bus, but I don't have a solution for the SPI bus.  

Also, on the I2C bus, the Master operates at 3.3v while the Slaves operate at 5v. 

Questions:

  1. Is there a single chip (or two chip) solution to support the data lines at this distance range?
  2. If we use the P82B715 can the Master and Slave operate at different voltages by just having Vcc-master at 3.3v and Vcc-slave at 5v? 

Thanks for the help!

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diego_charles
NXP TechSupport
NXP TechSupport

Hello, mark@paradoxrooms.com

For long distances, the general approach is to use the RS-485 protocol. You could take a look out Bridges | Protocol Bridge | Bridge IC Solutions | NXP portfolio to find a chip to translate SPI/I2C to RS-485 and vice-versa.

Have a nice day. 

Best regards Diego.

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bobpaddock
Senior Contributor III

Personally I would never pursue this path.
This is the kind of thing RS485 was meant for.
That being said:

"If we use the P82B715 can the Master and Slave operate at different voltages by just having Vcc-master at 3.3v and Vcc-slave at 5v? "

Section 9.1 of the data sheet addresses that:
" the logic-signal levels at Sx/Lx are independent of the supply voltage. They remain at the levels presented to the chip by the attached devices "  So it depends on if your devices are 5V tolerate, or will work with a 3.3V input threshold with a Vcc of 5V.

I would also replace the passive pull-up resistors with active current sources.

I don't see any reason, other than size and power, that the P82B715 could not also be used for SPI if there are no speed issues.

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