PCF8574 max distance to Arduino board

Using Arduino Networking, Protocols, and Devices
1

Hi All.

Hope I got the right forum category here.

How far can a PCF8574 I2C expansion boards be away from the Arduino board for communication to work? My idea is to connect light multiple switches (Or other goodies) to the expansion board which communicates with an Arduino nano with max 25-30m of 2x2x0.8 twisted KNX cable for a home automation system. Voltage supply is not the issue as I will be using >12v with regulators at each expansion board.

example board: https://www.banggood.com/20pcs-PCF8574-PCF8574T-IO-For-I2C-IIC-Port-Interface-Support-Cascading-Extended-Module-Expansion-Board-High-L-p-1463716.html

Thanks
Mike

2

Hi,
I2C is not designed for communication other than at PCB level.
25 to 30m is not what it is designed for.

You could look at RS485 comms which would work well with your twisted pair.
You would need a controller at each point, a Nano or ProMini for example.
https://create.arduino.cc/projecthub/moreirarobotics/how-to-communicate-3-or-32-arduinos-via-rs-485-36888f

Tom.... :smiley: :+1: :coffee: :australia:

3

Thanks Tom.

RS485 combined with a controller ... would work but I might run into new issues, small space and issues with multiple devices trying to communicate at the same time.

I did however find articles this morning on I2C range extenders using P82b715 or an Adafruit alternative. As I do have about 40+ cases where I could need this, I might design a board which has the I2C range extender and PCF8574 combined on 1 board.

4

Hi,

That wouldn't happen as you have a master the polls the slaves, the slaves only respond when called.

It will be interesting to see how an extender goes, you will need to choose your connecting cable with possible shielding as I2C is not a balanced wire system.

Tom... :smiley: :+1: :coffee: :australia:

5

True. But I have been able to use a 1602 LCD over 20+ meter twisted pair cable without problems (just testing it). But I would not recommend it.

6

Thanks guys.

Yes, definitely will be using a shielded bus cable.

Just for those interested in creating range extenders, I did find 3 reference designs using PCF8574

https://www.arrow.com/en/reference-designs/application-circuit-for-the-p82b715-i2c-bus-extender-for-i2c-bus-extenders-interfacing-standard-i2c-bus-signals-to-twisted-pair-cables-up-to-30m-long/afdcd2c22172a52eb0f02d0e34fdf366

https://www.arrow.com/en/reference-designs/application-circuit-for-the-p82b715-i2c-bus-extender-for-i2c-bus-extender-paired-with-2-wire-bus-buffer-provides-bus-release-on-vcc-failure/0ba9a4c139ae8c2367241b2bda85ecec

https://www.arrow.com/en/reference-designs/application-circuit-for-the-p82b715-i2c-bus-extender-for-i2c-bus-extender-driving-twisted-pair-cables-for-very-large-signal-distribution-system/be18a0e5a1402206d32a25934ac835d1

7

Hi,
This may help, re I2C extender;

Tom... :smiley: :+1: :coffee: :australia:

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