Multiple BOB-14589 on one line

Qwiic System
1

Hi,

I bought 8pcs of the BOB-14589 dev boards.

made a simple two-way board test with success. :slight_smile:

now, getting more complex, I’m trying to hook it up as seen in page 12 of the spec sheet; https://cdn.sparkfun.com/assets/a/5/1/3/6/PCA9615.pdf

I removed the " termination resistance " on one board as mentioned in the spec sheet and also schematic

https://cdn.sparkfun.com/assets/c/4/7/0 … _Qwiic.pdf

this will be my middle

https://i.imgur.com/j78t3G1.png

https://i.imgur.com/kX0AC8V.png

can someone please let me know if it’s normal to have i2c pull-ups resistor on each boards ?

let say having four boards, exactly like the drawing on spec sheet page 12. I think it makes no sense to have pull-ups on each… or maybe the PCA6915 is doing something to the i2c line that I’m not understanding well.

also, I’ve bought 4 https://www.sparkfun.com/products/13021

and I realize that all of them have pin 1 & 2 and pin 7 & 8 ! --is this normal ?

I guess I’ll have to use; https://www.sparkfun.com/products/716 instead…

but I really think the pins should not be shorted out. :S

2

Hi gmtl,

I am not sure we have much information to go on regarding stringing four or more of the Differential I2C Breakouts but we will look into that and get back to you with any information we can dig up. You will most likely need to disable the I2C pull-up resistors as you add more of these to the bus so your master can still pull those lines low. As you add more, the resistance will hit a point where it is too high for the master to effectively toggle those lines for sending data.

This is just a quick response in relation to your final question regarding the MagJack Breakouts you are trying to use. Those breakouts are designed for ethernet only and are not a standard RJ45 connector like what is populated on the Differential I2C Breakouts. The pins are netted together internally like the [MagJack Datasheet shows and it will not work for your application. You are correct in your thinking. You will need to use the [RJ45 Breakout and [connectors. If you have some of these you do not wish to use and have not soldered to yet, please submit an RMA on [this page.

I hope this helps explain any confusion caused by the MagJack Breakouts and gives some insight into how you can use multiple I2C Differential Breakouts on the same bus.](Return Policy - SparkFun Electronics)](RJ45 8-Pin Connector - PRT-00643 - SparkFun Electronics)](SparkFun RJ45 Breakout - BOB-00716 - SparkFun Electronics)](https://www.sparkfun.com/datasheets/Prototyping/MagJack.pdf)

3

Thanks Mark,

Yeah I found out just after posting that the pins are tied together. I went ahead and bought the https://www.sparkfun.com/products/716 at my favorite hobby store witch happen to have inventory! :slight_smile: no RMA needed

corrected my board and came up with this setup; (as a proof of concept)

https://i.imgur.com/nL117E3.png

https://i.imgur.com/QFFJYzz.png

will not test and let you know how it goes with four BOB-14589 on one line as a starting point.

if your or your team finds out suggestions about this setup please let me know !

4

@gmtl any followup?

5

I think the problem here will be too many termination resistors. Each BOB-14589 has a set. Only the first and last should in a multidrop arrangement. It would be nice if the breakout included solder pads and traces to cut to enable/disable the termination resistors, similar the how the I2C pullup resistors are set up.

Also, the I2C pullup resistors are needed on each down stream breakout since the sensor side of the breaker is effectively a new I2C bus at each breakout. On the first breaker at the master, you also need to the pullup resistors unless you already have pullup resistors on the bus elsewhere. For example, raspberry pi’s already have pullup resistors onboard for the I2C pins, so you would need to cut the pullup traces on the breakout connected to a pi.